Essential of Oral Biology oral anatomy, histology, physiology & embryology [2 ed.] 9789387742611

Table of contents :
Half Title Page
Title Page
Copyright Page
Foreword by BH Sripathi Rao
Contributors
Preface to Second Edition
About The Book
About The Author
Section 1: Oral Embryology
1. General Embryology
2. Development of Orofacial Structures
Section 2: Oral Histology
3. Development of Tooth
4. Enamel and Amelogenesis
5. Dentin and Dentinogenesis
6. Pulp
7. Cementum and Cementogenesis
8. Periodontal Ligament
9. Alveolar Bone
10. Oral Mucosa
11. Salivary Glands
12. Temporomandibular Joint
13. Maxillary Sinus
Section 3: Oral and Dental Anatomy
14. Introduction to Dental Anatomy
15. Deciduous Maxillary Anterior Teeth
16. Deciduous Mandibular Anterior Teeth
17. Deciduous Maxillary Molars
18. Deciduous Mandibular Molars
19. Comparison between Deciduous and Permanent Dentition
20. Permanent Maxillary Central Incisors
21. Permanent Maxillary Lateral Incisors
22. Permanent Mandibular Central Incisors
23. Permanent Mandibular Lateral Incisors
24. Permanent Maxillary Canines
25. Permanent Mandibular Canines
26. Permanent Maxillary First Premolars
27. Permanent Maxillary Second Premolars
28. Permanent Mandibular First Premolars
29. Permanent Mandibular Second Premolars
30. Permanent Maxillary First Molars
31. Permanent Maxillary Second Molars
32. Permanent Maxillary Third Molars
33. Permanent Mandibular First Molars
34. Permanent Mandibular Second Molars
35. Permanent Mandibular Third Molars
36. Occlusion
Section 4: Oral Physiology
37. Eruption
38. Shedding
39. Saliva
40. Physiology of Taste and Speech
41. Mastication
42. Deglutition
43. Calcium Phosphorus Metabolism
44. Mineralization
45. Hormonal Influence on Orofacial Structures
46. Age Changes of Oral Tissues
Section 5: Allied Topics
47. Tissue Processing
48. Microscope
49. Muscles of Orofacial Region
50. Vascular and Nerve Supply of Orofacial Region
Appendix

Citation preview

Essentials of Oral Biology Oral Anatomy, Histology, Physiology and Embryology Second Edition

Essentials of Oral Biology Oral Anatomy, Histology, Physiology and Embryology Second Edition

Maji Jose MDS, PhD Professor and Head Department of Oral Pathology Yenepoya Dental College and Hospital Yenepoya University Deralakatte, Mangalore 575018 Karnataka, India Email: [email protected]

CBS Publishers & Distributors Pvt Ltd New Delhi • Bengaluru • Chennai • Kochi • Mumbai • Kolkata Hyderabad • Pune • Nagpur • Manipal • Vijayawada • Patna

Disclaimer Science and technology are constantly changing fields. New research and experience broaden the scope of information and knowledge. The authors have tried their best in giving information available to them while preparing the material for this book. Although, all efforts have been made to ensure optimum accuracy of the material, yet it is quite possible some errors might have been left uncorrected. The publisher, the printer and the authors will not be held responsible for any inadvertent errors, omissions or inaccuracies. eISBN: 978-93-877-4261-1 Copyright © Authors and Publisher First eBook Edition: 2017 All rights reserved. No part of this eBook may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system without permission, in writing, from the authors and the publisher. Published by Satish Kumar Jain and produced by Varun Jain for CBS Publishers & Distributors Pvt. Ltd. Corporate Office: 204 FIE, Industrial Area, Patparganj, New Delhi-110092 Ph: +91-11-49344934; Fax: +91-11-49344935; Website: www.cbspd.com; www.eduport-global.com; E-mail: [email protected]; [email protected] Head Office: CBS PLAZA, 4819/XI Prahlad Street, 24 Ansari Road, Daryaganj, New Delhi-110002, India.

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Foreword

O

ral biology, which includes oral anatomy, histology, physiology and embryology is one of the most important and useful subjects among the various basic science subjects, in the dental curriculum. Understanding of this subject widens the mental comprehension and strengthens the basic concepts of different dental science specialties. A thorough knowledge of this subject is sure to mould a dental student into an effective and efficient clinician. I am happy that Dr Maji Jose is bringing out a textbook Essentials of

Oral Biology

(Oral Anatomy, Histology, Physiology and Embryology) for students pursuing dentistry. Visualizing the integrated perspective of the subject, she has been successful in gathering together the diverse elements of oral biological sciences, which in the past had been scattered throughout many textbooks. I am sure that this book will be useful for dental students to improve their knowledge in oral biological sciences as well as to help them to confidently face the exam. I hope that this work will receive the deserved attention and encouragement from both dental students and teachers. I wish the author and this book all success.

BH Sripathi Rao MDS Principal Yenepoya Dental College and Hospital Former Executive Committee Member; Dental Council of India

Contributors ▪

Dr Heera R

Professor, Department of Oral Pathology Government Dental College Thiruvananthapuram

▪

Dr Girish KL

Professor, Department of Oral Pathology Sri Mookambika Institute of Dental Sciences Kulasekaram, Kanyakumari Tamil Nadu

▪

Dr Rajeesh Mohammed PK

Professor, Department of Oral Pathology KMCT Dental College Mukkam, Kozhikode Kerala

▪

Dr Usha Balan

Assistant Professor College of Dentistry King Khalid University Abah, KSA

▪

Dr Ajeesha Feroz

Department of Oral Pathology

Mahe Institute of Dental Sciences Mahe, UT of Puducherry

Preface to Second Edition

T

he textbook Essentials of Oral Biology presents all subsections of Oral Biology in single book, described in five sections: Oral Embryology, Oral Histology, Oral and Dental Anatomy, Oral Physiology and Allied Topics. This text provides a comprehensive coverage of all the topics included in the curriculum specified by Dental Council of India and various Indian universities. Different topics are dealt in detail in 50 chapters with flowcharts, tables and color diagrams to make learning more simple and pleasant. This book also includes an additional section on expected questions from each chapter, commonly asked in examinations of various Indian universities, to assist the students in examination preparations. This book has been designed in a way to keep the characteristics of a standard textbook for undergraduate students. The topics are explained in simple and lucid language. Concepts are presented in a simple and clear manner to help an undergraduate student develop a comprehensive knowledge in this basic science subject which makes a sound base for learning pathologic basis of diseases. I am gratified that original edition has received a good response. A positive feedback on the first edition of the book and various encouraging comments received from students and teachers, who have used the book, has encouraged me to come out with second edition. The second edition is a revised and updated version with flowcharts, more tables and color diagrams to further ease the learning process. A discussion on clinical considerations is added to each chapter in order to guide the students to clinical application of oral biology. While preparing the second edition, I have followed the original policy “simple presentation and lucid language” which enables a self-study. I offer this book to the dental students, hoping that this will ensure an enjoyable and rewarding study of oral biology.

Maji Jose

Acknowledgements

I

thank God Almighty for all the blessings He has showered on me in this venture. The preparation of this textbook was possible only with the help and cooperation of a number of people. I would like to express my gratitude to Dr Sripathi Rao, Principal, Yenepoya Dental College, for his kind words of encouragement and moral support, received at every stage of the preparation of this book. I also thank him for writing the Foreword to the book. I would like to express my heartfelt thanks to Dr Heera R, faculty of Oral Pathology, Government Dental College, Thiruvananthapuram, my teacher and friend, for giving me all guidance, moral support, and for sharing her knowledge at different stages of my work, and also for contribution in the book. I gratefully acknowledge the constant support of Dr Rajeesh Mohammed PK, Dr Girish KL, Dr Usha Balan and Dr Ajeesha Firoz who have also contributed chapters to this book. I am indebted to Head of the Department and all my colleagues of Department of Oral Pathology, Yenepoya Dental College, Mangalore, especially Dr Joshy, Dr Meera and Dr Haziel Diana Jenifer, for their constructive suggestions and timely support. The talented staff of CBS Publishers & Distributors deserve praise for their role in shaping this book. I owe a great deal of regard and gratitude to my parents, teachers and beloved students who have played a major role in making me what I am today. I thank my husband Mr Ajoy S. Joseph, and my children, Joe and Jiya, who stood by me at all the stages and exhibited patience and affection which enabled me to carry on with the work smoothly. We would like to thank Mr S.K. Jain (CMD), Mr. Varun Jain (Director), Mr. YN Arjuna (Senior Vice President – Publishing and Editorial), and Mr. Ashish Dixit (Business Head – Digital Publishing, Marketing & Sales) and

his team at CBS Publishers & Distributors Pvt. Ltd. for their skill, enthusiasm, support, patience and excellent professional approach in producing and publishing this eBook. Finally, I thank each and everyone whose contribution, direct or indirect, has made the preparation of this book a pleasant task.

Maji Jose

Syllabus Oral Biology course includes instructions in the subject of Dental Morphology, Oral Embryology, Oral Histology and Oral Physiology.

I. TOOTH MORPHOLOGY 1.

2.

3.

4.

Introduction to tooth morphology: Human dentition, types of teeth, function, Palmer’s and binomial notation systems, tooth surfaces, their junctions—line angles and point angles, definition of terms used in dental morphology, geometric concepts in tooth morphology, contact areas and embrasures—clinical significance. Morphology of permanent teeth: Description of individual teeth, including a note on their chronology of development, differences between similar class of teeth and identification of individual teeth. Morphology of deciduous teeth: Generalized differences between deciduous and permanent teeth. Description of individual deciduous teeth, including their chronology of development. Occlusion

II. ORAL EMBRYOLOGY 1. 2.

Brief review of development of face, jaws, lip, palate, and tongue, with applied aspects. Development of teeth: Detailed study of different stages of development of crown, root and supporting tissues of tooth and detailed study of formation of calcified tissues. Applied aspects of

3.

4.

disorders in development of teeth. Eruption of deciduous and permanent teeth: Mechanisms in tooth eruption, different theories and histology of eruption, formation of dentogingival junction, role of gubernacular cord in eruption of permanent teeth. Shedding of teeth: Mechanisms of shedding of deciduous teeth. Complications of shedding.

III. ORAL HISTOLOGY 1. 2. 3.

4. 5. 6. 7.

Detailed microscopic study of enamel, dentin, cementum and pulp tissue: Age changes. Detailed microscopic study of periodontal ligament and alveolar bone: Age changes, histological changes in periodontal ligament. Detailed microscopic study of oral mucosa: Variation in structure in relation to functional requirements, mechanisms of keratinization clinical parts of gingiva, dentogingival and mucocutaneous junctions and lingual papillae and age changes. Salivary glands: Detailed microscopic study of acini and ductal system. TM joint: Review of basic anatomical aspects and microscopic study. Maxillary sinus: Microscopic study, functions and clinical relevance of maxillary sinus in dental practice. Processing of hard and soft tissues for microscopic study: Ground sections, decalcified sections and routine staining procedures.

IV. ORAL PHYSIOLOGY 1. 2.

Saliva: Composition of saliva—formation of saliva and mechanisms of secretion, functions of saliva. Mastication: Masticatory force, need for mastication, peculiarities

3. 4.

5. 6. 7.

of masticatory muscles, masticatory cycle, masticatory reflexes and neural control of mastication. Deglutition: Review of the steps in deglutition, swallowing in infants, neural control of deglutition. Calcium and phosphorus metabolism: Source, requirements, absorption, distribution, functions and excretion, clinical considerations. Theories of mineralization: Definition, mechanisms, theories of mineralization. Physiology of taste: Innervation of taste buds and taste pathway, physiologic basis of taste sensation, age changes. Physiology of speech

About the Book Essentials of Oral Biology Oral Anatomy, Histology, Physiology and Embroyology is the completely rewritten, thoroughly revised, fairly enlarged and prudently updated edition of the popular book. All topics of oral biology are described in five sections: • Oral Embryology, • Oral Histology, • Oral and Dental Anatomy, • Oral Physiology and • Allied Topics. This text provides a comprehensive coverage of all the topics included in the curriculum specified by Dental Council of India and various Indian universities. Various topics covered in 50 different chapters, revised and updated with flowcharts, tables and colour diagrams to make learning more simple and pleasant. A discussion on clinical considerations is added to guide the students to clinical application of oral biology. An additional section on expected questions of each chapter, commonly asked in examinations of various Indian universities, will assist the students on examination preparations. This book is based on more than two decades of experience of the author as a teacher and examiner of oral biology. This edition maintains the hallmark of the earlier edition: Lucid language and simple presentation.

About the Author Maji Jose

is currently Professor and Head, Department of Oral Pathology, Yenepoya Dental College and Hospital, Yenepoya University, Deralakatte, Mangalore, Karnataka. She has more than two decades of experience in teaching oral biology and oral pathology to undergraduate and postgraduate students in various distinguished dental colleges such as Manipal College of Dental Sciences, KVG Dental College, Sullia, and Yenepoya Dental College. She has been examiner for both undergraduate and postgraduate students at various universities. MDS, PhD

Dr Jose, well acknowledged as a teacher, examiner and researcher, obtained PhD in 2013 from Yenepoya University and has to her credit over 50 scientific publications in Indian and international journals of repute. Her other textbook Manual of Oral Histology and Oral Pathology (CBS) is well accepted and widely used by dental students and teachers of many Indian and foreign universities.

Contents

Foreword by BH Sripathi Rao Contributors Preface to Second Edition About The Book About The Author

Section 1: Oral Embryology 1.

General Embryology

2.

Development of Orofacial Structures

Section 2: Oral Histology 3.

Development of Tooth

4.

Enamel and Amelogenesis

5.

Dentin and Dentinogenesis

6.

Pulp

7.

Cementum and Cementogenesis

8.

Periodontal Ligament

9.

Alveolar Bone

10. Oral Mucosa

11. Salivary Glands 12. Temporomandibular Joint 13. Maxillary Sinus

Section 3: Oral and Dental Anatomy 14. Introduction to Dental Anatomy 15. Deciduous Maxillary Anterior Teeth 16. Deciduous Mandibular Anterior Teeth 17. Deciduous Maxillary Molars 18. Deciduous Mandibular Molars 19. Comparison between Deciduous and Permanent Dentition 20. Permanent Maxillary Central Incisors 21. Permanent Maxillary Lateral Incisors 22. Permanent Mandibular Central Incisors 23. Permanent Mandibular Lateral Incisors 24. Permanent Maxillary Canines 25. Permanent Mandibular Canines 26. Permanent Maxillary First Premolars 27. Permanent Maxillary Second Premolars 28. Permanent Mandibular First Premolars 29. Permanent Mandibular Second Premolars 30. Permanent Maxillary First Molars 31. Permanent Maxillary Second Molars 32. Permanent Maxillary Third Molars 33. Permanent Mandibular First Molars

34. Permanent Mandibular Second Molars 35. Permanent Mandibular Third Molars 36. Occlusion

Section 4: Oral Physiology 37. Eruption 38. Shedding 39. Saliva 40. Physiology of Taste and Speech 41. Mastication 42. Deglutition 43. Calcium Phosphorus Metabolism 44. Mineralization 45. Hormonal Influence on Orofacial Structures 46. Age Changes of Oral Tissues

Section 5: Allied Topics 47. Tissue Processing 48. Microscope 49. Muscles of Orofacial Region 50. Vascular and Nerve Supply of Orofacial Region Appendix

Section 1

Oral Embryology 1. General Embryology 2. Development of Orofacial Structures

1 General Embryology

Formation of blastocyst Germ layers Neural crest cells Pharyngeal arches and pouches

E

mbryology is the study of growth and differentiation which an organism undergo during its development from a single fertilized cell to a complex independent living being. Every animal starts life in the form of a simple cell, i.e. the fertilized egg or zygote. Zygote is formed by two cells, namely the germ cells of parents. Fertilization occurs when male and female gamates (spermatozoon and ovum) unite to form zygote. The intrauterine life of human beings can be devided into embryonic period which lasts for 8 weeks after fertilization which will be followed by fetal period which continues throughout pregnancy that ends in birth approximately after 280 days. After fertilization, rapid proliferation of cells takes place leading to formation of a cell mass called morula. This morula is a “golf ball” like a little mass of cells and consists of a group of centrally placed cells termed as inner cell mass, surrounded by a peripheral layer of cells (Fig. 1.1). Once morula enters into the uterine cavity by 7 to 8 days, it turns into a fluid filled structure due to seepage of fluid, which separates the inner cell mass from peripheral layer of cells. The resultant structure is called blastocyst (Fig. 1.2). This blastocyst is lined by a layer of cells called trophoblasts. The trophoblasts are derived from the outer layer of morula, which later gives rise

to placenta and is also involved in implantation of the embryo. Within the blastocyst, the inner cell mass can be seen attached to one side of the inner aspect. This inner cell mass or embryoblasts forms the embryonic stem cells that gives rise to embryo.

Fig. 1.1: Morula

Fig. 1.2: Blastocyst

At this stage, the blastocyst has two different types of cells. The inner cell mass that occupies the center portion and an outer layer that surrounds this cell mass. As the blastocyst develops further, some cells of the inner cell mass differentiate into flattened cells and line the free surface while the other cells change into columnar cells. The flattened cells constitute the endoderm while the columnar cells forms the ectoderm. Thus, by 8th day of gestation the embryo appears like a ‘bilaminar circular disc’. As the development proceeds, in a localized area close to the future cephalic end of the disc, flattened cells of endoderm changes into columnar cells. This circular area where the changes takes place is called ‘prochordal plate’. The region where the prochordal plate is formed is the head end and opposing end is tail end of the embryo. Prochordal plate provides the disc an antero-posterior axis and a bilateral symmetry. After the formation of the prochordal plate, the cells of ectoderm proliferate near the tail end, forming another structure called the primitive streak. These proliferating cells initially form a thickening and later spread

sideways between ectoderm and endoderm forming a third layer called mesoderm. This mesodermal layer spreads and separates the ectoderm and endoderm throughout the disc except for the circular region of prochordal plate. So by the 16th day, the embryonic disc has three layers: Ectoderm, endoderm and mesoderm. These three primary germ layers give rise to different tissues and organs of our body.

Germ Layer Derivatives Structures of Ectodermal Origin are Cutaneous structures • • •

Skin and its appendages Oral mucous membrane Enamel of teeth

Neural system-central and peripheral nerve systems

Structures of Mesodermal Origin are Cardiovascular system—heart and blood vessels Locomotor system—bones and muscles Connective tissue Components of teeth other than enamel

Structures of Endodermal Origin are Lining epithelium of respiratory tract Lining epithelium of alimentary tract Secretory cells of liver and pancreas As the development progresses, the circular disc shaped embryo becomes elongated and pear shaped. The region of prochordal plate where ectoderm and endoderm remain in contact forms the ‘buccopharyngeal membrane’. The cranial end of the primitive streak thickens to form primitive node. The cells proliferate from primitive node and extend between the ectoderm and endoderm, along the central axis up till the prochordal plate. This forms

notochordal process or head process. Ectoderm over the notochord differentiates to form neural plate which develops an invagination and forms the neural tube. This neural tube extends from primitive node to prochordal plate. The cranial part of neural tube forms the brain and caudal part forms the spinal cord. The enlarging embryonic disc develops folds at its head end (cranial fold), tail end (caudal fold) and laterally, making the embryo entirely covered by ectoderm.

Neural Crest Cells Neural crest cells are a group of pleuripotent cells that develop from ectoderm along the lateral margins of neural plate. These cells migrate extensively in the developing embryo between ectoderm and endoderm and intra-mesodermally and differentiate into different types of cells that forms various tissues of the body. The neural crest cells move around the sides of the developing head beneath the surface of ectoderm as sheets of cells. They migrate and form the entire connective tissue of upper facial region; while in the lower facial region they migrate into already existing mesenchyme. Therefore the connective tissue beneath the developing ectoderm in this region is called ectomesenchyme. Derivatives of the branchial arches, pharyngeal pouches and cranial somites

The Structures that Develop from the Neural Crest Cells In the head and neck region neural crest cells differentiate to form most of the connective tissue components including bone, cartilage, dermis and tissues that form tooth except enamel and also contributes to formation of muscles and arteries of this region. Neural crest cells migrate to the trunk region giving rise to neural, endocrine and pigment producing cells. In the trunk sensory ganglions, Schwann cells and neurons are also derived from neural crest cells. Neural crest cells have a significant role in craniofacial development and formation of teeth. A developmental disorder called Treacher Collin syndrome which manifest with various craniofacial developmental defects is caused due to defective migration of neural crest cells. Defective migration of neural crest cells can also cause defective dentition.

Branchial Arches and Pouches The developing oral cavity, stomatodeum is situated between the developing brain and pericardium. In the early stages, neck is not present. Later, series of mesodermal thickenings develop in the wall of the cranial part of foregut resulting in the formation of neck between stomodeum and pericardium. These cylindrical thickenings are called branchial arches or pharyngeal arches (Fig. 1.3). Pharyngeal arches are six in number and extend from lateral wall of pharynx, towards the medial direction, to approach its counterpart extending from other side. The inner aspect of each arch is covered by endoderm and outer aspect by ectoderm. The central core is made up of mesenchyme, which is surrounded by ectomesenchyme, which is of neural crest origin. The endoderm extends outwards between the branchial arches in the form of pouches called pharyngeal pouches. The pharyngeal pouches meet the ectodermal clefts which are formed by invagination of ectoderm lining the outer surface of the pharyngeal arches.

Fig. 1.3: Pharyngeal arches and pouches

The mesoderm of each arch gives rise to a skeletal element (which can be either a cartilage or bone), muscle and an arterial arch. Each pharyngeal arch has a nerve which supplies the structures that develop from that arch. There are six pharyngeal arches. 1st arch is named as mandibular arch, which plays a very important role in craniofacial development. 2nd arch is hyoid arch and the 5th arch disappears soon after formation. The remaining 3, 4, 6 arches do not have specific names.

2 Development of Orofacial Structures Formation of orofacial structure

O

rofacial structures develop primarily from first, second and third branchial arches by fusion of various processes.

Formation of Face Brain and pericardium forms two prominent bulgings on the ventral aspect of the embryo after the head fold is formed. These two prominences are separated by a central depression called stomatodeum which is the developing oral cavity and is formed by an invagination of ectoderm on the ventral surface of future head of the embryo. In the deepest part of the stomatodeum, the lining ectoderm is in contact with endoderm of the foregut. This combined ectoderm and endoderm constitute the buccopharyrngeal membrane which separates the developing oral cavity from foregut. The mesoderm of the forebrain proliferates and forms a bulge that overlaps the upper part of stomatodeum. This downward bulge is called frontonasal process. Face develops from the frontonasal process and the 1st pharyngeal (mandibular) arch of each side. The ectoderm lining the frontal process forms thickenings on both inferolateral borders. These are called nasal or olfactory placodes. These nasal placodes invaginate to form nasal pit. This nasal pit is surrounded by a horseshoe shaped ridge which is formed by rapid proliferation of underlying mesoderm. The medial edge of this ridge is called medial nasal process and lateral edge is called lateral nasal process and the depressed area between the two medial nasal processes is called frontonasal process.

At the same time the mandibular arches that form the lateral wall of stomatodeum gives off a bud-like projection called maxillary process (on either side). The remaining part of the mandibular arch forms the mandibular process. The face is derived from the five prominences that surround the stomatodeum. These prominences are frontonasal process, pair of maxillary processes and a pair of mandibular processes (Fig. 2.1).

Lower Lip Lower lip develops from the mandibular processes which grow medially towards each other and fuses at midline. This forms the lower margin of stomatodeum. As the development continues an ectodermal proliferation occurs which extends into the ectomesenchyme. The structure developed is called vestibular lamina and it gives rise to a V-shaped sulcus that separates the lip from the tooth bearing area.

Fig. 2.1: Development of face

Upper Lip Mandibular arch on either side gives rise to process called maxillary processes. These processes grow forward and medially towards one another above the stomatodeum. As they do so, these processes first fuse with lateral nasal process and later with medial nasal process. The frontonasal process grows downwards at a faster rate and reaches the same level that of maxillary process. The inferolateral part of the frontonasal process is now called as globular process. As the maxillary process grows, the frontonasal process becomes narrower and the external nares formed by the fusion of medial and lateral processes come closer. Both maxillary processes form the major part of lip except for philtrum region. In this region mesoderm is derived from frontonasal process. The ectoderm of the maxillary process overgrows this mesoderm to meet that of the opposite side. The upper lip is separated from the developing jaw in the same manner as that of lower lip.

Cheek After formation of upper and lower lip the lateral margins of maxillary and mandibular processes fuses with each other to form cheek.

FORMATION OF PALATE During the medial growth of maxillary processes, they not only form the upper lip but also extend backward on either side of stomatodeum. From this backward extension of maxillary process, two plates like shelves grow medially. These are called palatal processes (Fig. 2.2). Meanwhile the primary palate is formed from the frontonasal process. Initially these three structures are widely separated because of the vertical orientation of palatal processes (lateral shelves) on either side of the tongue. During 8th week of intrauterine development after the descent of tongue, the palatine shelves alter their position from vertical to horizontal direction as a preparation to their fusion. Two palatal shelves, which grows medially towards each other and fuse in the midline and with the posterior margin of the primary palate to form a flat and unarched roof of the mouth, separating nasal cavity from oral

cavity. Palatal shelves also fuse with nasal septum to separate two nasal cavities. The fusing palatal shelves overlap the primary anterior palate and the junction of union of these three palatal components is marked by incisive papilla overlying the incisive canal.

Fig. 2.2: Development of palate

Ossification of palate starts at the 8th week of intrauterine life by intramembranous ossification of mesoderm. The hard palate grows in length, breadth and height and changes into an arch shaped roof for the mouth. The apposition growth of the alveolar process also contributes to deepening as well as widening of the vault of palate. Ossification does not occur in the most posterior part of the palate giving rise to the region of soft palate. Myogenic mesenchyme from the 1st, 2nd and 4th arches migrate to this region giving rise to musculature of soft palate.

Development of Tongue

The tongue develops in the ventral wall of the primitive oropharynx from the inner lining of 1st, 2nd, 3rd and 4th pharyngeal arches (Fig. 2.3). The mucous membrane lining the oropharynx rises into the developing oral cavity as swellings as a result of invasion by muscle tissue from occipital somites.

Fig. 2.3: Development of tongue

During 4th week of intrauterine life, from the internal aspect of both mandibular arches (1st branchial) mesenchymal thickenings develop which are called lateral lingual swellings. Between and behind these lateral swellings a median swelling named tuberculum impar appears. Immediately behind tuberculum impar the epithelium proliferates to form a down growth from which the thyroid develops. This structure is called thyroid diverticulum or thyroglossal duct. The region where the thyroglossal duct originates is marked by a depression called foramen caecum.

Lateral Lingual Swellings Anterior 2/3rds of the tongue is formed from the mandibular arch by the

fusion of two lateral lingual swellings and tuberculum impar. As the lingual swellings grow and fuse with each other, they over grow the tuberculum impar and therefore the ectodermal lining of entire anterior 2/3rds is derived from these two swellings and is of ectodermal origin. After these structures fuses the epithelium at the periphery proliferates into the mesenchyme to form a horseshoe shaped lamina all around. The central cells of this lamina degenerate to form linguo-gingival groove which separate the body of the tongue from floor of the mouth except for the region of frenum of tongue. The posterior 1/3rd of the tongue develops from another swelling known as hypobranchial eminence. This hypobranchial eminence is derived from 2nd, 3rd and 4th arches. The epithelial lining of posterior 1/3rd is endodermal in origin. As the development progresses the mesoderm of the 3rd branchial arch overgrow the mesoderm of 2nd arch and joins with mesoderm of 1st arch. The second arch mesoderm remains buried below the surface (Fig. 2.3). A V-shaped ‘sulcus terminalis’ demarcate the anterior 2/3rds and posterior 1/3rd of tongue. The posterior most part of the tongue is derived from the 4th arch. The epithelium of the tongue is derived partly from both ectoderm and endoderm and is single layered initially which later turns to stratified squamous epithelium. Circumvallate papillae develop by 2nd to 5th months of intrauterine life. Fungiform papillae develop at an earlier stage by 11th week of intrauterine life while filiform papillae develop later and development is completed only postnatally. The taste buds develop by the inductive interaction between epithelial cells and invading gustatory nerve cells from chorda tympani, glossopharyngeal and vagus nerves. The mucosa lining the posterior part of the tongue becomes pitted by deep crypts that develop into lingual tonsil. The muscles of the tongue have a dual origin. The intrinsic muscles probably arise in situ in the pharyngeal arch mesenchyme while the extrinsic muscles arise in the occipital somite region opposite to origin of hypoglossal nerve. The muscle mass migrates forward beneath the mucosal layers of the tongue which also carries the hypoglossal nerve. In the initial stages of development, tongue enlarges rapidly and occupies the whole of stomatodeum. Later as the stomatodeum increases in size the tongue descends down allowing the palatal shelves to become horizontal. The entire tongue is in the mouth at birth and by the 4th year posterior 1/3rd descends down to pharynx. The size of the tongue doubles in length, width

and thickness after birth, reaching its maximal size by 8 years.

DEVELOPMENT OF MANDIBLE Mandible develops from the mandibular process of first branchial arch. The cartilage and the bone of the mandibular skeleton are formed from neural crest cells. 1st branchial arch has its cartilage namely Meckel’s cartilage; which is a solid rod of hyaline cartilage surrounded by fibrocellular tissue. Meckel’s cartilage attains its full length after six weeks of intrauterine life and extends from midline to the developing ear region. Meckel’s cartilage of each arch shows an upward curve at the ventral end and is separated at the midline by mesenchyme. A great portion of Meckel’s cartilage disappears without contributing to formation of mandible. A small part of the ventral end (near mental foramen) forms the accessory endochondral ossicles that are incorporated in the mandible. The part of the cartilage extending from the mental foramen to the lingula is not incorporated into ossification of mandible. A part of Meckel’s cartilage transforms into sphenomandibular and malleolar ligament. The dorsal end of Meckel’s cartilage ossifies to form incus and malleus, two of the auditory ossicles. The mandibular branch of trigeminal nerve is found in close association with Meckel’s cartilage. This nerve divides into two branches: Lingual nerve and inferior alveolar nerve. The lingual nerve travels along the medial aspect of the cartilage and inferior alveolar nerve along the lateral aspect. More anteriorly the inferior alveolar nerve again divide giving rise to mental and incisive branches. First sign of mandibular development is seen as a condensation of ectomesenchyme in the fibrocellular tissue in the region of division of inferior alveolar nerve to mental and incisive branches. An osteogenic membrane is formed from this condensed ectomesenchyme that is located lateral to cartilage where the ossification of the mandible begins. A single ossification center for each half of the mandible arises in the 6th week of intrauterine life. The ossification spreads from this primary center below and around the inferior alveolar nerve and incisive branch and upwards to form outer and inner plates with a trough between them for the nerve. The spread of intramembranous ossification ventrally and dorsally forms the body and ramus of mandible. As the ossification proceeds, the trough is converted into

a canal containing the nerves. As a result of formation of buccal and lingual bony plates above the level of roof of alveolar canal the developing teeth are found in a bony troughs which is subsequently partitioned by transverse bony septae to form individual bony crypts. Ossification stops dorsally near the point of division of mandibular nerve. The ramus of the mandible develops by the spread of ossification posteriorly into the mesenchyme turning away from Meckel’s cartilage. Between 10th and 14th week of intrauterine life, secondary cartilage develops, which are not related to Meckel’s cartilage which include the condylar cartilage, the coronoid and symphyseal secondary cartilage. The condylar cartilage gives rise to condyle and secondary cartilage of coronoid process form part of coronoid and the secondary cartilage in mental region form variable number of mental ossicles that are incorporated into the bone in symphysis region. The two halves of mandible are united at midline only by 4th-12th month postnatally. The mass of cartilage is converted into bone by endochondral ossification. A thin layer of cartilage persists in the condylar head till the 2nd decade of life which helps in development of mandible while the cartilage component of coronoid disappear before birth.

Alveolar Bone By 2nd month of intrauterine life when the mandible and maxilla is being formed the ossification extend to form a trough-like structure; to protect the developing tooth buds. As the bone formation continues, the part of trough occupying the tooth buds are separated from the nerve by a horizontal plate of bone. Bony septa develop in the trough separating each tooth germ. As the growth continues part of the alveolar bone gets incorporated into the basal bone, adding to its height and thickness. The development of the alveolus depends on the teeth. Alveolar process fails to develop when teeth are absent and undergoes resorption when teeth are lost.

Maxilla Maxilla develops from the mesenchyme of maxillary process of the 1st branchial arch. A primary intramembranous ossification center appears for each maxilla in the 7th week at the termination of infraorbital nerve just above the dental lamina of developing canine. From this center, the ossification proceeds in all directions to form different processes of maxilla.

Ossification also spreads posteriorly to the palate, forming hard palate. A medial alveolar plate develop from the junction of body of maxilla and palatal process which together with the lateral alveolar plate forms a trough around the developing tooth. By the formation of bony septa these troughs are converted into separate bony crypts occupying the developing tooth germs.

Temporomandibular Joint The temporomandibular joint develops from temporal and condylar blastema; which are widely separated initially. Temporal blastema develops from the otic capsule, a component of basicranium that forms the petrous part of the temporal bone. The condylar blastema arises from the secondary condylar cartilage of the mandible. Initially the temporal articular fossa is convex which later turns to concave shape. Temporal and condylar blastema are widely separated by mesenchyme which gradually become closer by the growth of condyle. By 10th week of intrauterine life two clefts develop in the interposed fibrovascular connective tissue resulting in formation of two distinct joint cavities. The remaining strip of connective tissue becomes articular disc. Condensation of mesenchyme around the developing joint forms the analage of joint capsule, progressively isolating the joint with its synovial membrane from surrounding connective tissue. Immediately after birth, the temporomandibular joint is a lax structure with the mandibular fossa and articular eminence forming the flat surfaces. The joint attains the adult form by the 12th year of life.

Maxillary Sinus/Paranasal Sinuses Paranasal sinuses include maxillary, sphenoid, frontal and ethmoid, which begin their development as outpouching of mucous membrane of the middle and superior nasal meatus and sphenoethmoidal recess at mound 4th month of intrauterine life. Expansion of paranasal sinus occurs in two stages. Primary pneumatization causes expansion of the sinus to the cartilage wall and root of nasal fossa by growth of mucous membrane sac into maxillary, sphenoid, frontal and ethmoid bone. Secondary pneumatization causes enlargement of the sinus into the bone, always retaining the communication with nasal fossa through ostea.

At the time of birth maxillary sinus is the only paranasal sinus which is large enough to be evident radiographically. Other sinuses are rudimentary at the time of birth. Development of all paranasal sinuses continues in postnatal life.

Salivary Glands The development of salivary gland begins with proliferation of epithelium to form a bud under the influence of underlying ectomesenchyme. The epithelial bud undergoes further proliferation and turns into a solid chord of cells. Multiplication of cells at the end of these chord leads to formation of bulbs, which undergo extensive branching to form numerous bulbs. As the development progresses canalization of the chords occurs forming a central tube or duct. The terminal secretary acini and intercalated ducts differentiate from the terminal ends of the branches. The connective tissue below the epithelial chord differentiates into a capsule which surrounds the entire glandular structure. Parotid buds are the first to appear at 6th week of intrauterine life on the inner cheek, near the angle of the mouth and then grow back towards the ear. As the maxillary and mandibular processes fuse the opening of the duct is pushed backwards. The submandibular gland bud appears later by 6th to 8th weeks of intrauterine life on either side of the midline in the linguo-gingival groove of the floor of the mouth at the site of future papillae. Sublingual gland arises in the 8th week just lateral to the submandibular gland bud. Minor salivary glands also develop nearly at the same time.

Section 2

Oral Histology 3. Development of Tooth 4. Enamel and Amelogenesis 5. Dentin and Dentinogenesis 6. Pulp 7. Cementum 8. Periodontal Ligament 9. Alveolar Bone 10. Oral Mucosa 11. Salivary Glands 12. Temporomandibular Joint 13. Maxillary Sinus

3 Development of Tooth

Introduction Dental lamina Stages in development of tooth – –

Physiological stages Morphological stages

Root formation Clinical considerations

D

evelopment of tooth is a complex process initiated, mediated and controlled by the interaction between ectoderm and supporting ectomesenchyme. The epithelium lining the developing oral cavity, i.e. stomodeum/stomatodeum, is derived from ectoderm of first branchial arch and is composed of stratified squamous epithelium and the supporting connective tissue is ectomesenchymal in nature which are derived from neural crest cells. Tooth development begins at 3rd week of intrauterine life after the developing oral cavity establishes a communication with developing pharynx. Rupture of buccopharyngeal membrane, i.e. a membrane formed by juxtaposition of stomodeal ectoderm with the foregut endoderm without supporting mesoderm, results in this communication. First sign of the tooth development is proliferation of oral ectodermal cells to form an epithelial thickening called primary epithelial band, that projects into the underlying ectomesenchyme along the future tooth bearing regions of each jaw (Fig. 3.1a). Primary epithelial band forms partly because of proliferation of

epithelial cells and partly due to change in orientation of mitotic spindles of dividing epithelial cells from parallel to a perpendicular direction. This primary epithelial band is formed by the 6th week of intrauterine life. The primary epithelial band continue the proliferative activity and by 7th week, two subdivisions arise: One buccal and one lingual. The lingual extension is dental lamina and facial or buccal extension which develop a little later from the primary epithelial band is called vestibular lamina (Fig. 3.1b). The vestibular lamina proliferates further and form a wedge shaped structure, the central cells of which enlarge and further undergo degeneration forming a ‘V’ shaped cleft or vestibule. This vestibule separates the lips and cheeks from the tooth bearing area. This vestibular lamina is also called lip furrow band. The lingual extension, the dental lamina contributes to formation of teeth (Fig. 3.1c).

DENTAL LAMINA Dental lamina is the lingual subdivision that forms from the primary epithelial band and is intimately concerned with tooth formation. The dental lamina proliferates into the underlying ectomesenchyme and forms a Ushaped band along the future dental arches in each jaw (Fig. 3.2). This structure that forms in the 7th teeth (Fig. 3.1c) Localized/differential week of intrauterine life, acts as the primor-proliferative activity at 10 specific regions of dium for the enamel organ of the deciduous dental lamina of upper and lower dental arches, between 6th and 8th weeks of intrauterine life, results in formation of round or ovoid structures that protrude into the ectomesenchyme. These dental placodes form along the dental lamina further develop into tooth buds that gives rise to enamel organ of 10 deciduous teeth in each arch. Later in prenatal life, permanent successors develop from the lingual extensions that proliferate from the developing deciduous tooth germs. These lingual extensions are called successional lamina. Successional lamina of central incisor develops at 5th month in utero and second premolar at 10th month of age. The permanent molars develop from the distal/posterior extension of dental lamina, referred to as accessional lamina or parent dental lamina or lamina of permanent molars. Permanent first molar buds develop at 4th month of intrauterine life and second molar at 1st year and third molar at 4th or 5th year of life.

Fig. 3.1a: Formation of primary epithelial band

Fig. 3.1b: Vestibular lamina and dental lamina

Fig. 3.1c: Formation of tooth bud from dental lamina and vestibule from vestibular lamina

Fig. 3.2: U-shaped dental lamina

The activity of dental lamina starts at the midline of each arch and progresses posteriorly. As the tooth development proceeds through various stages, the dental lamina related to those particular tooth breaks up by mesenchymal invasion and eventually degenerates, while it is still active in the region of posterior teeth. Average period of activity of dental lamina is 5 years. A few remnant cells may persist even after the degeneration of dental lamina. These remnants may be seen in connective tissue of gingiva or in the jaw bones and are named as cell rests of Serres. These cell rests may proliferate under certain conditions giving rise to odontogenic cysts or tumors.

STAGES OF DEVELOPMENT OF TOOTH Proliferation at 10 specific locations of dental lamina in each arch gives rise to 10 knob like small swellings that project into the ectomesenchyme, each refers to as a bud. These buds along with underlying ectomesenchyme form the tooth germ of 10 different deciduous teeth in each dental arch.

The tooth germ is an aggregate of different types of cells, derived from the ectoderm of the first branchial arch and the ectomesenchyme containing the neural crest cells. This tooth germ is the primordia for developing tooth and eventually contribute to formation of different tissues of a tooth. These cells of tooth germ are organized into three distinct parts, i.e. the enamel organ, the dental papilla and the dental sac or follicle. Thus, the components of tooth germ are: Enamel organ: This develops from the dental lamina and hence is ectodermal in origin and is primarily involved in formation of enamel. Dental papilla: This component is ectomesenchymal in origin and gives rise to dentin and pulp. Dental follicle or sac: This is also an ectomesenchymal component and is responsible for formation of cementum, periodontal ligament and part of the alveolar socket. As the tooth development proceeds, various changes are observed in different components of the tooth germ. The enamel organ enlarges and changes its shape to determine the shape of the tooth to be developed. Based on the shape of the enamel organ, the developmental stages of tooth can be divided into various morphological stages, namely bud stage, cap stage and bell stage. As the tooth development proceeds through various morphological stages, many physiological changes also take place in different components of tooth germ which are named as physiological processes.

Morphological Stages Bud stage

Cap stage • •

Early cap stage Late cap stage

Bell stage • •

Early bell stage Advanced bell stage

Physiological Phases Initiation Proliferation Morphodifferentiation Histodifferentiation Apposition

PHYSIOLOGICAL PHASES AND ITS IMPORTANCE As the morphological changes progress in different components of tooth germ, physiological process goes on simultaneously to ensure formation of a normal tooth of appropriate size, shape and structure. Different physiological changes occur concurrently in each morphological stage and this overlaps makes a direct correlation of both these stages difficult. The physiological process happen in a tooth germ can be considered under the following subheadings: Initiation: The evidence of tooth formation is observed as early as the 6th week of intrauterine life with formation of dental lamina. At ten specific regions of this dental lamina, bud-like structures develop, which forms the primordium of ten deciduous teeth in each arch. Similarly, permanent teeth also develop from lingual and distal extensions of the dental lamina. The process of initiation is the result of epithelial mesenchymal interaction and this decides the commencement of tooth formation. It decides the number of

teeth to be formed and their location in the dental arch.  Any disturbance in initiation can lead to disturbance in number of teeth. Congenital absence of teeth is due to lack of initiation which can be single or multiple. Similarly, development of extra tooth, i.e. supernumerary tooth will be formed due to continued budding off from the dental lamina. Proliferation: The process of proliferation of cells begins at bud stage and continues through bell stage. This process helps to provide adequate cells for the further development of tooth germs and also contribute to the determination of shape of crown.  As with defect in initiation, a defect in proliferation in early stages of tooth formation results in failure of tooth germ to develop further and leads to formation of less number of teeth than normal. Lack of proliferation in later stages may lead to formation of a tooth which is defective in size and shape. Excessive proliferation of cells may lead large and defective teeth and continued inherent proliferative potential of epithelial rests, may lead to formation of odontogenic cysts and tumors. Morphodifferentiation: It is the physiological process that ensure the normal shape and size of the developing tooth. This process begins in cap stage and becomes maximum at early bell stage. The process of morphodifferentiation, as it proceed through different stages, make sure that the tooth germ is changed from an undifferentiated stage (bud stage) to more differentiated bell stage. Enamel organ develops an invagination in the deeper part in cap stage which further deepens in bell stage. The shape of the tooth crown is defined and established in early bell stage when the formative cells, i.e. ameloblasts derived from inner enamel epithelium and odontoblasts derived from dental papilla are arranged to outline the future dentino-enamel junction designated by basement membrane. At this stage the basement membrane separating the inner enamel epithelium/ameloblasts from dental papilla/odontoblasts is called ‘membrana preformativa’ and is considered as the blue print for crown formation as this indicate the form and size of the tooth to be formed.  The process of morphodifferentiation continues in advanced bell stage during root formation, when the Hertwig’s epithelial root sheath determines the shape, size and number of roots of forming tooth.  Disturbance in morphodifferentiation lead to abnormal form and size of

teeth. Examples are peg-shaped laterals, microdontia, macrodontia, etc. Histodifferentiation: It is the physiologic process by which the cells undergo morphologic and functional changes to perform their function. This stage is the forerunner of appositional activity and the differentiated cells loose the capacity to proliferate. Histodifferentiation begins in the cap stage and is maximum in bell stage. During this stage, the inner enamel epithelial cells differentiate into ameloblasts, the enamel forming cells and dental papilla cells into odontoblasts, the dentin forming cells. Interaction between inner enamel epithelium and dental papilla is essential for proper histodifferentiation.  Amelogenesis imperfecta, characterized by defective enamel formation is caused by defective histodifferentiation of ameloblasts. Vitamin A deficiency can also affect differentiation of ameloblasts leading to defective enamel and dentin formation. Similarly, defective histodifferentiation of odontoblasts results in a condition called dentinogenesis imperfecta. Apposition: The process of rhythmic deposition of dental hard tissue is called apposition. Once the dentino-enamel junction is established, the formative cells start successive deposition of organic matrix which gets mineralized to form dental hard tissues.  Any systemic or local factors that affect the activity of ameloblasts can cause arrest or interruption of matrix deposition or mineralization, leading to a condition called enamel hypoplasia. Since the ameloblasts are highly sensitive cells, enamel hypoplasia is a common condition while hypoplasia of dentin occurs only in case of severe systemic disturbance.

MORPHOLOGICAL STAGES OF TOOTH DEVELOPMENT Bud Stage (Fig. 3.3) The primordia for teeth are seen as structures budding off from the basal layer of the oral ectoderm, lining the dental lamina. These buds form the enamel organ of tooth germs. In this stage, the enamel organ is round or ovoid resembling a bud and hence this stage is referred to as bud stage. The

enamel organ is separated from the surrounding connective tissue by a distinct basement membrane. At this stage, two types of cells are seen histologically. The cells lining the periphery of the bud are cuboidal and the central cells are polyhedral. The cuboidal cells are continuous with the basal layer of oral epithelium.

Fig. 3.3: Bud stage of tooth development

The ectomesenchymal cells adjacent to enamel organ undergoes proliferation and condensation forming dental papilla which gives rise to dentin and pulp. The peripheral portion of the condensed ectomesenchymal cells encloses the dental papilla and enamel organ and is called dental sac or dental follicle, which later contributes to formation of cementum, periodontal ligament and part of alveolar socket. During this stage the dental papilla and dental sac cannot be differentiated as distinct parts. The cells of the enamel organ in this stage are in the physiologic phase of proliferation.

Cap Stage (Fig. 3.4) The cells of the enamel organ proliferate further resulting in increased size of enamel organ. Since the growth is unequal, the tooth bud does not expand uniformly and it gradually evolves into a cap shaped structure. This results from an invagination that develops at its deeper part. The convex surface of

the cap faces the oral cavity. At this stage the tooth germ appears like a cap of enamel organ sitting on a ball of dental papilla, both enclosed in a sac of dental follicle. The physiological changes that can be appreciated are proliferation, histodifferentiation and morphodifferentiation. Cap stage of the tooth development is arbitrarily divided into early and late stages solely based on phase of development of stellate reticulum, i.e. the central cells of enamel organ are polyhedral in early stage and form a network of stellate shaped cells in later stage.

Histology of Cap Stage Enamel organ: In this stage the cells of the cap shaped enamel organ exhibits distinct arrangement and shows three different types of cells. Inner enamel/dental epithelium: The cells lining the invaginated portion of enamel organ changes into low columnar cells and are named as inner enamel/inner dental epithelium. This layer is separated from dental papilla by a distinct basement membrane. The cells are attached to each other and to the cells adjacent layers by desmosomal junctions and to the basement membrane by hemidesmosomes.

Fig. 3.4: Cap stage of tooth development

Outer enamel/dental epithelium: The cells lining the convex portion of the

cap remain cuboidal and are named as outer enamel/outer dental epithelium. This layer is separated from dental follicle by a distinct basement membrane. The cells of this layer are also attached to each other by desmosomal junctions and to the basement membrane by hemidesmosomes. Stellate reticulum: The central cells are polyhedral in early cap stage and later turn into star-shaped cells called stellate reticulum.

Stellate Reticulum Stellate reticulum is layers of star-shaped cells present at the center of enamel organ of cap and bell stage of tooth development. In cap stage, the central cells proliferate and increase number. When this happens, the central cells move away from its source of nutrition. So, these cells start secreting glycosaminoglycans into the intercellular spaces. Since glycosaminoglycans are hydrophilic, water is attracted to the intercellular spaces, leading to widening of the intercellular compartment. Hence, the cells are forced apart while retaining their intercellular junctions which results in the change of polyhedral cells into starshaped cells. This gives the appearance of a network of star-shaped cells at the center portion of the enamel organ referred to as stellate reticulum. Stellate reticulum is also called enamel pulp. This stellate reticulum cells undergo degeneration and collapses during bell stage.

Functions of Stellate Reticulum Mechanical protection: The stellate reticulum with its rich content of water and glycosaminoglycans, act as a shock absorber and protect the ameloblast layer (inner enamel epithelium) from any type of mechanical insult. Nutrition: Because of high glycosaminoglycans, this layer may act as a source of nutrition to the neighboring cells, especially at the time when there is a change in source of nutrition of ameloblasts.

Transitory Structures of Enamel Organ The enamel organ shows the presence of some transitory or temporary structures during the cap stage which may disappear in bell stage. These structures may or may not be present and may not have any specific role in tooth development. These structures are:

Enamel knot: It is a transitory cluster of non-dividing ectodermal cells, present as knob-like projection at the deepest part of invagination of enamel organ which partly project into the dental papilla. The primary enamel knot forms in both incisors and molar tooth germs at the cap stage of tooth development. Secondary and tertiary enamel knots only develop in molar tooth germs and are seen at the sites of future cusp tips from the early bell stage of tooth development. Enamel knot cells do not show cell division and after their transient organizing role is complete, they undergo apoptosis at the end of the bell stage.  The enamel knots are thought to have significant role in tooth development and morphogenesis. Cells of enamel knot express several signaling molecules, together with mesenchymal signals, play important roles in regulating the patterning of the cusps and hence the shape of the tooth crown. Enamel cord: This structure is seen as a condensation of ectodermal cells in a linear pattern extending from enamel knot to the outer enamel epithelium. The attachment of the enamel cord to outer enamel epithelium is close to the attachment of dental lamina to enamel organ. This is composed of cells with elongated nuclei and can be easily differentiated. This enamel cord may also be playing role in determination of crown pattern. Enamel septum: Sometimes the enamel cord becomes thick in a buccolingual direction forming a septum partly dividing the enamel organ which is called enamel septum. The above three structures are formed due to the rapid proliferation of the enamel organ cells and these structures act as reservoirs of extra cells, later contributing them to the growing enamel organ. The enamel knot and cord may also be playing a role in determination of crown pattern. A small invagination is seen in the area where the enamel cord joins the outer enamel epithelium and is named as enamel navel. Enamel niche: In histological sections of cap stage and early bell stage of tooth development, another apparent structure is seen called as ‘Enamel niche’. Niche by definition is a defect in an otherwise even surface. The dental lamina is a sheet of cells with irregular depressions into which the surrounding ectomesenchyme is packed in. So when the sections of developing teeth are prepared in a single plane, it appear as the enamel organ is attached to the oral epithelium by two dental laminae; one buccal and one

lingual separated by area filled with mesenchymal tissue and is referred to as enamel niche.

Dental Papilla The dental papilla cells undergo further proliferation and condensation during cap stage. As the enamel organ invaginates, the dental papilla becomes partly enclosed in the invaginated portion. Dental papilla also shows active proliferation of blood vessels. At this stage of tooth development, dental papilla is the main source of nutrition to the inner enamel epithelium.

Dental Follicle The marginal condensation of the ectomesenchymal cells enclosing the dental papilla and enamel organ becomes more conspicuous at this stage. This layer becomes denser and fibrous, forming a well-formed structure that encloses the enamel organ and dental papilla. This layer gives rise to cementum, periodontal ligament and a part of alveolar socket.

BELL STAGE Early Bell Stage (Fig. 3.5) Different components of the tooth germ undergo further changes, which include proliferation, histodifferentiation and morphodifferentiation so that it enters the bell stage. During this stage the enamel organ enlarges and the invagination deepens further to resemble a bell. Various synthetic cells undergo differentiation in bell stage. The shape of the tooth to be formed is also determined during this stage. Therefore histodifferentiation and morphodifferentiation occurs in a rapid manner in bell stage. Dental lamina which was providing an attachment of enamel organ to the oral ectoderm undergoes degeneration and the enamel organ looses its connection to oral ectoderm.

Histology of Early Bell Stage Enamel organ: Enamel organ shows four distinct groups of cells during early bell stage.

Inner enamel epithelium: The inner enamel epithelial cells undergo histodifferentiation to form ameloblasts; the cells that synthesize enamel. These cells are separated from dental papilla by a distinct basement membrane, which is called membrana preformativa, which is considered as the blue print of crown. Differentiated inner enamel epithelial cells are columnar in shape with a length of 40 microns and width of 4 to 5 microns. As preparation to amelogenesis, these cells develop rich cytoplasmic organelles required for protein synthesis. The nucleus is situated at the center of the cell. Shortly before the beginning of amelogenesis the nucleus of these cells shifts to the proximal end (away from base), allowing the synthetic organelles to move to the secretory end of the cell, i.e. basal region. This phenomenon is called reversal of polarity. As the tooth germ enters into advanced bell stage, these inner enamel epithelial cells become fully functional ameloblasts and later changes its shape and structure according to the function it performs. Once the enamel formation is completed it becomes a part of reduced enamel epithelium. Stratum intermedium: During bell stage a new layer composed of 2-3 layers of squamous cells appear in the enamel organ, immediately above the inner enamel epithelium. These cells are attached to each other and to adjacent layers by desmosomes and are rich in synthetic organelles and high alkaline phosphatase content. Therefore these cells are thought to be supporting the ameloblasts in formation of enamel and both these cell layers are considered as single functional unit contributing to enamel formation. Furthermore, the absence of this layer in Hertwig’s epithelial root sheath that helps in formation of root; also support their possible role in enamel formation. The inability of the inner enamel epithelial cells of Hertwig’s epithelial root sheath to perform a secretory function is probably due to the absence of stratum intermedium in Hertwig’s epithelial root sheath. Enamel knot is thought to be contributing cells for formation of this layer.

Fig. 3.5: Early bell stage of tooth development

Stellate reticulum: During early bell stage this layer is well recognizable as a network of star shaped cells with long processes anastomosing with those of adjacent cells. The cells contain nucleus and other cytoplasmic organelles and they are tightly attached to adjacent cells by desmosomes. Alkaline phosphatase content is also present in these cells.  As the tooth development progresses the stellate reticulum collapses and reduce the distance between inner enamel epithelial cells and nutrient capillaries located in dental follicle adjacent to outer enamel epithelium. This is of importance as the source of nutritional supply changes after the initiation of apposition. Later stellate reticulum completely collapses and becomes a part of reduced enamel epithelium. Outer enamel epithelium: The cuboidal cells forming the outer enamel epithelium changes to flattened cells. In the early bell stage this layer forms a regular smooth convex outer boundary of enamel organ. Before the enamel formation starts, as stellate reticulum collapses, the outer enamel epithelium becomes irregular and folded, allowing the capillaries in dental follicles to become more closer to ameloblasts to ensure adequate nutritional supply. Adjacent cells are attached to each other by desmosomes and are separated from dental follicle by a distinct basement membrane. Cervical loop or zone of reflection: In the cervical part of enamel organ, the

outer enamel epithelium loops inwards and joins the inner enamel epithelium to form cervical loop. The cervical loop has only two layers; stellate reticulum and stratum intermedium are absent between these layers of outer and inner enamel epithelium. In advanced bell stage, after enamel formation is complete, cervical loop proliferates giving rise to Hertwig’s epithelial root sheath that helps in formation of root. Dental lamina that attaches the enamel organ to the oral ectoderm starts to degenerate in early bell stage. The remnants of this dental lamina may persist, which are called ‘cell rests of Serres’. From the bell stage onwards the enamel organ is not connected to oral ectoderm. Successional lamina During early bell stage, before dental lamina begin to degenerate, a lingual extension of dental lamina develop by proliferation of cells in the region closer to attachment of dental lamina to the enamel organ. Table 3.1: Layers of enamel organ in different morphologicai stages Bud stage Peripheral cuboidal cells Central polyhedral cells Cap stage Inner enamel epithelium Stellate reticulum Outer enamel epithelium Bell stage Inner enamel epithelium Stratum intermedium Stellate reticulum Outer enamel epithelium This structure is called successional lamina and these forms the primordia of permanent successors which go through various morphological stages and physiological processes as the primary tooth bud, to give rise to permanent

successors. Dental papilla In bell stage, the dental papilla becomes completely enclosed in the invagination of enamel organ. The ectomesenchymal cells are closely packed and interspersed with fine collagen fibers and capillaries. Dental papilla is separated from enamel organ by basement membrane and the basement membrane at this stage is referred to as membrana preformativa, the blue print of crown. During bell stage, the inner enamel epithelial cells exert an organizing influence on the dental papilla cells adjacent to them so that a peripheral layer of cells undergo histodifferentiation into odontoblasts, the synthetic cells of dentin. Initially the peripheral cells arrange themselves to a distinct layer. Later they change their shape to cuboidal and then become columnar. As a preparation to dentin deposition, these cells develop rich cytoplasmic synthetic organelles and nucleus shifts away from the secretory end of cells. The remaining portion of dental papilla becomes the pulp of the formed tooth.

Dental Sac/Follicle The dental follicle becomes more distinct at this stage of tooth development; with more dense fibrous component. Three distinct layers, i.e. the inner vascular fibrocellular condensation of two to four cell layer thick, middle loose connective tissue and outer vascular mesenchymal layer can be appreciated in the dental follicle. The dental follicle gives rise to three important entities: Cementoblasts forming the cementum of a tooth; fibroblasts of developing periodontal ligament which connect teeth to the alveolar bone and osteoblasts forming the alveolar socket.

Advanced Bell Stage (Fig. 3.6) Once the apposition process begins, the tooth germ enters into advanced bell stage. Dentin is the first hard tissue formed in a tooth and enamel formation can be initiated only after a layer of dentin is deposited. During this stage, histological evidence of enamel and dentin formation can be appreciated. As the hard tissue formation continues the outer enamel epithelium becomes more irregular and stellate reticulum collapses further. Dentin deposition by differentiated odontoblasts begins at dentino-enamel junction in the region of cusp tip, progresses inwards/pulpally and cervically. The differentiated

ameloblasts deposit enamel over dentin which starts at the incisal edge or cusp tip at dentino-enamel junction and progresses outward and cervically. Once the enamel and dentin formation reaches the cervical region of tooth, root formation begins by formation of Hertwig’s epithelial root sheath from the cervical loop of enamel organ.

Fig. 3.6: Advanced bell stage of tooth development

Once the formation of enamel is completed the columnar ameloblasts shorten to cuboidal and along with other collapsed layers of enamel organs form a 2-3 layered stratified epithelium which is termed as reduced enamel epithelium (REE). This reduced enamel epithelium covers the newly formed enamel and protects it till the tooth erupts into oral cavity and also play an important role in establishing the dento-gingival junction.

ROOT FORMATION Root formation (Fig. 3.7) begins in advanced bell stage after the enamel and dentin formation reaches the cervical region at future cemento-enamel junction. At this stage, the enamel organ at the cervical loop proliferates giving rise to a structure called Hertwig’s epithelial root sheath (HERS).

This Hertwig’s epithelial root sheath determines the number, size and shape of the root. As it is developing from a bilayered cervical loop, the Hertwig’s epithelial root sheath has only two layers; inner layer of columnar cells derived from inner enamel epithelium and outer layer of cuboidal cells derived from outer enamel epithelium. The Hertwig’s epithelial root sheath is supported by a basement membrane which separates this structure from the dental papilla present at inner aspect and dental follicle present at the outer aspect. As it proliferates, Hertwig’s epithelial root sheath bends to attain a horizontal position to form a structure termed epithelial diaphragm. This structure extends between dental papilla and dental sac separating both, except for a small portion at the center. This part is the future apical foramen. Once the epithelial diaphragm is formed, further proliferation of HERS occurs at the proximal end adjacent to the cervical part of the tooth. This proliferation results in the downward shift of epithelial diaphragm which maintains the same horizontal plane. As the HERS proliferates, the cells of dental papilla also proliferate to fill the gap created by the apical shift of epithelial diaphragm. Meanwhile, the inner enamel epithelial cells lining the inner aspect of HERS exert an organizing influence on adjacent dental papilla cells to differentiate into odontoblasts. These odontoblasts begin to secrete dentin and once a layer of radicular dentin is formed, in that region HERS loose continuity due to invasion by proliferating dental follicle cells. At this stage HERS appears as network of cells which eventually undergoes degeneration to leave only few remnants. Degeneration of HERS allows the dental follicle cells to come in contact with newly formed dentin. These dental follicle cells that come in contact with newly formed dentin differentiate into cementoblasts and begin to deposit cementum on the outer surface of dentin. Radicular dentin formation continues apically and inward while cementum formation continues apically and outward till the entire length of the root is formed. As the cementum formation proceeds the collagen fibers of the dental follicle get inserted into the cementum which becomes a part of periodontal ligament. Formation of HERS and deposition of dentin and cementum are step-by-step processes. So the entire length of HERS cannot be appreciated in a histological section of root formation. Once the desired length of root is formed, the lengthening of HERS stops. After this, the inner cells of epithelial diaphragm causes differentiation of odontoblasts adjacent to them. These odontoblasts deposit dentin along the inner aspect of epithelial diaphragm narrowing the opening of the apical

foramen.

Fig. 3.7: Root formation

Fig. 3.8: Multiple root formation

(Note the tongue-like extentions growing towards each other and fusing to divide the single root into 2 or 3 roots)

The Hertwig’s epithelial root sheath do not undergo complete degeneration, instead remnants may persist, which move away from the root surface and remain in the periodontal ligament and are called cell rests of Malassez. These cell rests may proliferate under certain conditions and can give rise to pathologies like odontogenic cysts or tumors. The roots of multirooted teeth develop in a manner similar to that of single-rooted tooth till the furcation area. To form multiple roots, tongue-like projections develop from the inner aspect of epithelial diaphragm, due to differential growth, which grow towards each other and then fuse. This results in division of single opening into two or three. The number and type of root formed depends on the number and position of the tongue-like projections. Thus, the HERS helps to determine the number, size and shape of the root and help in differentiation of odontoblast that deposit radicular dentin. All three components of the tooth germ function together, to give rise to various tissue component of teeth. Enamel organ, the only ectodermal component of tooth germ perform many functions other than giving rise to enamel.

Functions of Enamel Organ Enamel organ determine the morphological form or the shape and size of the crown Inner enamel epithelial cells differentiate to form ameloblasts that deposit enamel Inner enamel epithelial cells helps in odontoblast differentiation Cervical loop of enamel organ proliferates to give rise to Hertwig’s epithelial root sheath; the structure that determines the size, shape, type and number of roots. Once the formative function is completed enamel organ assumes a protective function by forming a reduced enamel epithelial layer around the newly formed enamel. Reduced enamel epithelium which is developed from enamel organ,

elaborates enzymes that have a role in eruption of tooth. Reduced enamel epithelium helps in establishing a dento-gingival junction.

Clinical Considerations The development of tooth is a complex process controlled by various factors. Therefore this process may be disturbed by defect in genetic control, nutritional or hormonal imbalances, infections or disturbances in local environment where the tooth development occurs, resulting in various anomalies. Developmental anomalies of teeth may be grouped into those affecting number, size, shape, structure, location, etc. 1. Supernumerary teeth or hyperdontia refers to a condition where extra teeth than normal are present in dental arch. This may develop from an additional initiation of dental lamina near the permanent tooth bud or by splitting of the permanent tooth bud itself. The most common supernumerary tooth is ‘mesiodens’ occur as an extra small conical tooth located between the two maxillary central incisors. Other supernumerary teeth include: Distomolar situated distal to the third molar and paramolars located either buccal or palatal to the molars. Multiple supernumerary teeth are present in disease conditions such as ‘Gardner’s syndrome’ and ‘cleidocranial dysplasia’. 2. Anodontia or hypodontia refers to absence of all teeth, i.e. total anodontia or some teeth, i.e. partial anodontia. True anodontia is congenital absence of teeth which occur due to lack of initiation. The absence of third molars is very common, followed by the second premolar and lateral incisor. Anodontia or hypodontia is usually a feature of a condition termed as hereditary ectodermal dysplasia. 3. Microdontia is a condition wherein the teeth are smaller and Macrodontia is larger teeth than normal. Pituitary dwarfism causes generalized microdontia and pituitary gigantism causes generalized macrodontia. Peg shaped lateral incisor is the most common single tooth that appear as microdont. The term Rhizomicry is used when the roots are smaller than normal and Rhizomegaly refers to abnormally larger roots. Tooth may have extra root or cusps than normally expected referred to as supernumerary cusps or roots.

4. Disturbances affecting the shape of the teeth: Talon cusp is an anomalous cusp-like structure projecting from the lingual aspect, in the region of cingulum of maxillary and mandibular incisors and Taurodontism characterized by rectangular shaped tooth resembling that of a Bull’s tooth. Gemination is a condition that occur when a single tooth germ divide, by an invagination resulting in incomplete formation of two teeth. If complete division occurs giving rise to two smaller teeth, identical in appearance, it is referred to as twinning. Similarly, two normally separated tooth germs may fuse (join) together to form a single large tooth. When fusion of two teeth occurs by the deposition of cementum, it is called concrescence. Dens evaginatus is a developmental anomaly characterized by the presence of a globule of enamel or an extra cusp on the occlusal aspect between the buccal and lingual cusps of premolars. Dens invaginatus (dens in dente) is a developmental anomaly affecting the shape of the tooth which occur due to invagination of enamel organ into the dental papilla during odontogenesis giving rise to a tooth within a tooth appearance. Trauma to a developing tooth germ may cause a bend or curve in the crown or root and is referred to as dilaceration. Congenital syphilis is a bacterial infection that may result in gross anomalies of incisors and first molars and collectively these defects are called Hutchinson’s teeth. Central incisors may assume a screwdriver shape and molars, a characteristic mulberry-like appearance. 5. Amelogenesis imperfecta, dentinogenesis imperfecta, dentin dysplasia and regional odontodysplasia are a few of the developmental structural defects. Various environmental factors such as fluorosis, nutritional deficiencies, infections, etc. may also disturb odontogenesis. 6. During root formation, some of the remnants of Hertwig’s epithelial root sheath remain attached to the root surface and may attain a capacity to form enamel and deposit a globule enamel on surface of root near cemento-enamel junction or close to furcation area, referred to as enamel pearl. Similarly, degeneration of Hertwig’s epithelial root sheath before radicular dentin formation may result in accessory root canals.

4 Enamel and Amelogenesis

Introduction Physical properties and chemical composition Amelogenesis Structure of enamel Clinical considerations

E

namel is the hardest calcified tissue of the body covering the anatomic crown of tooth. Enamel is a unique calcified tissue which is different from other calcified tissues of the body.

Characteristic Features of Enamel Ameloblasts, the enamel forming cells are ectodermal in origin Enamel formation occurs only for a limited period of time till the desired thickness is formed In an erupted tooth enamel is not lined by formative cells Enamel does not have the capacity to repair or regenerate Enamel is a nonliving tissue not containing cells or cellular components Enamel is avascular and insensitive Organic matrix of enamel is unique composed of enamel protein and is noncollagenous.

Physical Properties

Color: Ranges from grayish white to yellowish white. Yellowish white color is appreciated where enamel is thin as it is translucent and allows the yellow color of dentin visible through it. Translucency of enamel is related to the high mineral content and homogeneity of enamel. Hardness: Enamel is the hardest biologic tissue of human body and the hardness is compared to mild steel. Hardness of enamel is 343 KHN (Knoop Hardness Number). The high mineral content and complex crystalline arrangement makes it very hard, suitable to resist heavy masticatory stress. Hardness varies in different parts of same tooth with maximum at the cusp tip and incisal edge and less in the cervical region. Similarly, surface enamel is more harder than in deeper portion. Brittleness: Enamel is highly brittle and tend to fracture because of less tensile strength. Therefore resilient dentin support is very essential for the integrity of enamel. Loss of dentin due to caries or improper cavity cutting leads to fracture of unsupported enamel. Thickness: Enamel thickness vary considerably over different parts of crown. Maximum thickness (2.5 mm) is observed at the cusp tip or incisal edge and thinnest at cervix where it ends at a feather edge. As a functional adaptation, thickness of enamel is reported to be more in lingual aspect of maxillary molars and buccal aspect of maxillary teeth, in relation to functional cusps. Permeability: Enamel is semi-permeable and allows the passage of certain molecules. Distribution of pores between and around the enamel rods is responsible for this property of enamel. These pores permit entry of some bacteria and bacterial products, may result in caries initiation. Density: Enamel density varies in different parts. Density decreases from surface to dentino-enamel junction and from incisal to cervical region. Refractive index: Enamel is birefringent and its refractive index is 1.62. Solubility: Enamel is soluble in acids. Solubility depends on the presence of certain ions like fluoride. Surface enamel is less soluble. Specific gravity: Specific gravity of enamel is 2.8.

Chemical Composition Enamel is a highly mineralized tissue with 96% of inorganic components, in

the form of hydroxyapatite crystals; 4% of organic components, forming a lace-like network between the crystals; and water, filling the pores between crystals and at rod boundaries.

Inorganic Components Calcium and phosphate in the form of hydroxyapatite crystals. Traces of strontium, magnesium, lead and fluoride.

Organic Components Amelogenin (90%) Non-amelogenins (10%) Tyrosine rich amelogenin polypeptide (TRAP) and non-amelogenin proteins make up the major organic components.

AMELOGENESIS Amelogenesis is the process of formation of enamel. The cells responsible for amelogenesis are ameloblasts, which are derived from inner enamel epithelium of enamel organ, an ectodermal component. Ameloblasts during their life time undergo morphological and physiological changes that are directly related to their function. These changes can be described as life cycle of ameloblasts (Fig. 4.1). According to the functions performed, the life cycle of ameloblasts can be divided into: Pre-secretory stage • •

Morphogenic stage Organizing/differentiating stage

Secretory stage •

Formative stage

Post-secretory stages •

Maturative stage

• •

Protective stage Desmolytic stage

Morphogenic Stage The function of ameloblasts during this stage is determination of shape of the tooth. The inner enamel epithelium interacts with underlying connective tissue and through differential growth helps to establish the dentino-enamel junction and thereby determine the shape of the tooth to be formed. The ameloblasts at this stage are low columnar in shape with centrally placed nucleus. Cytoplasmic organelles are not abundant, the centrioles and Golgi complex are located at the apical part of cytoplasm and mitochondria is evenly distributed throughout the cytoplasm.

Organizing Stage or Differentiation Stage During this stage, the inner enamel epithelial cells undergo differentiation to ameloblasts as a prerequisite for enamel formation. This stage is also named as organizing stage because during this stage, the ameloblasts exert organizing influence on dental papilla cells which are adjacent to them and help in their differentiation to odontoblasts.

Fig. 4.1: Life cycle of ameloblast

In the differentiation stage, the ameloblasts increase in length to attain a length of 40 microns and also develop abundant cytoplasmic organelles necessary for protein synthesis. As the cells elongate, the nucleus shifts to the apical or proximal end of cell. The centrioles and Golgi apparatus also move from apical cytoplasm to basal or distal part of the cell and mitochondria becomes concentrated at the proximal end. This change in position of nucleus and the other organelles is called reversal of polarity. This is a preparation to secretion because the organelles are moved to the secretory end of the cell which is at the basal region. The cells also develop intercellular junctions at

the proximal and distal ends which are termed as proximal and distal terminal bars. During the terminal phase of organizing stage the dentin formation begins and the basal lamina supporting the ameloblast layer disintegrates. Deposition of dentin is an important event in life cycle of ameloblasts because the ameloblasts can attain the secretory function only after a layer of dentin is deposited. The interdependence between ameloblasts and odontoblasts is referred to as reciprocal induction. Ameloblasts also derive alternate source of nutritional supply from dental sac because the dentin deposited blocks the nutritional supply from the dental papilla.

Formative or Secretory Stage In this stage, ameloblasts perform the function of secretion of enamel matrix and partial mineralization. The ameloblasts which are fully differentiated starts secretory function only after a layer of dentin is deposited. The secretory ameloblasts are structurally suited for synthesis and secretion of enamel proteins. The cells have many mitochondria, well developed Golgi complex and extensive cisterns of rough endoplasmic reticulum. Cytoplasm also shows many secretory granules, vacuoles, free ribosome, various types of vesicles, microtubules, etc. Microtubules are involved in the movement of secretory granules to the basal plasma membrane.

During this stage, ameloblasts synthesize enamel protein. Steps involved are Messenger RNA carries the message from nucleus to cytoplasm ↓ Ribosomes translate the message ↓ Protein is synthesized in rough endoplasmic reticulum ↓ Protein undergoes post-translation modification in Golgi complex ↓ Packing of protein into secretory granules The basal portion of cytoplasm of ameloblasts contains numerous secretory

granules packed with enamel proteins. The secretory granules move towards the basal plasma membrane, fuse with it and release the matrix protein into the extracellular space against the newly formed dentin by a process called exocytosis. Secretory ameloblasts have several junctional specializations at basal and lateral cell surfaces. At the proximal and distal end of cell body the adjacent cells are attached to each other by junctional complexes. These intercellular junctions help to maintain organization of ameloblast layer and also to control the metabolite diffusion along extracellular spaces. The proximal intercellular junctions are relatively leaky while distal ones act as a permeability barrier to macromolecules such as enamel proteins and calcium. Therefore calcium is prevented from reaching the matrix through extracellular space. In the initial stage of secretory phase the ameloblasts have a flat basal region. After a little thickness of enamel matrix is deposited, ameloblasts develop a conical process at the base, which is called Tomes’ process. Tomes’ process is partially separated from cell body by an incomplete septa formed by the microfilaments and tonofilaments extending from the distal terminal bars. Cytoplasm of cell body is in continuation with that of Tomes’ process. The cytoplasm of Tomes’ process does not contain any organelles other than secretory granules, microtubules, microfilament and a few mitochondria. After the Tomes’ process is formed, the secretion of enamel takes place from two different sites and is responsible for the rod structure of enamel. Tomes’ process is lost before the last phase of secretory stage, before the surface layer of enamel is deposited.

Maturative Stage During this stage, ameloblasts helps in the mineralization and maturation of enamel. Ameloblasts enter into the maturative phase only after the desired thickness of enamel matrix is laid down. In this stage, ameloblasts have to introduce the inorganic material necessary for maturation and also reabsorb proteins and water to provide space for the minerals. Ameloblasts performing these dual functions shows morphological alterations. Ameloblasts are ruffle ended when they are performing the function of introducing inorganic components and smooth ended when they are reabsorbing proteins and water. The series of repetitive morphological changes that occur in ameloblasts of

maturative stage, from ruffled ended to smooth ended is referred to as ameloblast modulation. During this process, tight junctions and deep membrane infoldings periodically appear (ruffle-ended), then disappear for short intervals (smooth-ended), from the apical ends of the cells. Ameloblasts in maturative phase shows slight reduction in height, decrease in volume and organelle content. Excess synthetic organelles are removed and the remaining organelles are shifted to the distal end of the cells. The basal plasma membrane of the ruffle-ended ameloblasts shows a brush border with many foldings, while that of the smooth ended ameloblasts is smooth. The cytoplasm of ameloblasts also has vacuoles which contain material resembling enamel matrix indicating the absorptive function of these cells.

Protective Stage In this stage, ameloblasts along with other layers of enamel organ has to perform a protective function. After the enamel formation is completed the basal plasma membrane of ameloblasts looses the brush border and become smooth. They secrete protein similar to basal lamina onto the surface of newly formed enamel. Ameloblasts develop hemidesmosomal attachments to these basal lamina structure which help in holding these firmly to the tooth surface. After this, the columnar ameloblasts shorten to cuboidal and along with other collapsed layers of enamel organs form a 2–3 layered stratified epithelium which is termed as reduced enamel epithelium (REE). This reduced enamel epithelium covers the newly formed enamel and protects it, till the tooth erupts into oral cavity. This also has an important role in establishing the dentogingival junction. If not protected, enamel may be resorbed or cementum deposition may occur on enamel surface.

Desmolytic Stage In this stage the REE secretes collagenase enzyme which destroy the connective tissue between oral mucosa and erupting tooth. This facilitates the eruption process. During this stage the reduced enamel epithelium proliferates and fuses with the oral epithelium to form a solid plug of epithelial cells. The central cells of this degenerate to form a canal through which the tooth erupts. Amelogenesis, the process of formation of enamel involves two steps:

Matrix deposition and mineralization.

Enamel Matrix Deposition The secretory ameloblasts which are structurally suited for synthesis and secretion of enamel proteins, start secretory function after a layer of dentin is deposited. The secretory granules packed with enamel proteins fuses with the basal plasma membrane and release the matrix protein against the newly formed dentin by a process called exocytosis. Enamel formation begins in the cusp tips and incisal edges, from which it progresses outward and cervically. As the matrix deposition progresses the ameloblasts move outward, away from the matrix. In early stages of amelogenesis the enamel matrix consists of 20–30% of protein and the proportion of this protein gradually decreases during the process of mineralization. Ameloblast synthesize and secrete the enamel matrix which is composed of enamel proteins such as amelogenin and nonamelogenin (5 to 20%). The non-amelogenin proteins include enamelin, tuftelin, ameloblastin (amelin or sheathlin), enzymes like proteinases, etc. Amelogenin is characterized by the presence of large amount of proline. The enamelin contains less amount proline than amelogenin, but greater quantities of glycine. Amelogenins are located in the intercrystalline spaces and are postulated to be involved in control of crystal growth. Enamelins are closely attached to the crystals surface and form an envelope around individual crystals and have been hypothesized to have role in nucleation of enamel crystals and crystal growth. Thus, the enamel proteins are participating in enamel mineralization either by promoting nucleation or by regulating crystal growth. In addition to these proteins, enamel matrix also contains sulfated glycoconjugates. Enamel proteinases present in the matrix help to cause degradation of Amelogenins to facilitate its reabsorption during maturation. Enamel matrix is devoid of both collagen and keratin. In the initial stage of secretory phase the ameloblasts have a flat basal region. After a little thickness of enamel matrix is deposited ameloblasts develop a conical process at the base which is called Tomes’ process. In secretory ameloblasts, after formation of Tomes’s process, a dual area of secretion becomes operational. The Tomes’s process contains many secretory granules, microtubules, microfilaments, a few mitochondria, etc. Once the Tomes’ process is formed it extends behind the distal junctional complex and

is the only cellular site to interact with the growing enamel surface; matrix deposition and mineral transport takes place only through this. Enamel secretion takes place through two sites in Tomes’ process: One is inter-rod growth region formed by the proximal end of Tomes’ process of contiguous co-operative ameloblasts. This site is located adjacent to the distal terminal bar all around the cell and these results in accumulation of enamel matrix between adjacent Tomes’ process to produce inter-rod enamel. Other site is rod growth region located at distal portion of Tomes’ process of each individual ameloblast and this is associated with formation of enamel rod. At the border between the rod and inter-rod enamel a sheath-like zone (prism sheath) with slightly more concentrated organic matrix persists and it demarcates the rod and inter-rod region. Thus, Tomes’s process is responsible for the rod structure of enamel (Fig. 4.2). Deposition from the proximal end precedes the deposition from distal surface. Therefore the rod enamel is deposited into a pit created by the adjacent inter-rod enamel.

Mineralization of Enamel Matrix Mineralization takes place in two steps: Immediate partial mineralization and maturation. Mineralization of enamel occurs extracellularly by the deposition of minerals in the secreted protein matrix. During enamel formation secretory ameloblasts are directly involved in both the production of enamel matrix and its mineralization. Calcium required for mineralization reaches through the circulation to tissue fluid. The calcium from tissue fluid is actively transported into the cell where it gets attached to the calcium binding proteins. The complex of calcium and calcium binding proteins move towards the distal cytoplasm, where it is released and then the free calcium ions are extruded to the enamel matrix by Ca-ATPase in the plasma membrane of Tomes’ process.

Fig. 4.2: Amelogenesis (deposition from site 1 forms enamel rod and from site 2 forms inter-rod enamel. Note the difference in crystal orientation)

Mineral deposition in enamel matrix occurs in four phases. Primary mineralization corresponds to immediate partial mineralization which accounts for 30% of mineral deposition. The secondary stage starts at the surface and proceeds toward dentino-enamel junction. In tertiary stage, mineral rebounds from inner layer of enamel outward. The fourth stage is responsible for further deposition of minerals in the surface enamel which makes it hypermineralized than the rest of enamel. Immediate partial mineralization: The enamel matrix undergoes immediate partial mineralization, immediately after it is laid down. During this immediate partial mineralization 25 to 30% of total mineral content is deposited in the matrix. Mineralization of enamel begins at dentino-enamel junction where the tuftelin, ameloblastin and/or enamelin proteins are deposited on the layer of dentin. The initial enamel crystallites form a needle or a ribbon of minerals generally oriented perpendicular to dentino-enamel junction. Whether the initial enamel crystals are formed de novo or originate from the dentin minerals remain controversial. After nucleation, the ameloblasts deposit matrix rich in amelogenin. The initial enamel crystallites elongate to produce enamel ribbons and the process continues till the entire thickness of secreted enamel matrix. Maturation: Maturation is characterized by gradual completion of

mineralization. During maturation, a massive re-absorption of matrix protein and water takes place by ameloblasts, concomitant with the rapid growth of crystallites which grow in width and thickness to make large mineral crystals that characterizes the mature enamel. Initially the amelogenin proteins are absorbed onto the specific crystal faces, thus controlling their growth. The amelogenin proteins are removed from the mineralizing front by proteolytic cleavage mediated by proteinase enzymes and is reabsorbed by endocytic action of ameloblasts to bring about crystal growth. In tertiary stage of mineralization, the maturation process starts at dentino enamel junction and progresses to the surface, similarly it proceeds from cusp or incisal tip to the cervical region.

STRUCTURE OF ENAMEL Enamel is composed of millions of enamel rods and varying amounts of inter-rod enamel between them.

Methods used to Study Structure of Enamel To study the structure of enamel, a ground section should be prepared. Unlike other hard tissues, decalcified sections cannot be used to study enamel as major portion of enamel would be lost during decalcification because of its high mineral content. Enamel can be studied using a light microscope or by a polarized microscope. Electron microscope may be used to study the ultra structure. Light microscopic examination of ground section of enamel shows enamel rods, various structural lines, hypocalcified structures (enamel lamellae, tufts and spindles), dentinoenamel junction, etc.

1. Enamel Rods Enamel rods are the fundamental structural unit of enamel; each rod is extending from its site of origin at the dentino-enamel junction (DEJ) to the outer surface of enamel. The enamel rods are separated by varying amounts of inter-rod materials. The number of enamel rods varies in different teeth. The rods are roughly cylindrical in the longitudinal section (Fig. 4.3) with an average diameter of around 3 to 4 microns near dentino-enamel junction,

which increases gradually to the surface at a ratio of 1:2, to cover the larger surface area of outer surface compared to DEJ. In transverse section, the enamel rods have a keyhole shape (Fig. 4.4) with a head formed by the rod and a tail formed by inter-rod enamel immediately cervical to it. The rounded heads are commonly directed towards the incisal or occlusal aspect while the tails are directed towards the cervical region of the teeth. In enamel, the enamel rods and inter-rod enamel are arranged in such a way that the heads abuts against the tail of adjacent rods. The width of body of the enamel rod is approximately 5 microns in head region and 1 micron in tail, and the total length (head + tail) is approximately 9 microns.

Fig. 4.3: Longitudinal section of enamel rod with cylindrical appearance

Fig. 4.4: Keyhole shape of enamel rod in cross section

Although the predominant pattern in transverse section of enamel is keyhole pattern, in some parts it may appear round, oval, hexagonal or even as series of arcades resembling fish scales. (Previously enamel rods were also referred to as enamel prisms.)

Sub-microscopic/Electron Microscopic

Structure/Ultrastructure of Enamel Rods Enamel rods and inter-rod enamel are composed of millions of tightly packed hydroxyapatite crystals. Apatite crystals are flattened hexagonal structures with length ranging from 600 to 1000 Å, width of 400 Å and thickness of 250 Å. These crystals are 30 times larger than that of dentin. The direction of the crystals is different in rods and inter-rod substance. In the rods the apatite crystals are arranged parallel to the long axis of the rod, especially close to the center. But as it goes to periphery the crystals show lateral flaring (Fig. 4.5). In the cervical 1/3rd portion of the rod, the lateral flaring goes to the extent that the crystals become confluent with that of the inter-rod enamel located immediately cervical to it. Thus, making the boundary between indistinct, and inter-rod enamel appear like a tail attached to rod. In the remaining 2/3rds portion of the rod, there is marked difference in crystal orientation between rod and adjacent inter-rod region. The inter crystalline space created by this abrupt change in direction of apatite crystals gets filled with organic components. The thin structure formed by the accumulation of organic material delineating the coronal 2/3rds boundary of enamel rod is called rod sheath. Thus, enamel rods with distinct rod sheath covering coronal 2/3rds portion and confluent inter-rod at cervical 1/3rd portion, present keyhole pattern in the cross section.

Fig. 4.5: Enamel rod with crystals parallel to long axis of rods and inter-rod substance with different crystal arrangement

In the innermost portion of enamel, close to the DEJ (5 microns thickness), enamel does not have rod structure. Similarly, in the outermost 30 microns thick enamel, the rod structure is absent or irregular. In these regions, the

crystals are arranged uniformly with their long axis perpendicular to the surface. This is due to the absence of Tomes’ process during the formation of innermost and outermost enamel.

Direction of Enamel Rod The direction of movement of secretory ameloblasts dictates the orientation of enamel rod in mature enamel. Enamel rods are set in rows arranged circumferentially around the long axis of the tooth. The enamel rods (Figs 4.6a and b) are horizontally arranged at the midportion of the tooth. From midportion as it goes to occlusal aspect, rods are obliquely arranged with inclination towards the occlusal surface and becomes vertical at the cusp tips and incisal edges. Enamel rods follow an oblique direction along the cusp slopes and appear to be converging in the pit and fissure region. From the midportion as it goes cervically, enamel rods again follows an oblique course but deviates cervically. This feature is specific for permanent teeth. In deciduous teeth the enamel rods at the cervical region shows horizontal arrangements or slopes occlusally (Fig. 4.7). The angle between the DEJ and the enamel rod is about 70° and increases to about 90° cervically, sometimes exceeding 90° in cervical region. In general, enamel rods follow a wavy or tortuous course as it travels from the dentinoenamel junction to the outer surface of the enamel. Therefore the actual length of the enamel rod is more than the thickness of the enamel. Enamel rods bend up and down in a vertical direction and right and left in a horizontal direction. A variation in course may be noted, between adjacent zones of rods (a zone consisting of 10–13 rods). In addition, the enamel rods of adjacent rods may intertwine with each other in the inner 2/3rds of enamel thickness. Any tendency of enamel rod to cleave is reduced by the wavy pattern and nonparallelism of adjacent rows.

Figs 4.6a and b: Direction of enamel rods in permanent teeth

Fig. 4.7: Direction of enamel rods in deciduous teeth in cervical region

2. Structural Lines Cross striations: In longitudinal ground sections, the enamel rods appear to be divided into uniform segments which are separated by fine dark lines. These periodic lines are called cross striations. Cross striations are arranged perpendicular to the enamel rods, at a regular distance of 4 μm giving them a striated appearance. Thus, each segment is of 4 microns length, representing the daily deposition of enamel.  Cross striations are better appreciated, in less calcified enamel or after application of mild acids. There are various views about this cross striations.

It reflects the daily rhythmic deposition of enamel; or created due to relation between enamel rods and inter-rod substance; or particular orientation of crystals within the rod. Scanning electron microscopy has revealed periodic constrictions along the length of a rod which is responsible for cross striation. Another view regarding these striations is these could be representing an area of higher organic content and less inorganic content, Incremental lines of Retzius or striae of Retzius: These are the incremental growth lines in enamel representing the rhythmic deposition of enamel. In longitudinal sections of enamel, striae of Retzius (Fig. 4.8) appear as series of brownish dark lines which extend from the DEJ to the outer enamel surface. These lines run obliquely across the enamel rods and show an occlusal deviation as they travel to the surface. Striae of Retzius appear as concentric circles in transverse sections of enamel, comparable to growth rings of a tree. Each striae is separated by varying distance ranging from 20–40 microns separating weekly increments of enamel deposition and appears to be the result of cyclic disturbance in the rod formation occurring at every 7 to 8 days. The striae are closer and numerous in the cervical region.

Fig. 4.8: Incremental lines of Retzius

 Incremental lines of Retizus are seen encircling the dentin in the incisal or cuspal region while at the cervical region, these striae extend to the surface creating wave like grooves on the enamel surface which are called

perikymata or imbrication lines of Pickerill.  Microradiographic studies have shown change in inorganic components in the region of striae of Retzius, and therefore, are considered as a hypomineralized structure of enamel. These lines become more prominent in case of disturbance in enamel deposition. Shift in rod direction is also been reported to be associated with these structures. Neonatal line (Fig. 4.9): In the deciduous teeth and permanent first molars, enamel is deposited partly before birth and partly after birth. The incremental line separating the enamel deposited before birth (prenatal enamel) and enamel deposited after birth (postnatal enamel) becomes accentuated because of disturbance in formation that has occurred at the time of birth due to the abrupt change in environment. This accentuated incremental line is called neonatal line. The prenatal enamel is more homogeneous than the postnatal enamel, probably due to more constant surroundings and good nutritional supply. This variation in nature of pre- and postnatal enamel also may contribute to making neonatal line prominent.

3. Gnarled enamel This is the term used to describe an optical appearance that is seen in the longitudinal section of the enamel at the incisal or cuspal regions. Enamel rods follow a wavy, tortuous course (Fig. 4.10) as it travels from the dentinoenamel junction to the outer surface. Enamel rods undulate back and forth in a vertical direction and right and left in a horizontal direction. In addition, the enamel rods of adjacent region may intertwine with each other in the inner 2/3rds of enamel. This irregular twisting and intertwining is more prominent and complex at the incisal or cuspal regions, creating this optical appearance. The irregular twisting and intertwining may be associated with increased strength of enamel enabling it to withstand the strong masticatory forces.

Fig. 4.9: Neonatal line in enamel

Fig. 4.10: Gnarled enamel

4. Hunter-Schreger Bands This term is used to describe alternate dark and light bands found to be extending from DEJ towards the enamel surface. These lines are curved with the convexities facing the cervical region. Hunter-Schreger bands (Fig. 4.11) are considered as optical phenomenon and are observed when longitudinal

section of enamel is examined under reflected light or polarized light.

Fig. 4.11: Hunter-Schreger bands

This optical effect is created due to variation in course of adjacent groups of enamel rods and each band consists of 10–13 enamel rods. Because of this when a section is taken some rods are cut longitudinally while some transversely. When light passes through enamel those rods which are parallel to light reflect light away from microscope and appear dark (diazones) and the rods which are less parallel to light reflect, light through the microscope and appear bright (parazones). Since the enamel rods are nearly parallel in the outer 1/3rd of enamel, Hunter-Schreger bands do not extend to the surface rather will be restricted to the inner 2/3rds. The position of Hunter-Schreger bands can be reversed by altering the direction of light. It is thought that the relatively complicated rod arrangement observed in the region of Hunter-Schreger bands serve to reduce the propagation of fracture.

5. Amelo-dentinal Junction or Dentino-enamel Junction (DEJ) The junction between the dentin and enamel is called dentino-enamel junction (Fig. 4.12). The union of dentin with enamel is intimate without any dividing plane between the two. The matrix of enamel intermeshes into the

surface of dentin. In microscopic sections of tooth, due to change in orientation and difference in size of crystals, the DEJ appears distinct. The dentino-enamel junction is scalloped with convexity facing the dentin. The dome shaped elevations on the dentinal surface of enamel fits into depressions on the surface of dentin. The scalloped pattern is occasionally indistinct or even absent and is best appreciated in regions where the stresses on tooth structure are the greatest. This scalloped junction increases the surface area of contact between enamel and dentin and therefore strengthens the adhesion and union between them. The scalloped dentino-enamel junction also serves to reduce the chance of development of cracks along the junction, because of the numerous changes in direction of DEJ.

Fig. 4.12: Dentino-enamel junction

6. Enamel Spindles These are the spindle shaped structures, extending from the DEJ into the enamel to a distance 10 microns. These structures are the odontoblastic processes that are entrapped in enamel matrix. This occurs because some of the odontoblastic processes penetrate between ameloblast cells before enamel formation and subsequently get entrapped in the enamel matrix. In longitudinal sections of teeth, the enamel spindles (Fig. 4.13) are seen as dark spindle shaped structures, because the organic matrix is lost while sectioning and is replaced by air. They are found mainly in the incisal or

cuspal region. Enamel spindles are arranged perpendicular to the dentinal surface and may not follow the direction of enamel rods. The enamel spindles are responsible for increased sensitivity at DEJ. These structures are found more in incisal or cuspal region and thought to be improving the attachment between enamel and dentin.

7. Enamel Tufts Developmental faulting occurs in enamel prior to full maturation, probably to release the built in strain resulting from an internal swelling pressure created by ongoing crystal growth. Enamel tufts develop where enamel matrix proteins migrate to fill in the faulting voids, which therefore contain reduced minerals and enhanced organic matrix concentration. Early faulting leads to formation of enamel tufts while late faulting produces enamel lamellae. Enamel tufts are hypocalcified structures extending from the DEJ to the enamel, to a distance of about 1/5th or 1/3rd of enamel thickness. These structures are better appreciated in transverse sections of enamel. In ground sections they appear as tuft of grass, therefore the name enamel tuft is given. Enamel tufts (Fig. 4.14) are ribbon shaped structures with free ends undulating to the sides. In a thick ground section, these structures originating at different planes and curving in different directions are projected to one plane giving the appearance of tuft of grass. Enamel tufts are seen in the region where the prism sheath is prominent and these structures contain more of organic contents which is similar to enamelin.

8. Enamel Lamellae These are hypocalcified structures that extend from the enamel surface towards the dentin to varying distance (Fig. 4.14). These structures can be well identified as leaf like structures in transverse sections of enamel and are seen more in the cervical half of the tooth than coronal half.

Fig. 4.13: Enamel spindles

The enamel lamellae can be grouped based on time of development into pre-eruptive or post-eruptive lamellae. Pre-eruptive lamellae are formed due to stress or tension that is created during formation of enamel. When enamel rod crosses regions of stress or tension, a small segment of the rod in that region may remain hypocalcified or uncalcified depending on the degree of stress. The regions remaining uncalcified manifest as a crack like defect in formed enamel and get filled with surrounding cells. The post-eruptive lamellae are formed due to physical or thermal insult to which the tooth is exposed to. This leads to formation of crack like defect in formed enamel and gets filled by organic material from saliva. Based on the nature or content present in the defect, enamel lamellae can be categorized into three types. They are type A, B and C. Type A: These are composed of hypocalcified enamel rods and are restricted to enamel. Type B: These are crack-like defects formed due to early developmental faulting caused by the internal swelling pressure that occur due to ongoing crystal growth. Since these defects are formed before the eruption of tooth they get filled with cells from surroundings. The cells in the deeper portion degenerate, while superficial cells form cornfield cuticle or cementum like material (depending on of the cells entering; either from enamel organ or connective tissue), which is found in these defects. Type B lamellae may

cross DEJ and reach dentin. Type C: These are also crack-like defects formed due to late developmental faulting or due to various physical or thermal insult. In contrast to type B, in this type the crack-like defects are filled with organic materials probably derived from saliva. Type C lamellae also may reach dentin. Enamel lamellae may be mistaken for cracks formed while making ground sections. To differentiate both, decalcification of the ground section can be done. The structure that remains after decalcification can be considered as true enamel lamella.

Fig. 4.14: Enamel tufts and lamellae

Enamel lamellae are considered to be weak points in enamel which may allow the penetration of microorganism and therefore a point of dental caries initiation.

9. Surface Structures of Enamel Surface enamel: Physical characteristics and chemical composition of surface enamel is slightly different from rest of enamel. Around 30 microns thickness of enamel found on the surface is called structureless enamel or aprismatic enamel because it is devoid of rod structure, instead apatite crystals are arranged parallel to each other and perpendicular to the surface. This occurs because ameloblasts loses the Tomes’ process, which is

responsible for rod structure, before the deposition of surface layer of enamel. This structureless enamel is found in all deciduous teeth and most of permanent teeth and is most commonly seen in cervical region. (A layer of structureless enamel is also seen near DEJ.)  The surface enamel is highly mineralized, harder and less soluble than the rest of enamel. Fluoride content of this enamel is more. This layer is of a great clinical importance because it resists the initiation and spread of caries. Perikymata: The surface of enamel, especially those not exposed to abrasive forces appear corrugated with alternating horizontal ridges and troughs. These troughs that form wave-like transverse grooves in the cervical region of the surface of enamel are called perikymata or imbrication lines of Pickerill (Fig. 4.15). These structures are parallel to the cemento-enamel junction and to each other. Perikymata are the external manifestation of striae of Retzius; therefore considered as the external manifestation of internal layering. Perikymata is prominent in the cervical region because the striae of Retzius in this region are incomplete and extend to the enamel surface. Around thirty perikymata per mm is seen in the cervical region, while only ten perikymata per mm is seen in other parts of teeth.

Fig. 4.15: Incremental lines of enamel and perikymata

Enamel rod-ends: On the surface of enamel, enamel rods may show concave depressions of varying depth. The depth of concavity is more in rod ends of

incisal and occlusal edges and less in the cervical region. Enamel lamellae and cracks: Since the enamel lamellae and cracks are structures extending from surface of enamel towards DEJ, they can be observed as surface structures, when present. Pits and fissures: Pits and fissures are the developmental defects found in the enamel surface associated with developmental grooves, which can act as a site for initiation of caries. Afibrillar cementum: Afibrillar cementum is seen on the surface of enamel in the cervical region of tooth in the region of overlap type of cementoenamel junction and also in other enamel surfaces due to premature loss of reduced enamel epithelium. Organic structures on the surface of enamel Enamel cuticle: Enamel cuticle is the delicate covering of organic material found on the surface of newly erupted teeth. This delicate covering is also called Nasmyth’s membrane. This enamel cuticle is structurally similar to basal lamina and is secreted by the ameloblasts on the surface of newly formed enamel which helps in attachment of the reduced enamel epithelium to the enamel. The enamel cuticle is lost shortly after eruption of tooth. Pellicle: Pellicle is a layer of organic materials found covering the enamel of erupted tooth which is derived from saliva. Salivary proteins are deposited on tooth surface within hours after mechanical cleansing. Micro-organisms from the oral cavity colonize onto the pellicle and convert it into bacterial plaque within one or two days. (Age changes, refer page 320–321)

Clinical Considerations Enamel hypoplasia: Ameloblasts are very sensitive type of cells and therefore the function may be affected by a number of environmental as well as hereditary conditions resulting in defective enamel formation, which is collectively referred to as enamel hypoplasia. a. Amelogenesis imperfecta is a hereditary type of enamel hypoplasia which may be, 1. Hypoplastic type caused due to defective matrix

deposition resulting in teeth with thin layer of normal enamel, 2. Hypocalcification type with defective calcification resulting in soft enamel that can be scrapped with blunt instrument or 3. Hypomaturation type with defective maturation resulting in enamel that can be scrapped with sharp instrument. b. Environmental enamel hypoplasia: A number of environmental conditions including nutritional deficiency, infections, endocrine disturbances, birth injury, etc. may cause defective enamel formation. Turner’s hypoplasia is one of the most common forms of enamel hypoplasia occurs in permanent successor tooth due to trauma or infection to deciduous predecessor tooth. In patients affected by congenital syphilis, enamel deposition may be defective resulting in characteristic Mulberry molar Ingestion of excess amounts of fluoride can result in enamel defect known as dental fluorosis/mottled enamel. c. Direction of enamel rods must be kept in mind during cavity preparation for restoration, to ensure that unsupported enamel is not left behind. Enamel rods which are not supported by dentin may break and leads to failure of restoration. For example, as the enamel rods at cervical part of permanent teeth are inclined cervically, a bevel has to be prepared at gingival seat to remove unsupported enamel. d. Enamel lamellae can act as pathway for entry of caries causing bacteria and act as a point of caries initiation.

5 Dentin and Dentinogenesis

Introduction Physical properties and chemical composition Dentinogenesis Microscopic structure of dentin Age changes in dentin Dentin sensitivity Clinical considerations

D

entin is a mineralized connective tissue component of the tooth that makes up the bulk of the tooth and is located between pulp and external surface tissues. Dentin is covered by enamel in the crown region and by cementum in root region. Physically and chemically, dentin resembles bone; but unlike bone, dentin is avascular and do not contain entrapped cells. Dentin together with pulp is considered as a functional complex and denoted as dentin-pulp complex. Dentin is different from enamel in various aspects (Table 5.1).

Physical Properties of Dentin Color: It is yellowish and this impart color to the tooth because it is visible through translucent enamel. Color of dentin darkens as age advances. Thickness: Usually ranges from 3 to 10 mm. Dentin thickness vary in different teeth and in different aspects of the same tooth; reported to be thicker on buccal aspect than lingual.

Hardness: Dentin is much softer than enamel and the hardness is only 68 KHN compared to 343 KHN of enamel. Therefore dentin wears off much faster than enamel. Dentin hardness varies in different parts, with more hard at the central portion than pulpal and peripheral regions. Dentin is harder than cementum and bone. Elasticity: Dentin has a high degree of elasticity which makes it flexible. The modulus of elasticity is approximately 1.79 × 106 lb/in. The peripheral layer of dentin which is more resilient, has significant functional importance because it allows dissipation of stress/forces and thus dentin function as shock absorber for the overlying brittle enamel. This explains why the enamel unsupported by dentin fracture when exposed to masticatory stress. Density: The average density of dentin is approximately 2.1 gm/ml. Radio density: Dentin is more radiolucent than enamel due to less mineral content. Permeability: Because of dentinal tubules, dentin is highly permeable and permeability depends on patency of the tubules. In conditions when the tubules are blocked, permeability decreases considerably.

Chemical Composition of Dentin Dentin consists of 65% inorganic components, 20% organic components, 15% water.

Dentin and Dentinogenesis Enamel

Table 5.1: Comparison between enamel and dentin Dentin

Ectodermal in origin, formed by ameloblasts derived from enamel organ

Ectomesenchymal in origin, formed by odontoblasts derived from dental papilla

Formation is limited to a limited period

Formation of dentin is a lifelong process

High mineral content: 96%

Mineral content is less: 65%

Hydroxyapatite crystals are larger

Apatite crystals are smaller

Organic content is unique with enamel proteins: Collagen is absent

Collagen is the main organic content

Non living tissue; do not contain cells or cellular components

Living tissue; contain cytoplasmic extensions of odontoblasts

Do not respond to stimuli, do not have capacity to repair or regenerate

Respond to stimuli, and have capacity to repair and regenerate

Found only in coronal region

Found in both crown and root

Inorganic components are mainly calcium and phosphate in the form of hydroxyapatite crystals. Small amounts of carbonate, sulfate, calcium hydroxide and trace elements such as copper, fluoride, iron, zinc, etc. are also present. Apatite crystals are described as calcium deficient carbonate apatite of which crystal size is smaller than that of enamel and larger than that of bone and cementum. Crystals are plate shaped having length of 200–1000 Å and width of about 30 Å.

Good to Know In the early stages of odontogenesis, the dental papilla cells will be actively dividing. During the last mitosis, the daughter cells located near or in contact with the basement membrane become pre-secretory odontoblasts. Initially these cells are roughly parallel to the basement membrane, but after a short period, they align with long axis, right angles to the basement membrane, as a palisade-like structure. The cells located beneath, form the subodontoblast/Hoehl’s layer which constitute a reservoir for the renewal of odontoblasts. When odontoblasts are differentiated, they undergo polarization with migration of nucleus to basal region and Golgi apparatus from the basal part to a supranuclear area. Cells also develop cytoskeletal proteins, and junctional distal complex comprising desmosome-like junctions,

junctions and in some species, tight junctions. These junctional complexes constitute a permeability membrane, and intercellular diffusions are restricted to molecules with small molecular weight. Fenestrated capillaries infiltrate the odontoblast layer which permits the precursors of intracellular and extracellular matrix molecules to cross the space between endothelial cells and the basement membrane. The terminal polarization leads to the partition between cell body containing rich synthetic organelles such as rough endoplasmic reticulum, Golgi apparatus, immature and mature secretory vesicles, lysosomes, and a long process protruding in the pre-dentin and adhering to the dentinal walls of the tubules. The role of odontoblasts in dentinogenesis is crucial. It is observed that the secretion occurs through two sites, i.e. the proximal pre-dentin or at the distal pre-dentin—inner dentin edge. The former site releases collagen fibrils and their associated proteoglycans in pre-dentin, while the latter discharges non-collagenous phosphorylated proteins and mineral associated proteoglycans that are secreted at the mineralization front. Some matrix components migrate directly from the serum to the dentin compartment. They follow mainly an intercellular pathway, albumin and phospholipids being implicated in the transport of minerals. [Goldberg M, Kulkarni AB, Young M, Boskey A. Dentin: Structure, Composition and Mineralization: The role of dentin ECM in dentin formation and mineralization. Front Biosci, 2011; 3: 711–735]. 90% of organic components of dentin comprises collagen fibers; majority being type I, with trace amounts of type III and V. The dentin matrix also contains proteins and proteoglycans referred to as non-collagenous proteins. Major non-collagenous proteins are phosphophorin, dentin matrix protein-I and dentin sialoproteins and dentin glycoproteins. In addition, osteonectin, osteopondin, bone morphogenic proteins and growth factors like transforming growth factors, fibroblast growth factors and insulin-like growth factors are also present. These non-collagenous proteins participate in initiation of dentin mineralization and control the growth of apatite crystals.

DENTINOGENESIS

The process of formation of dentin is called dentinogenesis. The cells that form dentin are odontoblasts, which are derived from dental papilla, which in turn, is an ectomesenchymal component of tooth germ. In the early bell stage the cells of dental papilla adjacent to inner enamel epithelium align to form a distinct layer. Initially these cells become cuboidal and later turn to columnar cells utilizing the acellular space between inner enamel epithelium and dental papilla. These cells develop rich cytoplasmic organelles for protein synthesis and the nucleus shift from the center to the basal region. These cells form odontoblast layer that deposit dentin later. The differentiation of odontoblasts occurs under the organizing influence of inner enamel epithelium. Formation of dentin involves two steps Deposition of matrix which includes collagen and ground substance Mineralization

Matrix Deposition Mantle dentin: During the initial stage of dentin deposition, odontoblasts are not grown to its full size and have space in between, containing ground substance of dental papilla (Fig. 5.1a). To this pre-existing ground substance of dental papilla, odontoblasts deposit collagen which together form the organic matrix of the first formed dentin. The first formed dentin is referred to as mantle dentin. The collagen fibers deposited are large diameter (0.1–0.2 pm), discrete and arranged perpendicular to the basement membrane. After the deposition of collagen odontoblasts leave out many matrix vesicles which help in initiation of mineralization. As the deposition of dentin matrix proceeds the odontoblasts move inwards pulpally, leaving behind its cytoplasmic process, referred to as Tomes’ fibers. These cytoplasmic extensions can be seen in mineralized dentin as odontoblast processes in dentinal tubules.

Good to Know The term ‘Von Korff fibers’ is used to describe silver-staining ‘fiber bundles’, presumed to be collagenous, seen with the light microscope, which seem to arise from the sub-odontoblast zone of the dental papilla,

pass between the odontoblasts, and fan out to form the fibrous matrix of the first formed, or mantle dentin, von Korff inl905 demonstrated these argyrophilic fibers and therefore the term Von Korff fibers’. These fibers were considered to be aligned parallel to the dentinal tubules in mantle dentin, whereas in the collagen fibers of circumpulpal dentin lie at right angles to the tubules. Later, Ten Cate et al. (1970) concluded that von Korff fiber is an artefact of light microscopy, created by the deposition of silver in the extracellular compartment. The reducing sugars in an extensive extracellular compartment between widely separated pre-odontoblasts, take up the silver stain, giving a false appearance of black argyrophilic fibers. On this basis, with continued hypertrophy of the odontoblasts, and the exclusion ofthis extracellular compartment, the well-recognized reduction in the number of the so-called von Korff fibers during circumpulpal dentinogenesis was justifiable. From his research findings, he concluded that classical collagenous von Korff fibers do not exist and are artefacts. However, mantle dentin has large diameter collagen fibrils, lying parallel to the dentinal tubules. Whether it is worth retaining the term von Korff fibers for these large diameter fibers in dentin is a matter of discussion, as recent evidence suggests that all dentinal collagen is the product of odontoblastic activity. (Ten Cate AR. A fine structural study of coronal and root dentinogenesis in the mouse: observations on the so-called Von Korff fibers’ and their contribution to mantle dentin. J. Anat. 1978; 125(1): 183–97)

Fig. 5.1a: Mantle dentin formation (Note: The large diameter collagen, perpendicular to dentino-enamel junction)

Circumpulpal dentin: After deposition of mantle dentin the odontoblasts enlarge and get fully differentiated, obliterating the space between them. This makes it essential for the odontoblasts to secrete both collagen and ground substance to form organic matrix (Fig. 5.1b). These collagen fibers deposited are small diameter (50–200 nm), arranged in closely packed bundles which are parallel to the basement membrane. Odontoblasts do not release matrix vesicles rather secrete further components such as lipids, phosphoproteins, etc. to the matrix which may have role in mineralization. The dentin formation proceeds in the same manner throughout life of the tooth. The rate of dentin formation is around 4 microns/day till the crown completion which slows down to 1 micron/day till crown completion. Afterwards it becomes a slow process which continues throughout life.

Fig. 5.1b: Formation of circumpulpal dentin (Note: The difference in the type and arrangement of collagen in mantle dentin and circumpulpal dentin)

Mineralization For proper mineralization of dentin, three components are necessary, namely (i) collagen which forms a scaffold, (ii) non-collagenous proteins that bind to the collagen template and function as a mineral nucleator, and (iii) crystalline calcium phosphate deposited in an ordered manner. Non-collagenous dentin

matrix proteins 1 and 2 and dentin sialoprotein are important during mineralized tissue formation. These highly phosphorylated dentin phosphoproteins (phosphophoryn) are capable of inducing the formation of hydroxyapatite and can also inhibit mineral growth and regulate crystal size. Different patterns of mineralization observed in dentin are linear pattern, globular pattern, and a combination of the two. Linear calcification primarily found in the mantle dentin, where the deposition of crystals occurs along an uninterrupted front. Globular, or calcospheric calcification refers to the deposition of crystals in several areas of the matrix at same time. Crystal growth takes place in the form of globular or calcopheric mass. These globular mass enlarges by addition of more crystals and eventually fuses together to form a homogenous mineralized dentin; failure of which leads to formation of interglobular dentin. This type of mineralization is seen principally in the circumpulpal dentin formed just below mantle dentin. The size of globular mass depends on rate of deposition. As the rate of dentin formation decreases the size of globules progressively reduces so that mineralizing front gets a linear pattern giving a relatively smooth surface. Thus, in the rest of the circumpulpal dentin, a combined pattern of calcification occurs with a globular phase alternating with a linear phase.

TYPES OF DENTIN Based on time of formation Primary dentin • •

Mantle dentin Circumpulpal dentin

Secondary dentin Based on stimulus that evokes dentin formation Physiological dentin: Primary and secondary dentin Response dentin/tertiary dentin • •

Reactive dentin Reparative dentin

•

Sclerotic dentin

Based on the relation to dentinal tubules Peritubular/intratubular dentin Intertubular dentin Other types Pre-dentin Interglobular dentin Osteodentin

Primary Dentin The physiological dentin that is formed till the root formation is completed is referred to as primary dentin. This primary dentin forms the major part of dentin, both in crown and root. Primary dentin consists of two different types. Mantle dentin: This is the portion of primary dentin found at the outermost portions adjacent to dentino-enamel junction and dentino-cemental junction. This is the first formed dentin and is roughly of 20 microns thickness. This layer extends from DEJ up to the zone of interglobular dentin. This layer is different from rest of primary dentin in that, it contains collagen fibers which are of large diameter, loosely packed and arranged perpendicular to dentinoenamel junction. The large diameter collagen bundles observed in early mantle dentin formation, which are extending from the region between odontoblasts and fanning out to end near the basal region of ameloblasts is referred to as von Korff’s fibers. The ground substance is derived from dental papilla which lacks phosphophoryn. The mantle dentin has high organic component and is slightly less mineralized than rest of dentin (around 4%). Mantle dentin is better formed with fewer defects. Circumpulpal dentin: Circumpulpal dentin forms the remaining part of primary dentin which makes up the bulk of dentin. Circumpulpal dentin is composed of organic matrix and closely packed, smaller (50–200 nm) diameter collagen fibers which are arranged parallel to dentino-enamel junction. The ground substance is also secreted by odontoblasts which contain phosphophoryn. Circumpulpal dentin is 4% more mineralized than mantle dentin and may show mineralization defects referred to as

interglobular dentin. Circumpulpal dentin also include the physiological secondary dentin.

Secondary Dentin Although at a slower rate, dentin deposition continues throughout the life of the tooth. The physiological dentin that is formed after root completion as a part of continuous, lifelong deposition of dentin is referred to as secondary dentin. This designation is specifically used for part of physiological dentin that is formed after root formation and is located internally to primary dentin in crown and root. Although the number of tubules is lesser than in primary dentin, secondary dentin has regular tubular structure; therefore the term regular secondary dentin is also used to designate this type of dentin. The continuous formation of secondary dentin reduces the size of the pulp chamber gradually. The rate of deposition of secondary dentin is more at the roof and floor of the pulp chamber causing reduction in the size of the pulp chamber and decrease in height of pulp horn.

Tertiary Dentin This is also referred to as irregular secondary dentin, reactive or reparative dentin which is formed in response to stimuli such as attrition, abrasion, erosion, cavity preparation, etc. Tertiary dentin in contrast to physiological secondary dentin is deposited on the pulpal surface of dentin only in the affected area.

STRUCTURE OF DENTIN Dentin is a mesenchyme derived mineralized tissue composed of numerous dentinal tubules surrounded by highly mineralized peritubular dentin, embedded within a relatively less mineralized collagen matrix called intertubular dentin. The microscopic structure of dentin can be studied using ground sections or decalcified sections. The structures that can he appreciated in dentin on microscopic examination are dentinal tubules, peritubular and intertubular dentin, interglobular dentin, Tomes’ granular layer, structural lines and other age and functional changes. Along with these, the dentino-enamel and

dentino-cemental junctions also can be appreciated.

1. Dentinal Tubules Dentinal tubules are basic structural and functional units of dentin. It is tubular or canallike branched structures extending from pulpal end to the dentino-enamel/dentino-cemental junction. Dentin is composed of numerous dentinal tubules housing protoplasmic processes of odontoblasts.

Characteristics of Dentinal Tubules (Fig. 5.2) Dentinal tubules do not follow a straight course but has a curved morphology. In longitudinal sections dentinal tubules have a shallow S-shaped curvature (Fig. 5.3). The S shape of tubule is described as primary curvature of dentinal tubules. The first convexity of this primary curvature from the pulpal side is in an apical direction and the second convexity towards the crown. The curved path is more clearly evident in coronal and cervical region of the teeth. In radicular dentin the curvature is less distinct and may be even absent. When observed under higher magnification dentinal tubules also show minute undulations or a wavy course all along its length. These are referred to as secondary curvatures which may be the result of spiral course taken by odontoblasts as it move pulpally during dentin formation. Dentinal tubules show a ‘Y’ shaped terminal branching near the dentinoenamel junction The branches fork off at an angle of 45°. Extensive terminal branching and looping of dentinal tubules are seen in root dentin. Dentinal tubules show lateral branches along its course at a distance of 1–2 microns and are referred to as canaliculi or microtubules. The lateral branches are of 1 micron diameter and are somewhat perpendicular to the main tubule. These branches may contain odontoblast process and they may communicate with those of adjacent ones or blindly end in the intertubular dentin.

Fig. 5.2: Characteristics of dentinal tubule

Dentinal tubules have a tapered morphology or inverted cone shape with the smallest diameter at dentino-enamel junction and larger diameter on the pulpal end near the cell body of odontoblasts. The dentinal tubules converge at a ratio of 5:1 with an average diameter at pulpal end is 3–4 microns and 1 micron at dentinoenamel junction.

The density of dentinal tubules is more at the pulpal end than near dentinoenamel junction with more number of tubules per unit area at the pulpal end compared to periphery. The ratio of tubules of peripheral dentin to the pulpal region may vary from 1 : 2 to 1 : 5. The tubule population is approximately 15,000/mm2 near the dentino-enamel junction which increases towards the pulp up to 30,000 to 75,000/mm2. The number of tubules in radicular dentin is lesser than of coronal dentin. In a transverse section dentinal tubules are seen as circular space surrounded by a hypermineralized zone of peritubular dentin. The inner aspect of the tubule is lined by an organic layer of extracellular in nature, with high content of glycosaminoglycan, which is referred to as lamina limitans. Dentinal tubules contain odontoblast process, nonmyelinated nerve fibers, and dental lymph (tissue fluid in the periodontoblast space). The dental lymph is hypotonic with relatively low Na+ and high K+ and contains a few collagen fibers, apatite crystals, plasma proteins, etc. Fluid filled dentinal tubules are important in hydraulically transferring and relieving stresses imparted to dentin through the supporting structures of the periodontium and enamel. This may explain why endodontically treated teeth are more brittle than vital teeth.

Odontoblast Processes In vital teeth, the odontoblasts are arranged as a continuous layer along the periphery of pulp adjacent to pulpal surface of dentin. Each cell has a protoplasmic process that extends for varying distance into the dentinal tubule. These extensions are referred to as odontoblast processes and are the major content of dentinal tubules. These processes are of 3–4 microns in diameter at pulpal end and taper to 1 μ near the periphery. Each process has many fine branches along its entire length and lie in the corresponding lateral branches of the tubules.

Fig. 5.3: S-shaped dentinal tubules

The odontoblasts processes contain a fine network of microfilaments (5– 7.5 μm) and microtubules (20 μm) running longitudinally. Cytoplasmic organelles such as ribosomes, endoplasmic reticulum, mitochondria, ly sosomes and micro vesicles are also seen in odontoblast processes especially in the portion closer to cell body. Presence of vesicles indicates a secretory function of odontoblast processes and is responsible for formation of peritubular dentin. The question of the length of the process remains unanswered. Formerly, it was assumed that the processes reach the dentinoenamel junctions. It is, however, possible that the processes withdraw, but some nonfunctional remnants of the process may remain, adhering to the tubule wall. The distance to which the odontoblast processes extend into dentin is subjected to much research. Recent electron microscopic studies have confirmed their presence up to the outer surface of dentin till the dentinoenamel junction. But they do not follow a regular pattern. In certain region of the tooth these processes may extend beyond the dentinoenamel junction and remain in calcified enamel and are referred to as enamel spindles.

2. Peritubular or Intratubular Dentin Peritubular dentin is a zone of hypermineralized dentin which surrounds the

dentinal tubule (Fig. 5.4). This is also referred to as intratubular dentin (dentin inside the tubule) because it is formed by the deposition along the inner aspect of dentinal tubules. Peritubular dentin is deposited by the odontoblast process and is 40% more mineralized and also harder than the rest of dentin. Microradiographs show the peritubular dentin to be more radiopaque than intertubular dentin indicating a higher mineral content. The peritubular dentin is more homogeneously mineralized with smaller tightly packed appetite crystals and organic components. Albumin, glycoprotein, choline-rich phospholipids contribute to the formation of a highly mineralized dense ring reinforcing the tubule, where there is a little or no collagen.

Fig. 5.4: Cross-section of dentinal tubules with peritubular and intertubular dentin

The width of peritubular dentin is highest near dentino-enamel junction (0.75 μm) and progressively decreases in a pulpward direction (0.4 μm). In pre-dentin the zone of peritubular dentin is absent. Because it is hypermineralized, in ground sections of teeth, the zone of peritubular dentin appear lighter when compared to somewhat darker intertubular dentin and is seen as a clear transparent ring around each tubule lumen. In decalcified section peritubular dentin is lost and is represented by a space because of which the dentinal tubules appear larger.

3. Intertubular Dentin The major bulk of dentin present between the dentinal tubules, i.e. between the zones of peritubular dentin is called intertubular dentin (Table 5.2). This dentin is less mineralized than peritubular dentin. The thickness of intertubular dentin is highest in the region of dentino-enamel junction where the dentinal tubules are widely separated.

Intertubular dentin is formed by cell body of odontoblasts and is composed of organic components and apatite crystals, and is arranged in bundles almost perpendicular to the dentin tubules and the apatite crystals are deposited along the fibers with long axis of crystals parallel to the long axis of fibers. Although highly mineralized, intertubular dentin is retained after decalcification. Table 5.2: Differences between infra- and intertubular dentin Peri-/Intratubular dentin Intertubular dentin Restricted to around/within the lumen Makes up the major bulk of of the tubules which makes up only dentin, located between the about 10–20% of total dentin bulk dentinal tubules Deposition occur through odontoblast Deposition occur through cell process body Contains a little or no collagen Collagen forms the major organic component (90%) Extracellular matrix consists of noncollagenous proteins and some plasma proteins High mineral content, therefore more Mineral content is slightly lesser radiopaque and harder than peritubular dentin, therefore radiodensity and hardness is lesser Apatite crystals are smaller closely Plate-like crystallites, 2–5 nm in packed, isodiametric structures about thickness and 60 nm in length 25 nm in diameter. located either at the surface the They form a ring around the lumen of collagen fibrils, parallel with the the tubules collagen fibril axis; or randomly fill interfibrillar spaces Peritubular dentin is lost after Demineralization of intertubular decalcification due to high mineral dentin reveals a dense network of content collagen fibrils, coated by noncollagenous proteins glycosaminoglycans Mineralization process does not The formation of intertubular involve transformation of pre-dentin dentin involves, the immature

into dentin, rather mineralization of amorphous matrix secreted by the odontoblast processes or taking origin from the serum (dentinal lymph)

pre-dentin layer formation by a layer of odontoblasts, followed by mineralization

4. Interglobular Dentin The mineralization of dentin begin as calcospheric or globular masses. These globular masses enlarge by peripheral addition of new crystallites and eventually fuses together to form a homogeneous calcified mass, i.e. the mineralized dentin. Sometimes, few of the globular masses remain discrete and fail to fuse with each other retaining areas of uncalcified or hypocalcified dentin matrix between them. The term interglobular dentin is used to describe these uncalcified or hypocalcified zone that exists in mineralized dentin matrix. Generally the interglobular dentin has a star shape or they have the curved outlines of globular masses (Fig. 5.5a). This type of mineralization defects are seen in the coronal circumpulpal dentin immediately beneath the mantle dentin and this follows an incremental pattern. It is not unusual for the interglobular dentin to extend to radicular dentin, to some extent especially in the cervical portion. In the region of interglobular dentin, the dentinal tubules traverse uninterruptedly (Fig. 5.5b) indicating that this is purely a mineralization defect and not a defect in matrix deposition. The dentinal tubules passing this zone do not show peritubular dentin covering that portion of its course. While preparing the sections, the organic matrix in the interglobular dentin is lost and these areas get filled with air. Therefore in ground sections, the interglobular dentin appear dark under transmitted light and bright under reflected light.

Fig. 5.5a: Interglobular dentin

Fig. 5.5b: Interglobular dentin (Note: The dentinal tubule and absence of peritubular dentin)

5. Granular Dentin or Tomes’ Granular Layer In longitudinal ground sections of tooth a peripheral layer of radicular dentin adjacent to cementum appears granular. This layer is termed as Tomes’ granular layer (Fig. 5.6). This layer increases in thickness from dentinoenamel junction towards the apex. The exact nature of this layer is not known. Previously it was thought that this layer is composed of many minute interglobular dentin caused by some interference with mineralization in this area. But electron microscopic examination failed to reveal any organic content in this area. A more recent view is that these granules represent true spaces created by extensive looping and coalescing terminal portions of dentinal tubules, possibly resulting from turning of odontoblasts on themselves during early dentin formation. While sectioning these dentinal tubules are exposed and get filled with air and therefore appear as small dark

spaces giving a granular appearance. Under transmitted light Tomes’ granular layer appear dark and under reflected light bright.

Fig. 5.6: Tonnes’ granular layer

6. Structural Lines Incremental lines: Dentin formation is a rhythmic process with alternating periods of activity and rest. This cyclic process is registered as incremental lines which are perpendicular to the dentinal tubules. These incremental lines in dentin are called imbrication lines/incremental lines of von Ebner. The dentin matrix is deposited in daily increments of approximately 4 microns/day. The incremental lines correspond to the rest period and separate each increment of dentin that is formed. Since the rate of deposition vary in different teeth and in different regions of the same tooth, the distance between the incremental lines also vary. These lines are closer in the radicular region than coronal region.  During dentinogenesis, the matrix that is deposited for four or more days enter into calcifying period at the same time. The incremental lines separating these adjacent bands of matrix calcifying at different times are more prominent and are termed as contour lines of Owen (Fig. 5.7). These contour lines of Owen are hypomineralized areas and some people believe that these are incremental lines that are accentuated due to disturbance in mineralization. Neonatal line: In all deciduous teeth and in permanent first molars, part of dentin is formed before and part is formed after birth. The prenatal and postnatal dentin is separated by a distinct incremental line called neonatal

line. This is formed due to disturbance in mineralization as a result of change in environment at the time of birth.

Fig. 5.7: Incremental lines of dentin

Good to Know The exact nature of Tomes’ granular layer is not known. Previously it was considered as hypomineralized area of radicular dentin, composed of many minute interglobular dentin. Later these granules were thought to represent true spaces created by extensive looping and coalescing terminal portions of dentinal tubules at cemento-dentinal junction. A recent study by Kagayama et al., using confocal microscopy, demonstrated fluorescent fibers running parallel to the surface of dentin in the longitudinal sections in the granules of Tomes’ layer. From these results the researchers concluded that Tomes’ granular layer may be the collagen fiber bundles that remained uncalcified or hypocalcified within the radicular dentin. [Kagayama M, Sasano Y, Tsuchiya M, Watanabe M, Mizoguchi I, Kamakura S, Motegi K. Confocal microscopy of Tomes’ granular layer in dog premolar teeth. Anat Embryol (Berl) 2000 Feb; 201(2):131–7.]

7. Pre-dentin The pulpal surface of dentin is lined by a layer of non-mineralized dentin matrix. This layer is comparable to osteoid of bone and is termed as predentin. The pre-dentin layer varies in thickness between 2 and 6 microns or even up to 20 microns depending on odontoblastic activity; thick during active dentinogenesis and decreases with age. This is the mineralizing front of dentin and is always present throughout the life of a vital tooth. This layer always exists because the mineralization process lags behind matrix deposition. This unmineralized pre-dentin layer acts as a protective layer separating odontoblasts from mineralized dentin. Presence of this layer also has a functional significance because it covers the mineralized dentin and protects it from being resorbed. In a decalcified section, predentin layer appears pale in color, compared to dark pink colored mineralized dentin.

8. Dentino-enamel Junction The junction between the dentin and enamel is called dentino-enamel junction (see Fig. 4.12). The union of dentin with enamel is intimate without any dividing plane between the two. The matrix of enamel intermeshes into the surface of dentin. In microscopic section of tooth, due to change in orientation and difference in size of crystals the DEJ appear distinct. The dentino-enamel junction is scalloped with convexity facing the dentin. The domeshaped elevations on the dentinal surface of enamel fits into depressions on the surface of dentin. The scalloped pattern is occasionally indistinct or even absent and is best appreciated in regions where the stresses on tooth structure are the greatest. This scalloped junction increases the surface area of contact between enamel and dentin and therefore strengthens the adhesion and union between them. The scalloped dentino-enamel junction also serves to reduce the chance of development of cracks along the junction, because of the numerous changes in direction of DEJ.

9. Cemento-dentinal Junction This is the junction between dentin and cementum and is relatively straight, in contrast to scalloped DEJ. The cemento-dentinal junction may be scalloped

in deciduous teeth. The junction between dentin and cementum is not very distinct in acellular cementum while is somewhat distinct in cellular cementum. In decalcified sections, cemento-dentinal junction can be identified easily because cementum stains more intensely than dentin. Collagen fibers of dentin are dispersed randomly, whereas those of cementum are more orderly arranged and aggregated into discrete bundles. At the cemento-dentinal junction, the fibers of dentin and cementum are found to be intertwining. Sometimes dentin and cementum are separated by a layer of 10 microns thickness and is termed as Hyaline layer of Hopewell Smith.

AGE AND FUNCTIONAL CHANGES OF DENTIN Vitality of dentin: Dentin is considered as a vital tissue because of the odontoblast processes in the tubules and it can respond to any stimulus. As age advances, the ability of dentin to respond to stimuli decreases. Secondary dentin formation: The secondary dentin resembles primary dentin in structure but contains less tubules. Primary dentin and secondary dentin are usually separated by a prominent contour line which is formed due to a bend that develop as a result of sudden curve in the direction of dentinal tubules. This accentuated curve is due to the gradual space restriction of odontoblasts, located at the periphery of a withdrawing pulp. Dead tracts: When the teeth are subjected to traumatic insult sufficient to injure or destroy the odontoblast process, death of odontoblasts occur. In the affected areas dentinal tubules become empty due to loss of odontoblast processes. These emptied dentinal tubules are referred to as dead tracts (Fig. 5.8). These empty tubules get filled with air and in a ground section appear dark under transmitted light and white under reflected light. The dead tracts have an inverted cone shape with apex facing the pulpal surface; this is due to progressive crowding of dentinal tubules from periphery towards the pulp.

Fig. 5.8: Dead tracts

 Dentin in the region of dead tract is less sensitive than those with tubules containing odontoblasts processes. The affected dentinal tubules are sealed off from the pulpal end by deposition of tertiary dentin. Dead tracts are also isolated from surrounding normal dentin by a layer of sclerotic dentin.  Occasionally dead tracts are also seen in teeth without obvious attrition or other damage. In such situations these changes are mainly observed in cusp tips and incisal edges. This could be due to death and degeneration of odontoblasts due to overcrowding in these regions and this is regarded as a pure age change rather than functional change. Reparative or reactionary dentin: The pulp dentin complex represents a unique organ capable of responding in a variety of ways to environmental stimuli. Extensive tooth wear as in case of attrition, abrasion, erosion, dental caries, cavity cutting procedures, etc. can result in substantial tissue injury. In such cases, the odontoblasts may either survive the injury and go on participating in the tissue response, or if the injury is sufficiently severe it may die. In the second case the progenitor cells of pulp give rise to new odontoblasts that participate in reparative response.  The term reparative dentin is used to describe the tertiary dentin secreted

by new generation of odontoblasts developed from progenitor cells in the subodontoblast/Hoehl’s layer, in response to appropriate stimulus, after the death of original odontoblasts responsible for primary and secondary dentin formation. The tertiary dentin formed by surviving odontoblasts in response to an appropriate stimulus is called reactionary dentin. Thus, the reparative and reactionary dentin are subdivisions of tertiary dentin.  Formation of reactionary or reparative dentin is a rapid process and therefore shows variation in structures. The dentin formed may show the same regular tubular structure as primary dentin or may have irregular and fewer tubules or even atubular (without tubules). The irregular nature of tubules is the reason for the name irregular secondary dentin. Since the formation is at a rapid rate, incorporation of cells in the tertiary dentin matrix is not uncommon, leading to formation of osteodentin. The term osteodentin is used because entrapped cells gives a structural similarity to bone.  Formation of this tertiary dentin is a protective mechanism because it seals off the injury and prevents the stimulus reaching the pulp. A line of demarcation is seen between physiological dentin and tertiary dentin. The dentinal tubules of both are not continuous. The atubular tertiary dentin blocks the irritant stimulus from reaching the pulp. Sclerotic dentin: Continuous deposition of intratubular dentin as a result of aging or in response to tooth wear or slowly progressing dental caries, results in progressive reduction in the lumen of dentinal tubules and if continues obliterates the tubules. This dentin with obliterated tubules is called sclerotic dentin.  As a protective change in the existing dentinal tubules, dentinal sclerosis is observed in the crown region of the tooth. The stimuli may induce an active response on the part of odontoblast process resulting in deposition of organic matrix followed by mineralization. The apatite crystals are deposited gradually reducing the diameter of lumen of dentinal tubules eventually causing obliteration. The dentin deposited is structurally similar to peritubular dentin. The sclerotic dentin is formed at the expense of odontoblastic process which is either retracted or shortened by the loss of its distal extremity.  When this dentin deposition obliterates many tubules in adjacent areas, the dentin assumes a glassy or transparent or translucent appearance. This occurs because the refractive index of this dentin becomes uniform and therefore this

sclerotic dentin is also referred to as transparent or translucent dentin. In ground sections, the sclerotic dentin appears translucent or light under transmitted light and dark under reflected light.  Sclerosis is a protective mechanism because dentinal tubules are blocked and this reduces the permeability of dentin. This blocks any irritant stimulus reaching the pulp and therefore may help to prolong the vitality.  Sclerotic dentin as an age change is mainly observed in the apical third of the root which makes the root apex appear transparent and this increases with age.

SENSORY MECHANISMS OF DENTIN AND DENTIN SENSITIVITY Dentin is sensitive throughout its thickness. It is also been observed that dentin close to DEJ is more sensitive than dentin at slightly deeper layer and the pain perception then increased as the dental pulp is approached. Different types of stimuli including heat, cold, mechanical, drying, and solutions of high osmotic pressure, etc. produce pain sensation. There is no evidence that any of these stimuli produce any sensation other than pain. The above stimuli except for thermal stimulus must be applied to an exposed dentin surface to produce pain and they are most effective when the ends of dentinal tubules are patent. Despite the lack of vital cellular elements in the outer ends of dentinal tubules this area of dentin is highly sensitive. This has raised many questions. Where are the receptors that respond to pain producing stimuli? And how these receptors are stimulated? The mechanism by which the sensitivity is perceived is not clearly understood. Three possibilities have been widely investigated and accordingly three theories of dentin sensitivity have been evolved. Direct nerve stimulation: According to this theory the nerves present in the dentinal tubule are responsible for dentin sensitivity. Although there is clear evidence that some nerve fibers enter into dentinal tubules, the nerve fibers are observed in only few dentinal tubules, and they travel to a short distance into dentin (50 μ distance from pulpal surface). Therefore this theory is

insufficient to explain certain facts • • • •

Extreme sensitivity which is not in proportion to the nerve supply. Marked sensitivity in the peripheral dentin Sensitivity in newly erupted teeth because the intratubular nerves are established only often some time after eruption. Application of local anesthetics or protein precipitants such as silver nitrate does not eliminate sensitivity indicating that nerves are not directly involved in sensitivity.

Transduction theory: This theory suggests that odontoblasts themselves can act as a receptor cell that can be stimulated by various stimuli and can transmit the impulse through the pulpal nerves which are functionally connected to them.  The receptor function is suggested because of the origin of odontoblasts from neural crest cells, and therefore would be retaining some properties of nerve cells. Studies have shown that the odontoblast processes extend into dentinal tubules and have gap junctions between odontoblasts and pulpal nerves suggesting the possibility of nerve like function of these cells. But experimental studies have shown that the membrane potential of odontoblasts and their processes was too low to permit conduction of nerve impulses. Also topical anesthetics and protein precipitants do not abolish the sensitivity which is against this theory. Hydrodynamic theory: According to this theory the receptors in the nerves distributed in the peripheral portion of pulp react to local changes brought about by mechanical factors such as fluid movement, in dentin.  It is understood that the dentinal tubules contain the tissue fluid: Dental lymph which is in continuation with extracellular compartment of pulp. Dentinal tubules are channels which can act as a capillary tube. When dentin is exposed the dentinal fluid is lost from the exposed surface. This results in rapid movement of fluid due to capillary action. Since this fluid in the tubules is in continuation with extracellular fluid of pulp, the fluid movement in dentinal tubules disturbs the peripheral pulpal environment by disturbing the hydrostatic pressure equilibrium in the peripheral extracellular compartment of pulp. The pressure changes in this area stimulate the nerve endings in the vicinity of odontoblasts and initiate pain impulse.

 This theory is able to explain the extreme sensitivity of peripheral dentin near dentino-enamel junction. Near DEJ the dentinal tubules branch extensively and any irritation of this area may result in sudden displacement of a large volume of intratubular fluid. This also explains why application of local anesthetic does not reduce sensitivity and the sensitivity on application of hypertonic solutions. (Age changes, refer page 321–322)

Clinical Considerations Developmental defects: Mutations of genes (DSPP and DMP-1) involved in dentin formation lead to different forms of developmental disturbances, namely dentinogenesis imperfecta or dentin dysplasia. Similarly environmental conditions affecting mineralization such as calcium deficiency or vitamin D deficiency also cause defective dentin formation. Dentin sensitivity: Normally dentin is protected from external environment by enamel in crown and cementum in root. When the dentin is exposed, patients experiences severe sensitivity, due to patent dentinal tubules. Exposed dentin becomes less permeable with time. Partial tubule occlusion occur due to the growth of intratubular crystals from salivary or dentinal fluid mineral, adsorption of plasma proteins to the inner surfaces of dentinal tubules, or formation of a smear layer on the exposed dentin surface. If the patients continue to have sensitive dentin therapeutic intervention is needed, i.e. use dentin desensitizing agents. Smear layer: Whenever dentin is cut using hand or rotary instruments, the mineralized tissue is shattered to produce considerable quantities of debris, comprising of very small particles of mineralized collagen matrix, and is spread over the surface to form what is called the smear layer. This layer extend a few micrometers into the dentinal tubules and may also contain bacteria and their by-products. This layer may partly block the tubules and help to reduce sensitivity. However, it need to be removed before placing restorations. Protection from injuries: The odontoblast processes in the dentinal tubules and pulpal tissue has to be protected from chemical, thermal or galvanic injury. Chemicals from the restorative materials can seep through

patent tubules in to pulp. To prevent these injuries insulating bases need to be placed under deep restorations.

6 Pulp Dr Rajeesh Mohammed PK and Dr Girish KL Introduction Morphological characteristics of pulp Zones of pulp Structure of pulp Functions of pulp Age changes Clinical considerations

P

ulp is an ectomesenchymal connective tissue that supports the dentin. It occupies the pulp cavity in the central part of the teeth. Because it is the central or innermost tissue of the tooth, it is sometimes called endodontium. It is surrounded by dentin on all sides except at the apical foramen and accessory pulp canal openings, where it is in communication with periodontal soft tissue. Even though the composition and structure of the dental pulp and dentin are quite different, they are closely related embryologically and functionally and are usually considered together as a functional complex, termed the dentin-pulp complex.

DEVELOPMENT Pulp is derived from dental papilla, an ectomesenchymal component of tooth germ. During bell stage of tooth development, highly cellular dental papilla becomes well organized and well vascularized. Under the organizing

influence of inner enamel epithelium the peripheral cells surrounding the dental papilla differentiate into odontoblasts which forms dentin. Once dentin formation starts, the dental papilla is designated as dental pulp organ. As the dentin formation proceeds the dental papilla becomes enclosed in the central space within the tooth and remain as pulp tissue. The pulp is considered as mature dental papilla and the term pulp is used after dentin forms around it.

Morphological Characteristics of Pulp It is a soft connective tissue which occupies center cavity of each tooth. Each person normally has 52 pulp organs (20 primary + 32 secondary). The shape, size and volume of the pulp organ vary in different teeth. The total volume of all permanent teeth pulp organs is 0.38 cc. The mean volume of a single adult pulp is 0.02 cc. Molar pulps are 3 to 4 times larger than incisor pulps. The portion of the tooth that houses the pulp is divided into pulp chamber and root canal. The pulp chamber is the area located in the crown of the tooth and the root canal is seen in the root portion. The portion of pulp that occupies the pulp chamber is called coronal pulp and the portion that occupies the root canal is called the radicular pulp. The pulp communicates with the peri-radicular tissue through the apical foramen and the accessory canals or lateral canals. The apical foramen is the opening from the pulp at the apex of the tooth. Accessory canals or lateral canals are extra canals located on the lateral portions of the root.

Coronal Pulp Coronal pulp is located centrally in the crown of the teeth (Fig. 6.1). In young teeth, the shape of the pulp chamber resembles outer surface of dentin. The coronal pulp has pulp horns (cornua), which are protrusions that extend into the cusps of the tooth. The number of pulp horns in most cases equals the number of cusps. The pulp horns can be inadvertently exposed during cavity preparation and is more common in case of deciduous dentition. The coronal pulp has six surfaces, namely the occlusal, mesial, distal, buccal, lingual and the floor. At the cervical region, the pulp organ constricts and at this zone coronal pulp joins the radicular pulp. The pulp chamber is large at the time of eruption, but decreases in size with advancing age due to continuous deposition of secondary dentin.

Radicular Pulp Radicular pulp (Fig. 6.1) extends from the cervical region of crown to the root apex. Depending on the tooth, they vary in size, shape and number. It may be seen as a single extension of the coronal pulp in case of anterior tooth which single root and as multiple extensions in case of multi-rooted teeth. It may be straight or curved depending on the shape of the root canal. The radicular pulp is continuous with periapical tissues through apical foramen or accessory foramen. The radicular pulp is initially tubular in shape, which later becomes narrower as it goes to apical region. The radicular pulp is continuous with periapical tissues through apical foramen.

Apical Foramen Apical foramen (Fig. 6.1) is the opening seen at the root apex, through which the radicular pulp communicates with the peri-radicular area. It is through this opening, that the blood vessels and nerves enter the tooth. They vary in location, size, shape and number. The average size is 0.4 mm in maxillary tooth and 0.3 mm in mandibular tooth. The apical foramen is wide in young tooth and becomes narrower with age. The location and shape undergoes changes as a result of functional influences on the teeth. In case of mesial migration of tooth, the apex tilts to the opposite direction leading to relocation of the foramen. Occasionally the opening is found on lateral side of the root apex. Sometimes there may be two or more foramen, separated by dentin and cementum or cementum only.

Accessory Canals or Lateral Canals Accessory canals or lateral canals (Fig. 6.1) are extra canals that are present in the root dentin. They may be seen anywhere along the length of root, but are more numerous in the apical third of the root. They are formed as a result of premature loss of root sheath or when a developing root encounters blood vessel and the developing tooth root winds around the blood vessel, which later forms the extra canal. Accessory canals may be seen in furcation area due to lack of complete fusion of tongue like extensions of epithelial diaphragm that helps in division of roots. They result in communications between the radicular pulp and periodontal tissue which can lead to pulpoperiodontal lesions and failure of conventional root canal treatment (RCT).

Fig. 6.1: Morphological characteristics of pulp

HISTOLOGICAL STRUCTURE OF DENTAL PULP The structure of pulp can be studied by microscopic examination of decalcified sections of tooth. Histologically four distinct zones can be distinguished which include odontoblastic zone, cell free zone, cell rich zone and pulp core (Fig. 6.2).

1. Odontoblastic Zone This zone is found at the periphery of the pulp and consists of the cell bodies of odontoblasts which lie in a continuous row near the dentinal end of the pulp. Many nerve fibers enter this zone and terminate between the odontoblasts. The odontoblastic layer and the subodontoblastic nerve network combine to form a sensory complex (peripheral sensory unit) that completely envelop or encapsulate the central pulp core.

Fig. 6.2: Histological zones of pulp

2. Cell Free Zone Beneath the odontoblastic zone a layer of approximately 40 microns width is seen which is relatively devoid of cells. This layer is called zone of Weil or subodontoblastic layer. The cell free zone is more prominent in the coronal pulp. The major components of this zone are ground substance with reticular fibers and it appears to be relatively free of cells. The cell free zone diminishes in size or temporarily disappears when the dentin formation

occurs at a rapid rate. This zone contains network of nerve fibers that have lost their myelin sheath and are known as subodontoblastic plexus or plexus of Rashkow. These terminal, naked, free fibers are dendrites of sensory nerves and are specific receptors of pain.

3. Cell Rich Zone Cell rich zone is situated just below the cell free zone. It is a narrow zone with increased density of cells and rich capillary network. Although the cell rich zone is present both in coronal and radicular pulp, it is more prominent in coronal pulp. It consists of fibroblasts, undifferentiated mesenchymal cells, macrophages, immunocompetent cells and young collagen fibers. It serves as a reservoir for replacing the destroyed odontoblasts.

4. Pulp Core or Pulp Proper The connective tissue located in the center of the coronal and radicular pulp is referred to as pulp core. It is a core of loose connective tissue with abundant cellular elements which also contains the larger nerves and blood vessels that branch out towards the peripheral pulp area. In young pulp, the core contains more cells while in older pulp, it contains more of fibrous components.

STRUCTURAL COMPONENTS OF DENTAL PULP Dental pulp is a delicate connective tissue and is composed of cells, collagen fibers and other connective tissue structures distributed in abundant gelatinous ground substance.

Cells in the dental pulp include Odontoblasts Fibroblasts Undifferentiated mesenchymal cells Immunocompetent cells

Extracellular components Fibers: Collagen Intercellular ground substance

Connective tissue structures Blood vessels Lymphatic channels Nerve fibers

CELLS IN THE PULP Odontoblasts Odontoblasts are dentin forming cells which are of ectomesenchymal origin and are the most distinctive and the second most prominent cells in the pulp. They have a constant location adjacent to the dentin, with their cell bodies in the pulp and the cell processes in the dentinal tubules, i.e. the odontoblastic zone of the pulp. The number of odontoblasts equals the number of dentinal tubules and the average number is about 59,000–76,000 per square millimeter in coronal dentin. They are numerous and larger in the coronal pulp than the radicular pulp. Morphologic variations of odontoblasts range from the tall columnar cells in the crown of the tooth to a low columnar type in the middle of the root and are flattened near the apex of the tooth.

Structure Odontoblasts have a cell body residing in pulp and cytoplasmic process extending to the dentinal tubules. The cells are approximately 5–7 μm in diameter and 25–40 μm in length. The odontoblastic cells lie very close to each other and are connected to adjacent cells by junctional complexes. The shape of the cell may be influenced by the degree of activity. More active cells are taller and contain rich synthetic organelles in cytoplasm such as rough endoplasmic reticulum, Golgi apparatus, mitochondria, vesicles, granules, etc. The apical part of the cytoplasm, that is near the pulpal—

dentin junction is devoid of cytoplasmic organelles. The cell body contains an oval nucleus situated at the pulpal end. The cytoplasmic processes begin at the apical end of the cell just above the apical junctional complex, where the cell gradually begins to narrow (3–4 μ) as it enters the pre-dentin. The odontoblastic process is devoid of major cell organelles but microtubules, filaments and vesicles are present in abundance. The size, shape and structure of odontoblasts in the pulp are variable according to the functional activity of the cells. Accordingly, odontoblasts in three different stages can be identified in pulp which includes synthetic or active odontoblasts, intermediate or transitional odontoblasts and resting or aged odontoblasts.

Synthetic or Active Odontoblasts The synthetic odontoblasts can be distinguished under light microscope and appears elongated and having a basal nucleus with a basophilic cytoplasm. These cells have abundant synthetic cellular organelles required for synthesis and secretion of dentin matrix. Numerous secretory granules are found near the secreting end.

Intermediate or Transitional Odontoblasts The intermediate odontoblasts show all features of synthetic cells, but the organellae are less in number and less prominent. The nucleus shows condensation of chromatin with the organelles distributed around the nucleus. Secretory granules are less in number. The difference between the synthetic and transitional odontoblasts can be appreciated only under electron microscope.

Resting or Aged Odontoblasts The resting odontoblasts are stubby cells and can be appreciated under light microscope. These cells have a little cytoplasm with a dark, close faced nucleus. They have less of cellular organelles at pulpal end. Vacuoles and secretory granules are scarce or absent.

Fibroblasts Fibroblasts are the most numerous cell types in the pulp, especially abundant

in the coronal pulp. The shape of fibroblasts vary from fusiform with long slender protoplasmic processes to stellate (star shaped) with shorter numerous branches. The fibroblasts are numerous in young teeth and decreases with age. They help in synthesis, maintenance and degradation of pulp matrix.

Undifferentiated Mesenchymal Cells Represent the pool of reserve cells from which the connective tissue cells of the pulp are derived. They are found along the pulp vessels in the cell rich zone and are scattered throughout the central pulp. They appear larger than fibroblasts and are polyhedral in shape with peripheral processes and a large oval nucleus. They are totipotent cells and can give rise to odontoblasts, fibroblasts, etc. They are more in young pulp and decreases with age, which reduces the regenerative potential of the pulp.

Immunocompetent Cells The immune-competent cells are predominated by macrophages, dendritic cells and lymphocytes. Apart from these, mast cells, plasma cells, neutrophils, lymphocytes, monocytes, etc. are also seen.

Macrophages Macrophages are distributed in the central part of pulp. They are large oval or spindle shaped irregular cells with a clear cytoplasm containing mitochondria, rough endoplasmic reticulum and free ribosomes and have a small round dark staining nucleus. They function as scavenger cells, helping in elimination of dead cells.

Dendritic Cells Dendritic cells are antigen expressing or antigen presenting cells and are found in and around the odontoblast layer with dendritic processes extending between the odontoblasts. They have a close relationship to vascular and neural elements. They are non phagocytic cells and participate in immunosurviellance of pulp by capturing and presenting the foreign antigen to T cells. The number of dendritic cells increases in carious teeth.

Lymphocytes and Eosinophils

They are found extravascularly in normal pulp, which increase noticeably in number during inflammation.

EXTRACELLULAR COMPONENTS Fibers Fibers present in the pulp are predominantly collagen type I and III in the ratio of 55:45. The collagen fibers are distributed throughout the pulp and forms a delicate network. Collagen fibers in pulp, exhibit typical cross striations at 64 nm. In young pulp the fibrils are of smaller diameter ranging from 10 to 12 nm and in older pulp the fibrils aggregate into fibers of greater dimension. The number of collagen fibers increases with age. They may appear scattered throughout the pulp or may appear in bundles; and accordingly termed diffuse or bundle collagen. In addition to collagen, the pulp also contains a few reticulin fibers and elastic fibers.

Ground Substance The ground substance is particularly abundant in young pulp and is composed of acid mucopolysaccharides and protein polysaccharide complex (glycosaminoglycans and proteoglycans). Ground substance provides a medium for distribution of cells and extracellular fibers and gives support to cells of the pulp. It serves as a means of transport of nutrients from the vessels to cells, as well as for transport of catabolites from cells to blood vessels. The amount of ground substance decreases with age.

CONNECTIVE TISSUE STRUCTURES Blood Vessels The pulp organ is well vascularized and is supplied by superior and inferior alveolar arteries. The blood vessels enter and exit the dental pulp through apical and accessory foramina. The arterioles entering the apical foramen follow a straight course up to the coronal pulp. In the coronal pulp the vessels undergo extensive branching and some travel to the periphery of the pulp to

form a subodontoblastic capillary network (Fig. 6.2). During dentinogenesis some of the capillaries even loop around the odontoblasts. The arterioles in the pulp vary in diameter; greatest of 50 to 100 microns to 10 to 15 microns for terminal arterioles. The arterioles divide to give rise to meta-arterioles, precapillaries and capillaries. Capillaries of pulp vary in diameter from 7 to 10 microns and shows pores or fenestrations to facilitate exchange of materials between vessels and its environment. Veins draining the pulp follow the same course as the arterioles. Arteriovenous anastomoses is also seen in coronal pulp.

Lymphatic Channels The lymph vessels that drain the pulp are thin walled having an irregular lumen composed of endothelial cells surrounded by an incomplete layer of smooth muscle cells. The anterior teeth drains into the submental lymph nodes and the posterior teeth drains into the submandibular and the deep cervical lymph nodes.

Nerves Nerve supply to pulp is abundant. Nerve bundles enter pulp through apical foramen. Pulp receives sensory supply from trigeminal nerve and superior cervical ganglion. The nerves in the pulp are non-myelinated—A δ and A β fibers which transmits sharp pain or nonmyelinated or “c” fibers which transmits dull pain. The non-myelinated fibers are sympathetic and are mainly controlling the luminal diameter of the vessels. The myelinated fibers entering the foramen follow a course similar to the arterioles. In the coronal pulp they undergo extensive branching and advance towards the cell rich zone, again branch and form a network of nerves in the cell free zone below the odontoblastic zone. This network of nerves are known as plexus of Raschkow (Fig. 6.2).

FUNCTIONS OF DENTAL PULP Dental pulp performs various functions such as nutritive, sensory, formative, defensive and protective functions.

1. Nutritive The blood vascular system of dental pulp nourishes and maintains the vitality of dentin by providing oxygen and nutrients to the odontoblasts and their processes, as well as providing a continuing source of dentinal fluid.

2. Sensory The pulp has both myelinated and non-myelinated nerve fibers. Sensory nerve fibers present in the pulp respond to stimuli such as changes in temperature, pressure, vibration and chemical agents that affect the dentin and pulp.

3. Inductive The dental papilla, the primordium of dental pulp performs an important function in determining the crown pattern and differentiation of ameloblasts through its inductive influence.

4. Formative The pulp performs the formative function because of the presence of odontoblasts which are the formative cells of dentin. These cells are involved in the support, maintenance and continued formation of dentin.

5. Defensive and Protective The pulp responds to irritation and protects itself and the vitality of the tooth by producing reparative dentin or inducing dentinal sclerosis which can block the dentinal tubules and prevent the irritating stimulus reaching the pulp. Pulp being a highly vascularized connective tissue may initiate an inflammatory reaction in response to an irritating stimulus. Various immune cells such as macrophages, lymphocytes, plasma cells, neutrophils, etc. are involved which aid in the process of repair of pulp.

AGE OR REGRESSIVE CHANGES IN PULP 1. Size

With age there is progressive reduction in pulp size due to continuous secondary dentin deposition. As a result the pulp horns become less prominent or even obliterated.

2. Cellular and Fibrous Components In aging pulp the fibrous component becomes more prominent. The number of collagen fibers increases with age and in older pulp the fibrils aggregate into fibers of a great dimension. They may be more diffuse and randomly arranged in coronal pulp but are in bundled form in radicular pulp. In older teeth, more fibrous appearance of the pulp may be apparent, possibly due to reduction in size of pulp reducing the space available for their distribution. The number of cells in the pulp including fibroblasts and odontoblasts decreases with age. The cells also show a decrease in the amount of cytoplasm and cytoplasmic synthetic organelles.

3. Changes in Blood Supply and Innervations Loss and degeneration of myelinated and unmyelinated axons occur which can be correlated with an age related reduction in sensitivity. As this progresses, the number of nerves gets greatly diminished. There is a decrease in the blood supply as the apical foramen is almost obliterated by secondary dentin and cementum which initiates most of the other changes in the pulp. Blood vessels may also show fibrosis or calcification of vessel walls or atherosclerotic changes, from the age of 40 years.

4. Reduction in Sensitivity and Healing Potential As age advances the sensitivity and healing or reparative capacity of pulp decreases. Decreased sensitivity can be directly related to nerve degeneration. Overall reduction in vascular supply and cellular component could be responsible for decreased reparative capacity of pulp.

5. Pulpal Calcifications Calcification may occur in pulp tissue as a result of aging or external stimuli. These may be nodular, calcified masses referred to as pulp stones or diffuse calcifications.

Pulp stones or denticles are nodular calcified masses present in coronal or radicular pulp. They are seen in functional as well as embedded or unerupted teeth. Incidence increases with age: 66% between the age group of 10–30 years, 80% between 30 and 50 years and 90% above 50 years. Various other etiological factors such as infection, trauma due to operative procedures, vascular injury resulting in thrombosis and systemic diseases (atherosclerosis) also have been considered (Flowchart 6.1).

Pulp stones are classified based on its relation to adjacent dentin into three sroups (Flowchart 6.2) Free pulp stones are those calcified structures lying free in the pulp without being attached to the dentin. Attached pulp stones: Those which are attached to the dentin. Embedded pulp stones: When pulp stone is completely surrounded by dentin it is called embedded pulp stone. They are believed to be formed as free pulp stones which later becomes attached or embedded due to progressive dentin formation. Flowchart 6.1: Pathogenesis of pulp calcifications

Flowchart 6.2: Types of pulp calcifications

Depending on structure pulp stones can be grouped into True denticles: True denticles are localized masses of calcified tissue having tubular structure containing odontoblast processes and thereby resembling dentin and hence called as true denticles. They are very small and are seen only rarely. The true denticles are thought to be formed due to entrapped remnants of root sheath in pulp. These cells may induce the differentiation of odontoblasts which form calcified structures; resembling dentin. False denticles: False denticles are localized masses of calcified tissue having a laminated structure made of concentric layers of calcium deposited around a central nidus, which could be dead cells. They do not have a tubular structure or structural resemblance to dentin. They are larger than the true denticles and may fill the entire pulp chamber.

Diffuse Calcification Diffuse calcification is composed of small calcified particles with a few larger masses. The calcified structures are arranged as linear strands parallel to the long axis of pulp. They are found to be closely associated with blood vessels with an orientation parallel to the vessels and nerves. It is usually seen only in radicular pulp.

Clinical Significance Affected tooth is vital, and usually symptomatic but sometimes manifest mild neurologic pain. Pulpal calcifications may cause difficulty in extripating the pulp during RCT.

Clinical Considerations

•

In young teeth pulp chambers are large with high pulp horns. Therefore care should be taken while cavity preparation to avoid inadvertent pulpal exposure.

•

Presence of multiple accessory canals in some teeth may cause failure of endodontic treatment. Similarly, presence of pulp stones also may cause difficulty in endodontic treatment.

•

Pulpal tissue is highly sensitive to various types of trauma which may be thermal, chemical or mechanical. Mechanical or thermal trauma during cavity preparation prior to restoration of teeth may permanently damage the pulp. Similarly, chemicals leached out from restorative materials or heat transmitted due to inadequate thermal insulation while restoring the tooth also may have adverse effect on pulp. Therefore precautions need to be taken while cavity preparation and restoration. Permanent damage to the pulp causes death of pulp and therefore loss of vitality of the tooth.

•

Vital teeth respond to thermal and electric stimuli and vitality testing is a basic procedure carried out in dental clinic to diagnose pulpal diseases. Routinely used pulp testing strategies may involve sensitivity tests such as thermal or electric pulp testing, which assess whether there is response to a stimulus.

•

Pulp is connective tissue and any type of insult resulting from dental caries or trauma can cause the inflammation as in case of any other tissues of the body. Inflammation of dental pulp is called pulpitis. Pulpitis may be reversible or irreversible. Irreversible pulpitis results in permanent damage to the pulp and if not treated, progresses further to infection of periapical tissue. Chronic mild infection of pulp may induce a proliferative reaction of pulp which is referred to as pulp polyp. A pulp polyp will present as a pink globular soft tissue mass filling a large carious cavity. Once the pulpal tissue is involved in disease process, the tooth needs root canal treatment.

•

As pulpal tissue is located in a closed chamber, surrounded by rigid dentin, pressure built up in pulp due to inflammation result in intense pain.

•

Dental pulp stem cells found within the cell rich zone of dental pulp

has gained significant importance as a potential resource of stem cells which may be used for regeneration and repair of a multitude of diseased and injured organs and tissues. These cells exhibit multipotency due to their embryonic origin, from neural crests. These mesenchymal stem cells are capable of extensive proliferation and differentiation, which makes them important. Because of their ability to produce and secrete neurotrophic factors, these cells may also be beneficial for the treatment of neurodegenerative diseases and the repair of motoneurons following the injury.

7 Cementum and Cementogenesis

Introduction Physical properties and chemical composition Cementogenesis Types of cementum Structure Functions Clinical considerations

C

ementum is a calcified connective tissue that forms the outer covering of the anatomic root of the tooth. Cementum is also considered as a part of periodontium, the attachment apparatus of the tooth, because it provides a medium for insertion of periodontal ligament fibers. The name cementum is derived from the word ‘caementum’ which means quarried stone or chips of stone. It is a specialized connective tissue that shares some physical, chemical and structural properties of bone. Unlike bone, cementum is avascular, insensitive, do not undergo remodeling under normal circumstances and is more resistant to resorption.

Physical Characteristics of Cementum Color: Cementum is yellowish in color which is lesser than that of dentin. Cementum do not have a shining surface, therefore it can be easily differentiated from enamel which is white and shiny.

Hardness: Cementum is softest of all the dental hard tissue components. Permeability: Cementum is permeable for certain substances and is more permeable than enamel. Cellular cementum is more permeable than acellular cementum due to high organic component. With age the permeability decreases. Resistance to resorption: Cementum is resistant to resorption when compared to bone and could be related to the avascular nature of cementum. This property is utilized in orthodontic movement of teeth. Thickness: Cementum thickness varies in different part of root. Thinnest at cervical region, with a thickness of 50 μ, gradually increases to 200 μ at the apical region. Insensitive: Cementum is insensitive to pain due to lack of nerve innervation. When root scaling is necessary, patient do not experience pain. However, when the thin cementum layer that seal dentinal tubules are lost sensitivity is experienced.

Composition of Cementum Cementum is composed of 45–55% inorganic components and 50–55% organic material and water. Inorganic components are mainly calcium and phosphate in the form of hydroxyapatite crystals which are of the same size as that of bone. In addition, cementum also contains some trace elements such as copper, iron, magnesium, potassium, silica, sodium, zinc and fluoride. Cementum has highest fluoride content of all mineralized structures of the body. Organic components include collagenous and non-collagenous matrix. Collagenous matrix primarily comprises type I collagen fibers (90%) and some type III fibers in extrinsic fibers. Collagen fibers of cementum are intrinsic and extrinsic fibers based on their origin (source). Intrinsic fibers are secreted by cementoblasts, the synthetic cells of cementum while extrinsic fibers are from outside the cementum, i.e. from periodontal ligament (Sharpey’s fibers) which get inserted to the cementum. Intrinsic fibers are smaller, of 1–2 μ diameter and are arranged parallel to the root surface. Extrinsic fibers are of 5–7 μ diameter, arranges perpendicular to the root surface. Noncollagenous matix of cementum contains various proteins, of which

major ones are bone sialoprotein and osteopontin, generally accumulate in cement lines and in the spaces among the mineralized collagen fibrils. In addition, cementum derived attachment protein, osteocalcin, osteonectin, tenascin, fibronectin, alkaline phosphatase, proteoglycans such as chondroitin sulfate, heparin sulfate, hyaluronate as well as several growth factors have been identified in cementum. The noncollagenous matrix has a significant role in initiation and regulation of mineralization process. The amount of non-collagenous proteins, depends on cementum types and with speed of formation of the tissue and packing density of collagen fibrils.

Cementogenesis Cementogenesis is the formation of cementum and is a rhythmic process. Cells responsible are cementoblasts that are derived from dental follicle of the tooth germ. The formation of cementum can be subdivided into a pre-functional and functional developmental stage. Pre-functional stage refers to formation of main cementum varieties that occur during root development. On the other hand, the functional development of cementum commences when the tooth is about to reach the occlusal level and continues throughout life. Biological responsiveness of cementum, i.e. adaptive and reparative functions of cementum is possible because of functional development, which in turn, influences the alterations in the distribution and appearance of the cementum varieties on the root surface with time.

Steps Involved in Cementum Formation The process of cementogenesis involves two stages; matrix deposition and mineralization.

Matrix deposition After the deposition of radicular dentin, Hertwig’s epithelial root sheath degenerates and loses continuity, exposing the newly formed dentin. This allows the cells from the inner part of dental follicle to come in contact with newly formed dentin. These infiltrating dental follicle cells differentiate into cementoblasts under the inductive influence of dentin or Hertwig’s epithelial root sheath. These cells develop rich cytoplasmic synthetic organelles and

increased hydrolytic and oxidative enzymes. They deposit cementum matrix (cementoid) which include both collagen fibers and ground substance. Cementoblasts deposit collagen onto the dentin matrix which is in the process of mineralization, permitting intermingling of fibers of these two tissues at the future dentino-cemental junction. Noncollagenous matrix proteins are deposited into the spaces between the fibers. Once the inner part of the cementum is formed, the periodontal ligament that get inserted into cementum matrix forms the collagen matrix and further, cementocytes deposit only noncollagenous matrix proteins.

Mineralization Mineralization of cementum begins after some amount of organic matrix has been laid down, by deposition of hydroxyapatite crystals in the form of plates and spicules. Noncollagenous matrix proteins play a significant role in mineralization. A calcium binding amino acid, known as Gla protein and osteocalcin and osteonectin act as nucleating substances to initiate mineralization, bone sialoprotein promote mineralization and osteopondin regulate crystal growth. Possible role of cementoblasts released matrix vesicles, in mineralization of the initial cementum has been suggested by Yamamoto et al. Based on experimental findings these researchers suggest that during the initial cementogenesis, cementoblasts release matrix vesicles which result in formation of calciferous spherules, that trigger the mineralization. After insertion of principal fibers, mineralization advances along collagen fibrils without matrix vesicles (Yamamoto et al. Mineralization process during acellular cementogenesis in rat molars: A histochemical and immunohistochemical study using fresh-frozen sections. Histochem Cell Biol 2007 Mar; 127(3):303–11). Cementoblasts that form cementum recedes outward as the formation proceeds. So the outer surface of cementum will always have cementoblasts lining the periphery. Since the mineralization process lags behind matrix formation, a layer of cementoid is seen lining the mineralized cementum at the inner aspect of cementoblast layer. The cemetoid layer is less distinct or even absent in relation to acellular cementum. Sometimes a few cementoblasts get entrapped in cementum matrix which remains in mineralized cementum in spaces called lacunae. This happens when the rate of formation of cementum is faster and the formed cementum is referred to as

cellular cementum. As the cementogenesis proceeds fibers from developing periodontal ligament get inserted into cementum and the portion of principal fibers embedded in cementum is called Sharpey’s fibers.

Classification of Cementum Depending on time of formation • •

Primary cementum Secondary cementum

Based on presence or absence of cells • •

Acellular cementum Cellular cementum

Good to Know Origin of cementoblasts It was generally accepted that cementoblasts originate by differentiation of the mesenchymal cells of the dental follicle. Recently, a different hypothesis for the origin of cementoblasts has been proposed, i.e. epithelial - mesenchymal transformation of Hertwig’s epithelial root sheath cells result in formation of cementoblasts. Accordingly, two types of cementoblasts have been identified 1. Cells derived from Hertwig’s epithelial root sheath that are involved in formation of acellular cementum; 2. Cells derived from dental follicle, that form cellular cementum. These cells were reported to be different in receptors expressed on cell surface as well as in their reaction to signaling molecules, e.g. receptor for PTH is expressed by cementoblasts derived from dental follicle, while those derived from Hertwig’s epithelial root sheath do not express. The former cells express extracellular protein osteopondin and osteocalcin, thus phenotypically similar to osteoblasts while latter express only osteopondin and partial osteoblastic phenotype. However, investigations carried out by Yamamoto et al., suggested that there is no intermediate phenotype transforming epithelial to mesenchymal cells, and that epithelial sheath cells do not generate mineralized tissue. They concluded that the epithelial-mesenchymal transformation does not occur in Hertwig’s epithelial root sheath in acellular or cellular

cementogenesis and that the dental follicle is the origin of cementoblasts, as has been proposed in the original hypothesis. (Yamamoto T, Takahashi S. Hertwig’s epithelial root sheath cells do not transform into cementoblasts in rat molar cementogenesis. Ann Anat 2009 Dec;191(6): 547–55.) (Yamamoto T, Yamamoto T, Yamada T, et. al. Hertwig’s epithelial root sheath cell behavior during initial acellular cementogenesis in rat molars. Histochem Cell Biol 2014 Nov;142(5): 489–96.) Based on presence or absence of fibers • •

Fibrillar cementum Afibrillar cementum

Based on type of fibers • • •

Intrinsic fiber cementum Extrinsic fiber cementum Mixed fiber cementum

Accordingly cementum can be of different types Primary acellular intrinsic fiber cementum Primary acellular extrinsic fiber cementum Secondary cellular intrinsic fiber cementum Secondary cellular mixed fiber cementum Acellular afibrillar cementum Intermediate cementum Mixed stratified cementum

STRUCTURE OF CEMENTUM Primary Acellular Cementum This is the first formed cementum covering the cervical 2/3rds of the root. The rate of deposition of primary cementum is slow during its formation, which allows sufficient time for the cementoblasts to move away. As a result

of this, cells are not entrapped in the matrix and therefore primary cementum is always acellular. Cementoid layer covering this cementum is indistinct. Cementum is thinnest at cervical region with thickness of 50 pm which gradually increases towards the root apex. In a ground section acellular cementum (Fig. 7.1) appears as a structureless layer without any entrapped cells. Incremental lines, indicating periodic rhythmic deposition of cementum, are seen as dark lines which are parallel to the root surface and are relatively closer to each other because of slow deposition. These lines are called incremental lines of Salter that can also be seen in decalcified sections as basophilic lines. Sharpey’s fibers (part of principal fibers of periodontal ligament inserted to cementum) may be seen as fine striations perpendicular to the root surface. Although the Sharpey’s fibers are more in number in acellular cementum than in cellular cementum, they are less distinct as they are fully mineralized.

Fig. 7.1: Acellular cementum

Good to Know The cellular cementum, generally consists of more of intrinsic fibers exhibiting alternate intensely and weakly stained lamellae (each about 2.5 microns thick). It has been suggested that this pattern results from periodic changes of arrangement of the intrinsic fibers. According to Yamamoto et al., the alternate lamellar pattern conforms to the twisted plywood model,

in which collagen fibrils rotate regularly in the same direction to form two alternating types of lamellae; one type consists of transversely and almost transversely cut fibrils and the other consists of longitudinally and almost longitudinally cut fibrils. The development of the intrinsic fiber arrangement may be controlled by cementoblasts; the cementoblasts move finger-like processes synchronously and periodically to create alternate changes in the intrinsic fiber arrangement, and this dynamic sequence results in the alternate lamellar pattern. (Yamamoto et al., histological review of the human cellular cementum with special reference to an alternating lamellar pattern. Odontology 2010 Jul; 98(2): 102–9.) Innermost portion of cementum of 15–20 microns adjacent to dentin has only collagen fibers deposited by the cementoblasts. So this portion is called primary acellular, intrinsic fibrillar cementum. The remaining part of cementum is formed after the establishment of periodontal ligament and in this portion the extrinsic fibers make up the bulk of collagen. Therefore this part of cementum is called primary acellular extrinsic fiber cementum.

Secondary Cellular Cementum Secondary cellular cementum is seen at the apical 1/3rd of root and the thickness gradually increases as it approaches the apex of the root. The rate of deposition is faster leading to entrapment of cementoblasts in the matrix and they remain as resting cementocytes in the mineralized cementum. This cementum is thicker up to 150–200 microns. Cementocytes (Fig. 7.2) are the entrapped cells found in cellular cementum and are located in lacunae (Fig. 7.3). They are spider shaped with an ovoid cell body of 8 to 15 microns diameter and up to 30 processes or canaliculi projecting from the cell body. These canaliculi branch and anastomose with those of adjacent cells. Most of these processes are directed towards the periodontal ligament from where the cells derive nutrition, while some are directed inwards and laterally. The cytoplasm of these cells contains only a few organelles and the cells in deeper portions; more than a distance of 60 microns from the source of nutrition shows degenerative changes. In ground sections, the cementocytes are lost and lacunae appear as dark spaces. In decalcified sections, cementocytes are clearly visible.

Fig. 7.2: Cementocytes (Note the direction of cell processes).

Fig. 7.3: Cellular cementum

Cellular cementum also shows incremental lines of Salter which are parallel to root surface and slightly far apart because of increased thickness of each increment resulting from faster deposition. Sharpey’s fibers are seen as striations at an angle to the root surface. The actual number of Sharpey’s fibers is lesser than that of acellular cementum, but they are more distinct as the fibers are not fully mineralized. They have a mineralized periphery and an unmineralized core. So in ground sections the organic component at the unmineralized core is lost and appears dark, making it more distinct. Cellular cementum always has a peripheral layer of cementoid lined by cementoblasts.

In decalcified sections, cementoblasts are found lining the surface of cementum, interposed between bundles of periodontal ligament fibers and are separated from mineralized cementum by a layer of cementoid. The cementoid or precementum layer provide a compatible environment for the cementoblasts and serve a protective function preventing odnotoclastic resorption. Cementoblasts maybe either active (formative) or inactive (resting). Active cementoblasts are round or ovoid plumb cells with slightly basophilic cytoplasm and open faced nucleus, while the resting cells have closed faced nucleus and a little eosinophilic cytoplasm. Because of the cementoblasts lining the surface, cementum formation can continued throughout life. Similar to acellular cementum, the portion of cellular cementum that is formed before the establishment of periodontal ligament, has mainly intrinsic fibers and therefore called secondary cellular intrinsic fiber cementum. The portion formed after the establishment of periodontal ligament has both extrinsic and intrinsic fibers, therefore called secondary cellular mixed fiber cementum.

Acellular Afibrillar Cementum This is the type of cementum found at the overlapping type of cementoenamel junction. Acellular afibrillar cementum do not contain any entrapped cells. Although this cementum is referred to as afibrillar cementum, it contains fibrillar component in the matrix. But the fibrillar component does not bear characteristic collagen periodicity. Gradually the thickness of afibrillar cementum increases by deposition of fibrillar cementum due to contact with the connective tissue. The afibrillar cementum is also called coronal cementum because it may be seen on the occlusal fissures and other sites where the break in the reduced enamel epithelium has occurred.

Intermediate Cementum The term intermediate cementum is used to describe a type of secondary cementum found near root tip region of molars and premolars, which shows some entrapped cellular debris derived from Hertwig’s epithelial root sheath or odontoblasts layer. This type of cementum is not generally observed in deciduous teeth and anterior teeth.

Mixed Stratified Cementum Generally the acellular cementum is distributed in cervical 2/3rds and cellular cementum at apical third. At times in the apical region or in furcation areas of multi-rooted teeth, these two types of cementum show an alternate layered arrangement where the cellular cementum is covered by a layer of acellular cementum to which in turn, may be added another layer of cellular cementum. This type of layered arrangement of cementum is referred to as mixed stratified cementum. This may represent cementum deposited at different rates in response to adaptive needs. Differences between acellular and cellular cementum Acellular cementum Cellular cementum Primary cementum Secondary cementum Thickness is less Thickness is more Rate of deposition is Faster rate of deposition slower No entrapped cells seen Entrapped cells (cementocytes) are seen Incremental lines of Salter Incremental lines are slightly far apart are closer Located at cervical 2/3rds Located at apical 1/3rds Contain more extrinsic Contain more intrinsic fibers fibers Sharpey’s fibers are less Sharpey’s fibers are more prominent because prominent and they are they are not fully mineralized and have fully mineralized central unmineralized core The junction between Cemento-dentinal junction is more distinct dentin and cementum is less distinct Cementoid layer is thin Definite thicker layer of cementoid seen and not distinct Function is mainly Function is mainly adaptation attachment

Cemento-enamel Junction

This is the junction between cementum and enamel at the cervical region of tooth (Figs 7.4 and 7.5). The relationship between cementum and enamel at cervical part of the tooth can be of three types (Fig. 7.4). More than one relationship may occur at different sites around the neck of a given tooth. Sharp junction or Butt joint or end-to-end approximating CEJ, where enamel and cementum meet at a sharp line. This type is reported in around 30% teeth. Overlap junction: In this type cementum overlaps the cervical region of enamel. This occurs due to degeneration of cervical region of reduced enamel epithelium allowing the dental follicle cells to come in contact with newly formed enamel. The follicle cells differentiate into cementoblasts and deposit cementum. This type of junction is seen in 60% of teeth. The type of cementum that is overlapping enamel is acellular, afibrillar type without any entrapped cells but containing fibrillar component that does not bear characteristic collagen periodicity.

Fig. 7.4: Types of cemento-enamel junctions

Gap junction: In this type, there is no actual junction between enamel and cementum. Instead, a cervical region of root devoid of cementum is seen. This occurs due to delayed degeneration of Hertwig’s epithelial root sheath preventing the contact between dental follicle cells and newly formed dentin which causes lack of differentiation of cementoblasts. In that region a gap between enamel and cementum is seen due to lack of deposition of cementum. This type of junction is seen in 15% teeth. Normally the cementoenamel junction is covered by gingiva. Gingival recession causes

exposure of cementoenamel junction leading to sensitivity due to exposed dentin in gap type junction.

Good to Know Another pattern of CEJ has been reported in about 1.6% of teeth, where the enamel overlap cementum at cervical region of tooth. (Arambawatta, et al. J Oral Sci 2009 Dec; 51(4):623–7.) The existence of this pattern is controversial as cementum formation is initiated only after completion of enamel formation. Some researchers consider it as an optical illusion, while rare micro-regions of enamel over cementum has been demonstrated by some researchers through scanning electron microscopic (SEM) analysis.

Fig. 7.5: Cemento-enamel junction

Cemento-dentinal Junction This is the junction between dentin and cementum and is relatively straight in contrast to scalloped DEJ. The cemento-dentinal junction may be scalloped in deciduous teeth. The junction between dentin and cementum is not very distinct in acellular cementum while is somewhat distinct in cellular cementum. In decalcified sections cemento-dentinal junction can be identified easily

because cementum stains more intensely than dentin. Collagen fibers of dentin are dispersed randomly whereas those of cementum are more orderly arranged and aggregated into discrete bundles. At the cemento-dentinal junction the fibers of dentin and cementum are found to be intertwining, which is more pronounced in cellular cementum. This intertwining fibers along with proteoglycans present between, contribute to attachment between cementum and dentin. It was thought that dentin and cementum are separated by 10 microns thickness layer termed as hyaline layer of Hopewell Smith.

FUNCTIONS OF CEMENTUM Attachment: Cementum is one of the components of periodontium which is the attachment apparatus of tooth. The periodontal ligament fibers are inserted into cementum therefore providing attachment of the tooth to alveolar bone. Acellular cementum is primarily involved in attachment. (Cellular cementum is at times absent in some teeth particularly in anterior teeth indicating that it is not essential for attachment.) Continuous deposition of cementum provides the new layers to keep the attachment apparatus intact. Functional adaptation: Due to masticatory force there is continuous wearing away of occlusal or incisal part of teeth causing decrease in length. This decrease in length is compensated by cellular cementum deposition in the apical region of tooth. Thus apical cementogenesis helps to maintain occlusal functional relationship of teeth. Repair: Any damage caused to the root is repaired by continuous deposition of cementum. In case of rapid repair, cellular cementum with small apatite crystals are deposited while in case of slow repair, acellular cementum is deposited. Cementum protects the dentin by forming a continuous layer covering it and thus preventing possibility of the direct exposure of dentin to oral environment, in case of gingival recession and root exposure. Exposure of dentinal tubules causes sensitivity. Cementogenesis assist in maintenance of width of periodontal ligament. Probably helping in the eruption procedure by deposition in apical region.

(Age changes, refer page 322)

Clinical Considerations ▪

Hypercementosis is the deposition of excessive amount of secondary cementum on the root surface. This may involve single tooth or multiple teeth. Excessive cementum deposition may be only at the apex or nearly over the entire root area. Hypercementosis may or may not be increasing the functional efficiency. If hypercementosis is associated with improved functional quality, it is called cementum hypertrophy and if it is not related to function as in a nonfunctional tooth, it is called cementum hyperplasia. Hypercementosis can occur due to local factors such as abnormal occlusal trauma, chronic periapical inflammation or unopposed teeth. As a functional adaptation to compensate for the occlusal wear there can be excessive cementum deposition in some teeth. Generalized hypercementosis involving multiple teeth is a finding in Paget’s disease of bone. Teeth affected do not show any clinical symptoms. Radiographs reveal thickening of root with a round apex. Hypercementosis may cause problems while extracting, therefore care should be taken.

▪

Avascular nature of cementum makes it more resistant to resorption than bone and this nature permits the orthodontic tooth movement without causing damage to tooth. However, excessive orthodontic force may result in resorption of cementum.

▪

Cementum resorption or even fracture can occur due to trauma or excessive forces, but the damage usually is repaired by formation of new cementum, either acellular or cellular cementum.

▪

Gingival recession or periodontal surgery leads to exposure of cervical cementum may result in sensitivity particularly in case of gap type of CEJ.

▪

Absence of cementum or defective cementum formation and therefore premature loss of deciduous teeth has been reported in conditions like hypophosphatemia. Congenital absence of cellular cementum in the deciduous and permanent dentition has been reported in cleidocranial dysplasia, an autosomal dominant disorder, in which this is related to

the failure in the eruption. ▪

Continuous rhythmic deposition of cementum throughout life is used for age estimation in forensic odontology.

▪

Cemetum, once exposed to oral environment undergo various changes: Due to incorporation of minerals from oral environment or adsorption of microbial toxins from oral microflora, etc.

8 Periodontal Ligament Dr Rajeesh Mohammed PK and Dr Girish KL Introduction Components of periodontium Structure of periodontal ligament – –

Cellular components Extracellular component

Functions of periodontal ligament Clinical considerations

T

he tissues which invest and support the tooth in its natural and functional state are collectively called periodontium. These tissues form the attachment apparatus of the tooth. The periodontium is comprised of two mineralized tissues and two fibrous tissues. The alveolar bone and the cementum form the mineralized components and the periodontal ligament and the lamina propria of gingiva which contains the gingival group of fibers forms the fibrous component of the periodontium.

Components of Periodontium Two mineralized tissues – –

Alveolar bone Cementum

Two fibrous tissues –

Periodontal ligament

–

Lamina propria of gingiva

PERIODONTAL LIGAMENT It is a soft fibrous connective tissue that is noticeably cellular and vascular, which surrounds the tooth root and anchors it to the bony socket. The periodontal ligament is interposed between the roots of teeth and the inner wall of the alveolar socket, the periodontal space. It is neither a true ligament nor a membrane. The periodontal ligament is continuous with the gingival connective tissue above the alveolar crest. It communicates with the dental pulp at the apical foramen and with bone marrow of the alveolar process through vascular channels. The infection from these tissues, i.e. gingiva and pulp, can involve the ligament if left untreated. Various synonyms are used in the literature to describe periodontal ligament such as desmodont, gomphosis (fibrous joint), pericementum, dental periosteum, alveolar ligament, periodontal membrane, etc.

Shape Periodontal ligament resembles the “Hour glass” in shape as it is narrowest in the middle third of the root and widens both apically and near the alveolar crest.

Width The width of periodontal ligament is variable, the average width ranging from 0.15 to 0.38 mm. The width of periodontal ligament decreases with age, which can be partly attributed to the reduced functional load.

Development The periodontal ligament develops from the dental follicle, an ectomesenchymal component of tooth germ. As the root formation progresses, the dental follicle cells differentiate into cementoblasts to produces cementum, osteoblasts to produces bone and fibroblasts to produce the fibers and ground substance of periodontal ligament. As the root formation proceeds the fibers get embedded in the forming cementum and alveolar bone.

In the initial stages of formation of periodontal ligament, the ligament space consists of unorganized short connective tissue fiber bundles which extend from the cementum and the alveolar bone. As the tooth erupts the fibers orient themselves in various characteristic planes.

Microscopic Structure of Periodontal Ligament The periodontal ligament is comprised of cellular components and extracellular substances. Various connective tissue structures such as neurovascular elements are also found to be distributed in the periodontal ligament. The cellular components of the periodontal ligament include synthetic cells and resorptive cells of various structural components of periodontium. A synchronized functioning of these cellular components helps to maintain the integrity of the attachment apparatus of tooth. In addition, progenitor cells, epithelial cell rests of Malassez and other defense cells are also present. Cells of periodontal ligament Synthetic cells • • •

Osteoblasts Cementoblasts Fibroblasts

Resorptive cells • • •

Osteoclasts Cementoclasts Fibroblasts

Progenitor cells Epithelial cell rests of Malassez Defense cells • •

Mast cells Macrophages, etc.

Osteoblasts

Osteoblasts are the bone forming cells derived from the multipotent mesenchymal cells. They cover the periodontal surface of alveolar bone and constitute a modified endosteum. The osteoblasts help in the formation of organic matrix of bone (osteoid) and in the mineralization of the matrix. Osteoblasts lining the periodontal surface of the alveolar bone may be either resting or active. Active osteoblasts are plump with abundant synthetic organelles while resting cells are flattened (for details refer page 95).

Cementoblasts Cementoblasts are the cementum forming cells which are derived from the undifferentiated ectomesenchymal cells of the dental follicle and they resemble osteoblasts and are most often in resting stage. These cells are distributed along the periodontal surface of cementum. The cementoblasts help in the formation of cementum which has a functional importance in maintaining the width of periodontal ligament (for details refer page 79).

Fibroblasts Fibroblasts are the most numerous and functionally important connective tissue cells in periodontal ligament. They may be plump, spindle-shaped or fusiform and are oriented parallel to the collagen fibers. They are large cells with extensive cytoplasm containing abundant cellular organelles associated with protein synthesis and secretion such as rough endoplasmic reticulum, Golgi complex, mitochondria, etc. Unlike other cells, the fibroblasts perform the dual function of synthesis as well as degradation of collagen fibers, thereby helping to maintain the turnover of collagen and homeostasis of periodontal ligament. Fibroblasts produces collagen and ground substance required for periodontal ligament. They participate in collagen degradation by secreting collagenase enzyme and by phagocytosing and degrading the collagen molecules. The fibroblasts in the ligament exist as different subpopulation, although they look alike at both light and electron microscopic levels. Fibroblasts in the periodontal ligament are also referred to as myofibroblasts because of the presence of contractile elements such as actin and myosin in their cytoplasm, to provide contractile force required for tooth movement.

Osteoclasts

Osteoclasts are multinucleated giant cells, approximately 20–100 microns in diameter (refer Figs 9.4a and b). These cells are found in Howship’s lacunae and have a brush or ruffled border towards the surface to be resorbed. The osteoclasts help in the resorption of bone (for details refer page 96–97).

Cementoclasts Cementoclasts resemble osteoclasts structurally and functionally and helps in the resorption of cementum and other dental hard tissues (for details refer page 289–290).

Progenitor Cells Progenitor cells are undifferentiated mesenchymal cells which have the capacity to undergo mitotic division. Pleuripotent undifferentiated mesenchymal cells are present in the periodontal ligament which can give rise to various synthetic cells. They have a perivascular location and are usually found in a quiescent state having a small close-faced nucleus and a little cytoplasm. These cells can enter the cell cycle when triggered by stimuli and undergoes cell division, giving rise two daughter cells, one of which differentiates into the synthetic cell type while the other remains in the progenitor compartment.

Epithelial Cell Rests of Malassez Epithelial cell rests of Malassez are the remnants of Hertwig’s epithelial root sheath (HERS) (Fig. 8.1). They are found in the periodontal ligament close to the cementum. These cell rests persists as network, strands, islands or tubulelike structures near or parallel to the root surface and are most numerous in the apical and cervical areas. These epithelial cells exhibits tonofilaments and are attached to one another by desmosomes. Cell shape varies from squamoid to cuboidal with round or ovoid hyperchromatic nucleus. These cells are surrounded by a periodic acid-Schiff (PAS) positive, argyrophilic, fibrillar material, from which they are separated by a basal lamina. These cell rests decreases with age by degenerating and disappearing or by undergoing calcification to form cementicles. The physiologic role of the epithelial cell rests of Malassez is unknown. They can undergo rapid proliferation when stimulated and give rise to certain pathologic conditions like cysts or tumors.

Fig. 8.1: Epithelial cell rest of Malassez

Mast Cells (Labrocyte, Mastocyte) Mast cells are defense cells found in periodontal ligament and are round or ovoid in shape with a small, pale and a centrally placed nucleus. The cytoplasm of these cells contains numerous metachromatic granules. The granules possess histamine, heparin, serotonin and other inflammatory mediators. The granules stain with metachromatic dyes like azure A and toluidine blue.

Macrophages Macrophages are the scavenger cells with a round or ovoid shape with a horseshoe or kidney shaped nucleus. The cytoplasm contains numerous lysosomes. These cells are derived from blood monocytes are usually located near the blood vessels. The extracellular components of periodontal ligament Fibers – – –

Collagen (type I, III and XII) Oxytalan Eluanin

Ground substances – –

Glycosaminoglycans Glycoproteins

Structures present in the connective tissue – – – – –

Blood vessels Lymphatics Nerves Cementicles —

Fibers of the Periodontal Ligament More than 90% of connective tissue fibers of periodontal ligament are mainly collagen. In addition, oxytalan and eluanin fibers are also seen.

Collagen Fibers Collagen is a high molecular weight protein to which small numbers of sugars are attached. Collagen fibrils have a transverse striation with a characteristic periodicity of 64 nm. Collagen is secreted mainly by fibroblasts, and are secreted as tropocollagen which aggregates into microfibrils which are arranged to form fibrils. These fibrils are packed to form fibers and the fibers are then packed to form bundles. Periodontal ligament has predominantly collagen type I, III and XII. The collagen fibers in the periodontal ligament are found to be gathered into bundles and organized as functional groups having clear orientation relative to the periodontal space. These fiber groups are termed as principal fibers and are assisted in function by a second group of fibers called accessory fibers.

The Principal Fibers of Periodontal Ligament There are five different principal fiber groups (Fig. 8.2) of which four groups are distinguished in all teeth and include alveolar crest group, horizontal group, oblique group and apical group. In addition, a fifth group of fibers called as inter-radicular fibers are seen in multirooted teeth.

1. Alveolar Crest Group The alveolar crest group of fibers radiates obliquely from the crest of alveolar bone to the cervical part of cementum just beneath the junctional epithelium. Function: Alveolar crest fibers help to secure the tooth in the socket and prevents extrusion of tooth. They also resist lateral tooth movements.

Fig. 8.2: Principal fibers of periodontal ligament

2. Horizontal Group These are found immediately apical to the alveolar crest group. The horizontal group of fibers, as the name indicate are oriented horizontally from the cementum to the alveolar bone, almost at right angles to the long axis of the tooth. They constitute only a minor group and are restricted to the coronal third of periodontal ligament space. Function: The horizontal fibers help to resist tooth displacement against lateral pressure. 3. Oblique Group The oblique group is the major groups of fibers in periodontal ligament and

have an oblique orientation within the periodontal space. They extend from the cementum to the alveolar bone with the insertion into the cementum in an apical position when compared to the insertion into the alveolar bone. Function: Since oblique fibers are the major group of fibers, they have a significant role in holding the tooth in its socket. The oblique orientation serves to resist apically directed masticatory forces. 4. Apical Group These fiber bundles radiate from apical region of root to the surrounding alveolar bone (base of the alveolar socket). These fibers are absent in teeth with incompletely formed roots. Function: Apical group of fibers function to resist forces of luxation and may prevent tooth tipping. These fibers may also have a role in protecting the vasculature and nerve fibers in the apical region. 5. Inter-radicular Group The inter-radicular group of fibers are seen only in multi-rooted teeth. The fibers fan out from the crest of inter-radicular septum and get inserted into the cementum in furcation areas of a multi-rooted tooth. Function: They may be help in resisting tooth tipping, torquing and luxation. The principal fibers run a wavy course from the cementum to the bone in various planes. These fibers straighten out under load, thereby helping in force transmission. The unique molecular configuration of collagen imparts flexibility and strength to the tissues. The periodontal ligament fibers are capable of functional adaptation, depending on the requirement. These fibers are attached to cementum on one side and alveolar bone on the other side. The portion of the principal fibers that is embedded into either cementum or bone is called Sharpey’s fibers. These fibers occasionally pass through the bone of the alveolar process to continue as principal fibers of an adjacent periodontal ligament and are referred to as transalveolar fibers. Sharpey’s fibers are associated with high levels of noncollagenous proteins (osteopontin and bone sialoprotein) mainly found in bone and cementum. Between and among the dense bundles of principal fibers, areas of loose connective tissue are found and are referred to as interstitial tissues. These regions contain blood vessels, and nerves and are responsible for providing nutrients to the periodontal ligament and cells of the cementum.

Accessory Fibers of Periodontal Ligament This group includes gingival fibers and transseptal fibers. Gingival fibers (Fig. 8.3) are group of fibers which vary in size and orientation and are found in the connective tissue of gingiva underlying the crevicular and junctional epithelium. These fibers are also called gingival ligament and play a very important role in maintaining the integrity of supporting apparatus of the tooth.

The gingival group of fibers include Dento-gingival fibers: These fibers extend from the cervical portion of the cementum to the lamina propria of gingiva. Dento-periosteal fibers: These fibers extend from the cervical part of cementum to the periosteum of the alveolar crest and the vestibular and oral surface of the alveolar bone.

Fig. 8.3: Gingival group of fibers

Alveolo-gingival fibers: These fibers extend from the crest of the alveolar bone to the lamina propria of gingiva. Circular fibers: These fibers are arranged in the gingival connective tissue, encircling the neck of the tooth like a collar. These fibers are also known as the marginal ligament and they play an important role in maintaining a tightly

fitting gingival collar. Trans-septal fibers: Trans-septal fibers which are also called interdental ligament are also found in gingival connective tissue as accessory fibers extending interproximally between two adjacent teeth. These fibers extend from cementum of one tooth to cementum of adjacent tooth over the interdental bony alveolar crest. Indifferent fiber plexus: Indifferent fiber plexus are formed by small collagen fibers that are seen in association with the larger principal collagen fibers and runs in all directions to form a fiber plexus. Intermediate plexus: Light microscopic examination of longitudinal section of periodontal ligament gives an appearance, as though fibers arising from cementum and alveolar bone are joined in mid-region of the periodontal space giving rise to a zone of distinct appearance. This is called intermediate plexus. It was believed that intermediate plexus provide a site where rapid remodeling of fibers occur, allowing adjustments in periodontal ligament to accommodate small tooth movement. After electron microscopic, radioautographic studies and surgical experiments, intermediate plexus is considered as an artifact arising out of the plane of sectioning. Oxytalan and eluanin fibers: Oxytalan and Eluanin fibers are immature elastic fibers found in the periodontal ligament. They run in an axial direction, one end being embedded in cementum or alveolar bone and the other end in the wall of a blood vessel. These fibers are numerous and dense in the cervical region. These fibers supports the blood vessels of the periodontal ligament and regulates the blood flow. Ground substance: The space between cells, fibers, blood vessel and nerves in periodontal ligament space is occupied by ground substance. The principal ground substance has been estimated to be water (70%). The ground substance is composed of two components; glycosaminoglycans such as hyaluronic acid and proteoglycans, and glycoproteins such as fibronectin and laminin. Ground substance acts as a gel-like base in which cells and fibers are arranged in an organized pattern. Cementicles: Cementicles are small foci of calcified tissue which lie free in the periodontal ligament. They represent areas of dystrophic calcification.

They are commonly seen in older individuals. The size varies from 0.2 to 0.3 mm in diameter and are too small to be seen in radiographs. Cementicles does not have any clinical significance. They may be lying free in periodontal ligament, attached to cementum or embedded in cementum. Cementicles may be formed by calcification of degenerated epithelial cell rests, Sharpey’s fibers or thrombosed blood vessels. Spicules of alveolar bone or cementum traumatically displaced also may remain in periodontal ligament as calcified mass.

FUNCTIONS OF PERIODONTAL LIGAMENT The functions of periodontal ligament can be broadly categorized into: Supportive or physical Sensory Nutritive Homeostatic/formative/developmental

a. Supportive Function The periodontal ligament fibers provide attachment of the tooth to the bone. It helps to transmit masticatory forces to the bone and acts as a shock absorber against external forces. By providing cushioning effect, the periodontal ligament protects the vessels and nerves from mechanical injury. The periodontal ligament also helps to maintain the proper relationship between gingiva and the tooth. Tooth support and shock absorption is explained on the basis of three theories. Tensional theory: Attributes the major role for the principal fibers of the periodontal ligament in supporting the tooth and transmitting forces to the bone. Viscoelastic system theory: According to this theory displacement of tooth is largely controlled by fluid movements while fibers have only secondary role. Thixotropic theory: Periodontal ligament has rheologic behavior of a

thixotropic gel.

b. Sensory Function The periodontal ligament through its nerve supply provides efficient proprioceptive mechanism. This mechanism is so effective that, it is possible to sense even a grain of sand that is caught between the teeth.

c. Nutritive Function The periodontal ligament transmits blood vessels which provide anabolites and remove catabolites from the cells of ligament, cementum, and alveolar bone. This is of particular importance in case of cementum, as it is avascular and has to depend entirely on the periodontal ligament for nutrition.

d. Homeostatic Function Periodontal ligament has synthetic cells and resorptive cells of various structural components of periodontal ligament. These cells synthesis and resorb the connective tissue components of periodontal ligament cementum and alveolar bone. Therefore they help in remodeling of these components which is very essential for maintaining functional integrity of periodontium. The activity of these cells are well controlled and balanced, therefore various components of periodontium are able to maintain their integrity and relationship to each other. Any disturbance in homeostatic function may disturb the functional efficiency of attachment apparatus of teeth. (Age changes, refer page 323)

Clinical Considerations ▪

Periodontal ligament thickness varies in different teeth, is thicker in functioning teeth than in non-functioning teeth. Abnormal occlusal forces can damage the periodontal ligament resulting in stretching of periodontal ligament and expressed as widening of periodontal ligament space. Abnormal thickening of periodontal ligament, expressed in a radiograph as uniform widening of periodontal ligament involving many teeth is a characteristic finding in a disease called scleroderma.

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Ankylosis is a condition in which the tooth roots become fused directly to the alveolar bone proper and poses difficulty in extraction. Trauma that damages the periodontal ligament may result in ankylosis.

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Tooth which is accidentally knocked out (avulsion) can be reimplanted only if the periodontal ligament cells are viable.

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Untreated gingival inflammation (gingivitis) may progress to involve the supporting structures. This condition is termed as periodontitis, which leads to destruction of periodontal ligament and supporting alveolar bone and mobility of teeth, eventually premature loss of tooth.

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Resorption (on the pressure side) and formation (on tension side) of both bone and PDL forms the basis for orthodontic tooth movement.

9 Alveolar Bone Dr Heera R Introduction Structure of alveolar bone Development Chemical composition Bone histology Bone remodeling Clinical considerations

T

he alveolar process is that part of the jaw bones in which teeth are found. It can also be defined as the part of maxilla or mandible that forms and supports the socket of the teeth in which the teeth are anchored. Alveolar bone is seen as an extension from the body of maxilla and mandible without any distinct boundary between them. But an arbitrary boundary can be drawn at the level of root apices of the teeth which separates the alveolar process and the basal bone. Like bones in other sites, alveolar bone function as a mineralized supporting tissue, giving attachment to muscles, providing frame work for bone marrow and acting as a reservoir of ions, especially calcium. Alveolar bone is dependent on the presence of teeth for its development and maintenance.

STRUCTURE The maxillary alveolar process extends interiorly and mandibular alveolar

process superiorly from their respective jaw bones. They support the teeth within the bony sockets. The alveolar process is composed of two parallel plates of cortical bone, the buccal and lingual or palatal alveolar plates, between which lie the sockets of teeth. The individual sockets are separated from the adjacent ones by plates of bone called interdental septa and the sockets of multi-rooted teeth are separated by inter-radicular septa. The floor of the socket is termed as fundus and its rim, the alveolar crest. The form and depth of each socket depends upon the form and length of the root and the functional demands placed upon the teeth. As an adaptation to its function, the alveolar bone (process) can be divided into two parts, alveolar bone proper and supporting alveolar bone. The parts of the alveolar bone include (Figs 9.1a and b) Alveolar bone proper Supporting alveolar bone • •

Buccal and lingual cortical plates Central spongy bone

Alveolar Bone Proper Alveolar bone proper is a layer of compact bone lining the tooth socket and it varies in thickness from 0.1 to 0.5 mm. It has been given various names.

Fig. 9.1a: Parts of alveolar bone—proximal view

It is referred to as cribriform plate due to the sieve like appearance produced by numerous vascular canals (Volkmann’s canals). These foraminae transmits the vessels from the alveolar bone into the periodontal ligament. Alveolar bone proper is composed of two parts; bundle bone (the portion adjacent to the periodontal ligament) and lamellated bone. Bundle bone is called so, because numerous bundles of Sharpey’s fibers from the periodontal ligament are inserted and cemented into it. The bundle bone contains only fewer number of intrinsic collagen fibers in the matrix which are arranged at right angles to Sharpey’s fibers. The decreased fibril density is associated by increase in ground substance. This increase of cementing substance with high amount of minerals is responsible for its dense opaque appearance in the radiographs. The lamellated portion of the alveolar bone proper shows lamellae which are arranged parallel to the root surface with few Haversian system. Based on radiographic appearance, alveolar bone proper is referred to as lamina dura because of increased radiopacity which makes it appear as a radio-dense layer. Lamina dura, appears as a continuous radiopaque lining of the socket and usually is continuous with buccal and lingual cortical bone at the alveolar crest.

Fig. 9.1b: Alveolar bone from occlusal view

In histological sections, the layer of cribriform plate appears to stain more

intensely. The bundle bone is the most important part of the alveolar process in terms of tooth support and is considered to be a very important structure in the radiographic interpretation of periodontal and periapical pathologies.

Supporting Alveolar Bone Supporting alveolar bone consists of two parts: Cortical plates, which is made up of compact bone and forms an outer (buccal) and inner (lingual) plates of alveolar process. Spongy (trabecular) bone, which fills the area between the cortical plates and the alveolar bone proper. The cortical plate consists of surface layers of lamellar bone supported by compact Haversian system. The outer surfaces of these cortical plates are covered by periosteum and different types of lamellae such as circumferential, concentric and interstitial lamellae are seen. The cortical plates are continuous with the compact bones of the maxilla and mandibular body. The cortical plate and the alveolar bone proper meet at the alveolar crest which is located usually 1.5 to 2 mm below the level of cemento-enamel junction of the tooth. The cortical plates are generally thinner in the maxilla than in mandible. The thickness of the cortical plate of mandible tends to increase from anterior to posterior region with the greatest thickness in the molar region. Generally the lingual cortical plates of both the arches are found to be thicker than the buccal cortical plate. The outer cortical plate shows numerous small openings (Volkmann’s canal) through which vessels and nerves enter the bone. Mandibular alveolar bone has fewer but larger Volkmann’s canals. Spongy bone is the cancellous bone, occupying the space between cortical plates and cribriform plates of the alveolar process. It is composed of irregular interlacing bony trabeculae, each consisting of one or more lamellae with osteocytes enclosed in lacunae. The inter trabecular spaces are generally filled with yellow marrow rich in adipose cells or sometimes red or hemopoietic marrow. Based on the radiographic appearance, the spongiosa is classified into two main types: type I and type II. In type I the interdental and interradicular trabeculae are regular and horizontal in a step ladder type arrangement. This architecture is most often seen in the mandible where trajectorial pattern of

spongy bone is seen. Type II shows irregularly arranged, numerous delicate interdental and inter radicular trabeculae. This arrangement is more common in maxilla and lacks a distinct trajectory pattern. In general the amount of spongiosa is less in mandible than maxilla and the distribution vary depending on the inclination of roots. In the anterior region of both jaws, the supporting bone is usually very thin with less or no spongy bone in between; hence the cortical plate may be fused with the alveolar bone proper in this region. The shape of the crest of the alveolar septa in radiographs depends on the position of adjacent teeth. The interdental and interradicular septa contain perforating canals of Zuckerkandl and Hirschfeld which house the interdental and interradicular arteries, veins, lymph vessels and nerves.

DEVELOPMENT Alveolar bone is formed during fetal growth by intramembraneous ossification. As the developing tooth germs reach the bell stage, developing bone becomes closely related to the tooth germ to form the alveolus. The size of the alveolus is dependent upon the size of the growing tooth germ. Resorption occurs on the inner wall of the alveolus and deposition occurs on the outer wall. The developing teeth therefore come to lie in a trough of bone. Later, the teeth become separated from each other by development of interdental septa. With the onset of root formation interradicular bone develops in multirooted teeth. Much later the primitive mandibular canal is separated from the dental crypts by a horizontal plate of bone.

CHEMICAL COMPOSITION OF BONE Bone is a mineralized connective tissue composed of: By weight 60% inorganic material 25% organic material 15% water

By volume 36% inorganic material 36% organic material 28% water Inorganic component is carbonated, hydroxyapatite in the form of small plates, most of which lodged in the holes and pores of collagen fibrils. Organic matrix of bone is about 90% collagen. Most of the collagen is secreted by osteoblasts and are considered as intrinsic collagen. However, collagen fibers in Sharpey’s fibers are extrinsic collagen formed by fibroblasts. Most dominant collagen in bone is type I collagen, but small amounts of type III and type V collagen are also found. Noncollagenous proteins: 10% of organic content of bone matrix is constituted by a heterogenous group of noncollagenous protein and about 200 of this type of proteins have been identified. Most of these are endogenous, produced by bone cells. Bone also contains exogenous proteins which circulate in the blood and become locked up in the bone matrix themselves. Some of the noncollagenous proteins are proteoglycans, which may regulate the collagen fibril diameters and may play a role in mineralization and glycoproteins like osteocalcin, osteonectin, osteopontin, bones sialoprotein, thrombopondin and fibronectin. Osteonectin with its ability to bind calcium may have role in mineralization. Osteopontin, ostonectin and fibronectin help in attachment of cells to the bone matrix. Osteocalcin is a calcium binding protein synthesized only by osteoblasts and odontoblasts.

BONE HISTOLOGY All bones have a dense outer sheet of compact bone and a central medullary cavity. The medullary cavity is filled with red or yellow bone marrow. The marrow is interrupted by a network of bony trabeculae and is known as trabecular, cancellous or spongy bone. In adult bone, histologically, both the compact and the trabecular bone consists of lamellae. Three types of lamellae have been recognized: circumferential, concentric and interstitial lamellae (Fig. 9.2).

Circumferential lamellae form the outer perimeter of the bone. The bulk of compact bone is made up of concentric lamellae. The interstitial lamellae fill the space between adjacent concentric lamellae. The interstitial lamellae are considered as the fragments of previous concentric lamellae as a result of remodeling and they contain old remnants of circumferential lamellae as well as osteonal remnants.

Fig. 9.2: Longitudinal section of bone

The concentric lamellae (Fig. 9.3) are arranged around a central vascular canal, the Haversian canal. The Haversian canal contains capillaries and is lined by a single layer of bone cells. The Haversian canal together with the concentric lamellae is known as osteon or Haversian system which is the basic structural and functional unit of bone. There may be 9–20 concentric lamellae with in each Haversian system. The collagen fibers with in each lamellae spiral along the length of the lamellae but have different orientation to those in adjacent lamellae. The change in orientation can be demonstrated by viewing the bone in polarized light. The longitudinally running Haversian canals are connected by horizontal interconnecting canals known as Volkmann’s canal which also contain blood vessels.

In spongy bone the lamellae are apposed to each other to form trabeculae. The trabeculae are about 50 microns thick and aligned along the lines of stress to withstand the force applied to the bone. The trabeculae usually do not have Haversian canal and they get their nutrients from the marrow spaces. In young bone the marrow is red and hematopoietic and contains stem cells of both mesenchymal type and blood cell lineage. In old bone, the marrow is yellow due to accumulation of fat cells and hence lose hematopoietic potential.

Fig. 9.3: Portion of osteon with concentric lamellae and osteocytes

Surrounding the outer aspect of every compact bone is the periosteum which contains two layers, an outer fibrous layer of dense irregular connective tissue and inner cellular layer of bone cells and their precursors. The periosteum has rich blood supply. The inner surface of compact bone and cancellous bone are covered by cellular endosteum. The periosteal surface is more active in bone formation than the endosteal surface.

Cells of Bone Different cell types are responsible for formation, resorption and maintenance of bone. Two cell lineages are present in bone: Osteogenic cells derived from mesenchymal (or ectomesenchymal) stem cells, including osteoprogenitors, preosteoblasts, osteoblasts and osteocytes which form and maintain the bone. Osteoclasts, which resorb bone are derived from monocytes and

macrophages and form part of hematopoietic system.

Osteoblasts Osteoblasts are mononucleated cells of mesenchymal origin and seen as a layer of cuboidal cells on the surface of bone where bone formation is taking place. The cells are polarized with a prominent, round nucleus located at the basal end. The active osteoblasts exhibit basophilic cytoplasm due to the presence of large amount of RNA content. The cytoplasm contains rich synthetic organelles such as, rough endoplasmic reticulum, numerous mitochondria, Golgi complexes and vesicles, etc. The cells contact one another by means of adherence and gap junction. These cells exhibit high levels of alkaline phosphatase on the outer surface of their plasma membrane. Osteoblasts are the synthetic cells of the bone which are involved in secretion of the organic matrix of bone, i.e. osteoid and also help in mineralization of uncalcified matrix. In addition the osteoblasts have a controlling influence in activating osteoclasts. They contain receptors for parathyroid hormone and regulate osteoclastic response to this hormone. They also participate in matrix degradation though the production of hydrolytic enzyme and interleukin-6. When the bone surfaces are neither in the formative nor resorptive phase, the layer of osteoblasts lining the bone surface flatten and these cells are called bone lining cells. These cells cover most surfaces in the adult skeleton and contain only few cell organelles with little sign of synthetic activity. They retain their gap junctions with the osteocytes. They are regarded as post proliferative osteoblasts and protect the bone from resorptive activity of osteoclasts. They can be reactivated to form osteoblasts.

Osteocytes During bone formation, some osteoblasts become entrapped with in the matrix of the bone; these entrapped cells are called osteocytes (Fig. 9.4a). The number of osteoblasts, that become osteocytes depend, on the rate of bone formation. The more rapid the formation the more osteocytes are present per unit volume. So the embryonic (woven) bone and repair bone have more osteocytes than does lamellar bone. Usually about 15% of osteoblasts become embedded in the organic matrix as osteocytes. Approximately 25,000 osteocytes can be seen per cubic mm of bone. The

space in the matrix, occupied by an osteocyte is called the lacuna. Many fine canals called canaliculae radiate from the lacunae in all directions which contain cell processes of the osteocytes (Fig. 9.3). Through this canaliculae, osteocytes maintain contact with adjacent osteocytes, osteoblasts and lining cells. As a result of this inter connections the osteocytes are regarded as the main mechanoreceptors of bone. Osteocytes are thought to be capable of taking part in bone resorption which is referred to as osteocytic osteolysis. At the ultra structural level, the appearance of osteocytes vary according to its position in relation to the surface layer. Osteocytes which are newly incorporated into the bone matrix contain larger amount of organelle like osteoblasts. With continued bone formation, the osteocytes become more deeply situated and the number of organelles shows reduction, reflecting decreased cellular activity.

Osteoclasts Osteoclasts are multinucleated giant cells responsible for bone resorption; they are derived from hematopoietic cells of monocyte or macrophage lineage by fusion of mononuclear precursors. They can be easily differentiated under light microscope because of their large size and multiple nuclei. The cells show considerable variation in size and shape. The cell body is irregularly oval and may show many branching processes. Usually osteoclasts contain 10–20 nuclei and the size is about 100 microns in diameter (Fig. 9.4a). Tissue culture studies indicate that osteoclasts are highly motile and is evident from the ‘snail tracks’ on the bone surface. The osteoclasts are recruited only when required since there is no significant reservoir of inactive osteoclasts. The life span of osteoclasts is thought to be about 10–14 days. Osteoclasts are characterized cytochemically by possessing tartrate resistant acid phosphatase within its cytoplasmic vesicles and vacuoles which, distinguishes it from other multinucleated giant cells. Typically osteoclasts are found occupying hollowed out depressions on the resorbing surface known as Howship’s lacunae that they have created. Scanning electron microscopy shows that the Howship’s lacunae are shallow troughs with irregular shape which reflect activity and mobility of osteoclasts during active resorption. Under electron microscopy, osteoclasts exhibit the following morphologic characteristics (Fig. 9.4b). The cell membrane of the osteoclast that lies

adjacent to resorbing bone surface is thrown into a number of deep folds that form the ruffled border. It is composed of many tightly packed microvilli. This ruffled border provides a large surface area for the resorptive process. The cytoplasm adjacent to the ruffled border is devoid of cell organelle but contains numerous contractile actin microfilaments and this zone is referred to as clear zone. At the periphery of the ruffled border, the plasma membrane is apposed closely to the bone surface. This sealing zone serve to attach the cell very closely to the surface of bone and create a microenvironment in which resorption can take place without diffusion of the hydrolytic enzymes produced by the cell into adjacent tissue.

Fig. 9.4a: Cells of bone

Fig. 9.4b: Osteoclast in Howship’s lacunae

There are several mechanisms by which the osteoclasts bind to bone surface. One of the mechanism is concentration of osteopontin on bone surface which may facilitate osteoclasts adhesion at the sealing zone due to the presence of cell membrane protein known as integrins (especially a2 b3 integrin). In addition to multiple nuclei osteoclast also contain various cytoplasmic organelles such as endoplasmic reticulum, Golgi complexes, many mitochondria, numerous vesicles of different sizes and types, some containing lysosomal enzymes, etc. distributed through out the cytoplasm except near the ruffled border.

Bone Resorption Once the osteoclast has been activated against the bone surface, bone resorption occurs in two stages. Initially, the mineral phase is removed and later the organic matrix. A sealed acidic microenvironment is created in the resorption lacunae which dissolves the mineral crystals in bone and exposes the organic matrix. To provide the low pH, the osteoclast secretes protons across the ruffled border by means of ATP dependent protein pump that pumps H+ ions to sealed compartment. The H+ ions are generated by the action of enzyme carbonic anhydrase II on the carbon dioxide and water to form carbonic acid. The organic matrix is then degraded by the action of enzymes like collagenase, lysosomal acid proteases (cathepsin B1). The inorganic and organic bone degradation products are taken inside the

osteoclasts by endocytosis at the ruffled border. These endocytosed products are translocated in transport vesicles and released extracellularly along the membrane opposite the ruffled border.

REMODELING OF BONE The process by which the over all size and shape of bone is established is referred to as bone remodeling and extends from embryonic bone development to the preadult period of human growth. During this phase bone is formed on the periosteal surface. Bone is laid down rhythmically; there are periods of active deposition and quiescence which result in formation of regular parallel incremental lines, called resting lines. The resting lines are formed in periods of relative quiescence (rest period). Simultaneous with bone deposition bone is resorbed along the endosteal surface at focal points. During the growing phase of a child, the amount of bone deposition exceeds that of resorption resulting in increase in bone mass. During adult phase, the amount of bone deposition is equivalent to that of bone resorption and bone mass is more or less constant. In old age and in diseases like osteoporosis bone deposition is generally less when compared to resorption. Therefore there is an overall decrease in bone mass. The replacement of old bone, by new bone is called remodeling or bone turn over. In rapidly growing children bone turn over is about 30–100%. The rate of remodeling decreases in adults. This bone turn over occurs in discrete focal areas involving groups of cells called bone remodeling units. The rate of cortical bone turn over is approximately 5% per year, where as trabecular bone and endosteal surface of cortical bone is 15% per year. As the bone deposition continue at the periphery by deposition of circumferential lamellae, the internal reconstruction of Haversian system take place to meet the functional and nutritional demands. During this process, osteoclasts differentiate in the peripheral Haversian canals to cause resorption of concentric lamellae. The leading edge of resorption is always towards the periphery and is called cutting cone or resorption tunnel. Initially, the resorbed area gets filled with loose connective tissue followed by migration of mononucleated cells onto the area. As these cells differentiate into osteoblasts, they produce a coating, a thin layer of glycoprotein (mainly bone sialoprotein and osteopontin) that, acts as a cohesive, mineralized layer

between the old bone and the new bone to be secreted. On top of this osteoblasts begin to lay down new bone matrix mineralizing it from outside in. The area of active formation is termed as the filling cone. As bone formation proceeds some osteoblasts become osteocytes. The old and new bones are separated by a distinct curved hematoxiphilic line with its convexity facing the old bone. These lines are called reversal lines and are indicators of continuous remodeling of bone. A considerable amount of internal remodeling occurs within the bone by means of resorption and deposition. The repeated deposition and removal of bone tissue accommodates the growth of a bone without losing function or its relationship to neighboring structures during remodeling phase. The remodeling is enabled by the coordinated action of osteoclasts and osteoblasts. Bone metabolism is directly under the control of various hormones (for details refer Chapter 43). (Age changes, refer Chapter 46)

Clinical Considerations •

Alveolar bone being part of jaw bone bearing teeth, existence of alveolar bone is significantly dependent on teeth. Alveolar bone may not be well developed in disease conditions where there is complete or partial absence of teeth. Similarly alveolar bone undergo resorption once the teeth are lost.

•

Alveolar bone undergoes continuous remodeling to maintain functional integrity. The response of alveolar bone to applied force and remodeling capacity forms the basis of orthodontic tooth movement

•

Alveolar bone undergoes destruction or resorption in cases of local conditions such as periodontitis or due to pressure from cysts or tumours. Alveolar bone loss may eventually results in mobility of teeth.

•

Alveolar bone or the basal bone can be common site of involvement of various bone disorders such as fibrous dysplasia, Paget’s disease, etc.

•

Radiographic examination of status of lamina dura and periapical bone tissue is a routine procedure carried out in the diagnosis of periapical diseases.

10 Oral Mucosa

Introduction Functions of oral mucosa Classification of oral mucosa Structure of oral mucosa Structural variations Clinical considerations

O

ral mucosa is the moist lining of the oral cavity. The mucous lining of oral cavity shares some features with skin as well as the mucosa lining the gastrointestinal tract.

Functions of Oral Mucosa Protection: Protection of underlying structures against mechanical trauma that may result from heavy masticatory stress or from hard food. Defense: Intact mucosa acts as a protective barrier against invasion of microorganisms and various bacterial products and toxins. Any breach in the epithelium permits the entry of microorganism in to the mucosal tissue and may initiate related disease process. Furthermore oral mucosa contributes to defense function due to the presence of Langerhans cells and lymphocytes, which are part of defense system of the body. These cells identify any foreign material entering the mucosa and present to the immune system and ensure the removal of the same. Sensory: Oral mucosa has a special sensory function, i.e. taste perception due

to the presence of taste buds. Oral mucosa also has receptors that respond to pain, temperature and touch. Secretory function: The presence of minor salivary glands within the mucosa aids in secretory function. Thermal regulation: Heat regulation of the body is one of the functions of oral mucosa which is mainly seen in animals especially dogs.

Classification of Oral Mucosa Based on function •

• •

Lining/reflecting mucosa: Lines the inner aspect of lips and cheeks, soft palate, floor of mouth, ventral aspect of tongue, alveolar mucosa, vestibule, faucial pillars, etc. Masticatory mucosa: Lines the hard palate and gingiva. This mucosa is subjected to considerable friction during mastication. Specialized mucosa: Lines dorsal aspect of tongue and this mucosa shares the characteristics of both masticatory mucosa and gustatory mucosa because of presence of papillae and taste buds.

Based on type of epithelium covering the mucosa • •

Keratinized mucosa: It is found in the region of hard palate, gingiva, vermilion border of lip and some papillae of tongue. Nonkeratinized mucosa: It is found in the region of lining mucosa and certain areas lining the dorsal aspect of tongue and parts of gingiva.

STRUCTURE OF ORAL MUCOSA Oral mucosa resembles skin in its structure and is composed of two components: epithelium and connective tissue (Fig. 10.1). The interface between epithelium and connective tissue is not flat, rather is irregular. Epithelium has many irregular projections that interdigitate with similar projections from connective tissue. The epithelial projections are called rete ridges, rete pegs or epithelial ridges and connective tissue projections are called connective tissue papillae. The epithelium and the connective tissue

are separated by a basement membrane of 1–2 microns thickness. The irregular epithelial-connective tissue junction increases the surface area of contact between these two components which helps in better adhesion, and transport of nutrients and other materials between the two. This also helps to disperse the forces applied on epithelium, over a great area of connective tissue. The number and configuration of rete ridges vary in different regions of oral mucosa. Masticatory mucosa has relatively long rete ridges compared to lining mucosa. In addition, more number of rete ridges in masticatory mucosa ensures stronger adhesion between epithelium and connective tissue.

Fig. 10.1: The structure of oral mucosa

CONNECTIVE TISSUE COMPONENT OF ORAL MUCOSA Connective tissue of oral mucosa is called as lamina propria. The loose connective tissue below the lamina propria is continuous with it and is called submucosa. Lamina propria, seen subjacent to the epithelium is arbitrarily divided into papillary portion occupying the region of papillae and reticular portion found beneath the papillary portion. Papillary portion contains loose connective tissue with many capillary loops and nerves. A few nerve fibers from here also enter into the epithelium and remain as free nerve endings, perceiving sensations such as cold, heat, touch, pain and taste. In reticular region connective tissue is less cellular and denser with fibers having more parallel arrangement to the epithelial surface. Lamina propria shows all the normal connective tissue components which include cells such as fibroblasts and defense cells, extracellular components including collagen fibers, elastic fibers, oxytalan fibers and ground substance. Reticular portion is always present, but papillary portion can vary depending on the presence and absence of rete ridges. Submucosa is the deeper connective tissue seen beneath the mucosa. The submucosa comprises of loose connective tissue and the texture and density of this determines the stretchability of the mucosa. In addition to the normal connective tissue components submucosa contains large blood vessels and nerves, minor salivary glands, fat cells, etc. Submucosa is divided into compartments by bundles of vertically arranged collagen fibers extending from the lamina propria to fascia of the muscle or periosteum. These bands of collagen along with elastic fibers attach the mucosa to the underlying structures and therefore prevent the folding of mucosa which might otherwise become entrapped between the teeth. Submucosa is absent in gingiva and some regions of hard palate. In these regions lamina propria is directly bound to periosteum of underlying bone. This type of attachment is called mucoperiosteal attachment and this makes mucosa tough, immovable and tightly bound to the bone.

ORAL EPITHELIUM The covering epithelium of the oral mucous membrane is stratified squamous variety. The cells are tightly bound to each other and arranged to form different layers or strata. The integrity of oral epithelium is maintained by a system of continuous renewal mechanism. Old cells are continuously lost from the surface by a process termed as desquamation and are replaced by new cells formed by the process of mitotic division. Stem cell population in the basal cell compartment of the epithelium undergo mitotic division giving rise to two daughter cells. One of the daughter cell remains in the progenitor compartment while the other cell enters in to the maturing compartment. The cells entering into the maturing compartment undergo further differentiation. As cells leave the basal layer and enter into differentiation, they become larger and begin to flatten and accumulate cytoplasmic protein filaments, representing the cytokeratins. Keratins represent 30 different proteins of differing molecular weights. All stratified oral epithelia possess keratins 5 and 14 in the basal cells, but changes as the cells undergo further differentiation. Orthokeratinized oral epithelium, such as the palate, contains keratins 1 and 10, whereas gingiva and parakeratinized palatal epithelium contains keratins 1 and 10 or keratins 4 and 13. Nonkeratinized epithelium, contains keratins 4 and 13. These cells passes through different layers till it reaches the surface layer from where these are desquamated. The maturation of oral epithelium follows two patterns, keratinization or nonkeratinization. Different types of maturation pattern are observed in different regions of oral mucosa. The cells in the progenitor compartment undergo mitotic division to maintain the structural integrity. The rate of cell proliferation vary in different types of epitheliam, but, in general, the rate is highest for cells in the thin nonkeratinized regions, such as floor of mouth and underside of tongue, than for the thicker keratinized regions, such as palate and gingiva. The mitotic index also correlated significantly with epithelial thickness, with the thicker regions showing a higher rate of proliferation. The lowest value is noted in skin. The time it takes for a cell to divide and pass through the entire epithelium till it desquamates is termed as turnover time of the epithelium and varies in different epithelia.Turnover times range from a median value of 34 days for

epidermis to 4 days for the small intestine with the values for oral and esophageal epithelium falling between. Nonkeratinized buccal epithelium turns over faster than keratinized gingival epithelium. Regional differences in the turn over time is reported as follows: Floor of mouth 20 days, buccal and labial mucosa 14 days, gingiva 40 days, attached gingiva 10 days, junctional epithelium 4 to 6 days, and hard palate 24 days.

HISTOLOGICAL STRUCTURE OF ORAL EPITHELIUM Microscopically oral epithelium shows different layers which vary in keratinized and nonkeratinized epithelium. Majority of cells of both keratinized and nonkeratinized epithelium have the capacity to produce keratin and therefore called as keratinocytes. These cells show some common features unique to epithelial cells which include presence of keratin tonofilaments as a component of cytoskeleton and intercellular attachment in the form of desmosomes.

Keratinized Epithelium Light microscopic structure: Four different layers are seen in keratinized oral epithelium (Figs 10.2a and b). Stratum basale/basal cell layer: This layer is composed of single layer of cuboidal or columnar cells that rest on the basement membrane. Basal and parabasal cells have the capacity to undergo mitotic division. So these cell layers are also called as proliferative or germinative layer (stratum germinativum). Basal cells have basophilic cytoplasm and centrally placed nucleus which is hyper chromatic and relatively larger, occupying 1/3rd of cytoplasm. The nucleus is arranged perpendicular to basement membrane. Stratum spinosum/prickle cell layer: It is seen above basal layer and composed of several rows of polyhedral cells. As the cells pass from basal layer to prickle cell layer, there is considerable decrease in basophilia, making the boundary between these layers distinct. Cells are larger than basal cells and have centrally placed round or ovoid nucleus. The nuclear cytoplasmic ratio of spinous cells is 1:6. This layer is also called prickle cell

layer because in histological sections, cells have a spiny or prickly appearance. While tissue processing, cells shrink away from each other remaining in contact only in the areas of intercellular attachment, resulting in a prickly appearance. In stratum spinosum as the cell mature and move superficially they increase in size and become more flattened with flattened nucleus in a plane parallel to the surface. Stratum granulosum/granular cell layer: This layer is composed of few layers of flattened cells seen immediately above stratum spinosum. The cytoplasm of the cells in this layer is filled with basophilic granules called keratohyaline granules and hence the name stratum granulosum. The nucleus of these cells are flattened with long axis parallel to the outer surface of epithelium.

Figs 10.2a and b: Keratinized mucosa

Stratum corneum/cornified layer: This is the most superficial layer found in keratinized epithelium and is composed of keratin squames which are larger and flatter than the cells of stratum granulosum. This layer appears as eosinophilic amorphous layer in histologic sections. As the cells reach the cornified layer nucleus undergoes degeneration. If the nucleus is completely absent in surface layer, the pattern of maturation is called as orthokeratinization. If pyknotic nucleus is retained in all or some squames it

is called as parakeratinization. Parakeratinized epithelium is mainly seen in gingiva. In parakeratinized epithelium the keratohyaline granules in stratum granulosum is less prominent.

Ultrastructure or Electron Microscopic Structure Basal cells Basal cells are the least differentiated cells of the epithelium. These cells contain nucleus occupying 1/3rd of the cells with evenly distributed chromatin and 2–3 nucleoli. Basal cells are involved in protein synthesis and therefore cytoplasm has rich cellular organelles like rough endoplasmic reticulum, mitochondria, Golgi complex, few lysosomes, etc. These cells synthesize the proteins of basement membrane and also proteins which form intermediate filaments of basal cells. Basal cell layer has two populations of cells. One group of cells is serrated with protoplasmic processes at basal region and is heavily packed with tonofilaments. These cells are adapted for attachment. Second population of cells are the stem cells which undergo division and provide cells for maturing compartment. The basal cells are attached to each other by desmosomes and to the basement membrane by hemidesmosomes. These cells also contain tonofilaments like any other epithelial cell but are few in number.

Prickle cells Overall size of the cell and nucleus increases as it passes to spinous cell layer (Fig. 10.3). Nucleus has evenly distributed chromatin with 2–3 nucleoli. Cytoplasm is rich in organelles for protein synthesis. The proteins synthesized by these cells are primarily the fibrilar proteins, known as cytokeratin and this indicate these cells are in the process of differentiation. The concentration of the tonofilaments increases and gets arranged to form bundles. The cells are attached to each other by desmosomes. The number of desmosomes and width of intercellular space is more in keratinized epithelium. The size of desmosome is wider in prickle cell layer than basal cell layer. As the cell passes to upper prickle layers the desmosomes become smaller.

Fig. 10.3: Ultrastructure of cells of keratinized epithelium

Cells in the upper part of prickle cell layer show new cytoplasmic organelles called Odland bodies. These are also known as membrane coating granules, cytoplasmic lamellated body, keratinosomes, microgranules or cementosomes. (Odland bodies are also present in nonkeratinized epithelium but are structurally different.) In keratinized epithelium Odland bodies appear as ovoid membrane bound organelles of 0.25 microns length, containing a series of parallel internal lamellae consisting of alternate electron lucent and electron dense bands. These organelles may be derived from Golgi bodies. The size of the Odland bodies do not increase but the density increases as the cell passes to more superficial layer and also these structures move closer of superficial cell membrane. Granular layer cells: In this layer the size of the cells still increases. The cells are flatter with long axis parallel to the epithelial surface. Nucleus is also flattened and shows pyknotic changes. The cells still retain the capacity for protein synthesis, only to a lesser extent. This is indicated by decrease in

number of cytoplasmic organelles. Although the cells show a decrease in cytoplasmic components, the amount of tonofilaments is found to be more. The cell surfaces become more regular and closely approximated with each other. Odland bodies are also present in these cells where they fuse with the superficial cell membrane and discharge the contents into the intercellular spaces. This discharged material provide lipid rich permeability barrier, at the junction of stratum granulosum and stratum corneum, that limits the movements of substances through intercellular spaces. Desmosomes maintain their structure in this layer while the intercellular contact layer of desmosomes becomes more condensed. Cytoplasm of stratum granulosum cells also shows keratohyaline granules. In keratinized epithelium these are variable in size ranging from 0.1–1.5 microns. Their size and number increases as the cell moves through the granular layer. Keratohyaline granules are usually angular or irregular and they are usually associated with ribosomes suggesting they are synthesized by ribosomes. Keratohyaline granules contain sulphur rich proteins fillagrin and loricrin which provide an embedding matrix for the tonofilaments and therefore help in aggregating the tonofilaments. They also contain a protein involucrin which provide constituents for the cell membrane thickening and makes it resistant to chemical solvents. Stratum corneum: Ultrastructurally stratum corneum is composed of cells resembling hexagonal discs called squames (Fig. 10.3). Large amount of bundles of keratin tonofilaments are found to be embedded in a matrix that is contained in a thick envelope. Keratin is a tough insoluble protein which more or less completely fills the interior of shrunken cells. Cellular organelles are almost completely lost and these cells do not produce protein. The nucleus may be completely lost in case of orthokeratinization or remain pyknotic in parakeratinization. The cell membrane is thickened. Desmosomes can be still recognized but they become less distinct. As the cell passes to the superficial layer, desmosomes tend to degenerate resulting in desquamation of cells.

Desquamation The physiological process of shedding off of the superficial cells of epithelium is called as desquamation. Mechanism of desquamation is not fully understood. The possible mechanisms include:

Release of hydrolytic enzymes from membrane coating granules causing destruction of desmosomes which leads to desquamation. Intercellular junctions have a physiological life span after which there will be rapid breakdown, leading to desquamation.

Nonkeratinized Epithelium Light microscopically three different layers are seen in nonkeratinized oral epithelium (Fig. 10.4) Stratum basale/basal cell layer: This layer is similar to that of basal layer of keratinized epithelium and is composed of single layer of cuboidal or columnar cells immediately adjacent to basement membrane. Basal cells have centrally placed nucleus which is hyper chromatic and relatively larger and occupies 1/3rd of cytoplasm. The cytoplasm of these cells shows significant basophilia due to high RNA content. Stratum intermedium: This layer is composed of several rows of polyhedral cells located above basal layer. The cytoplasm of these cells takes up eosinophilic stain and therefore this layer can be easily differentiated from basal cells exhibiting basophilic cytoplasm. Cells are larger than basal cells and have centrally placed round nucleus. The nuclear cytoplasmic ratio of spinous cells is 1:6. In contrast to stratum spinosum of keratinized epithelium, the cells of this layer are closely apposed to each other and prickly appearance is not distinct. As in stratum spinosum, these cells increase in size when they mature and move superficially and also become more flattened with flattened nucleus in a plane parallel to the outer surface. Stratum superficiale: This is the most superficial layer found in nonkeratinized epithelium and is composed of few layers of flattened cells. The nucleus of these cells are flattened with long axis parallel to the outer surface of epithelium. These cells ultimately undergo desquamation.

Ultrastructure or Electron Microscopic Structure Basal cells: Ultrastructurally basal cells of nonkeratinized epithelium resemble the basal cells of keratinized epithelium in all respects (Fig. 10.5). These are the least differentiated cells of the epithelium. These cells contain nucleus occupying 1/3rd of the cells with evenly distributed chromatin and 2–

3 nucleoli. These cells are involved in protein synthesis and therefore cytoplasm has rich cellular organelles like rough endoplasmic reticulum, mitochondria, Golgi complex, few lysosomes, etc.

Fig. 10.4: Nonkeratinized mucosa

Fig. 10.5: Ultrastructure of cells of nonkeratinized epithelium

As in keratinized epithelium, basal cell layer has two populations of cells, serrated cells with protoplasmic processes at basal region and cytoplasm

heavily packed with tonofilaments, which are adapted for attachment. Second population of cells are the stem cells which undergo division and provide cells for maturing compartment. The basal cells are attached to each other by desmosomes and to the basement membrane by hemidesmosomes. These cells also contain tonofilaments like any other epithelial cell but are few in number.

Stratum Intermedium Overall size of the cell and nucleus increases as it passes to stratum intermedium (Fig. 10.5). Relative increase in size of the cell and nucleus is more in nonkeratinized epithelium than in keratinized epithelium. Nucleus has evenly distributed chromatin with 2–3 nucleoli. Cytoplasm is rich in organelles for protein synthesis. The concentration of tonofilaments is more than that in basal cells but in contrast to the cells of stratum spinosum, tonofilaments are found in unbundled form. The cells are attached to each other by desmosomes. The number of desmosomes and width of intercellular space is less in nonkeratinized epithelium. The size of desmosome is wider in stratum intermedium than basal cell layer but as the cells move more superficially the number of desmosomes becomes lesser. Superficial cells of stratum intermedium show cytoplasmic organelles called Odland bodies which are structurally different from that of the keratinized epithelium. In nonkeratinized epithelium Odland bodies appear as spherical membrane bound organelles of 0.2 microns diameter. These structures have an electron dense core from which delicate radiating strands are observed. The size of the Odland bodies do not increase but their density increases as the cell passes to more superficial layers and also these structures move closer to superficial cell membrane.

Stratum Superficiale In this layer, there is further increase in the size of the cells. The cells are flatter with long axis parallel to the epithelial surface (Fig. 10.5). Nucleus is also flattened and shows pyknotic changes. The cytoplasmic organelles decrease in number indicating a lesser capacity to produce protein. Although the cells show a decrease in cytoplasmic components, the amount of tonofilaments is found to be more but in unbundled form. The cell surfaces

become more regular and closely approximated with each other. Desmosomes decrease in size and number and intercellular space becomes wider and irregular and maintain their structure in this layer while the intercellular contact layer of desmosomes become more condensed.

Fig. 10.6: Junctions of epithelium

In contrast to superficial layer of keratinized epithelium, the superficial cells of nonkeratinized epithelium show nucleus and various cytoplasmic organelles. These cells undergo desquamation.

Intercellular Junctions Intercellular junctions are cell junctions that bind the cells to one another and allow intercellular communication. Three different types of junctions may be seen between the epithelial cells, which include desmosomes, tight junctions and gap junctions (Fig. 10.6).

Desmosomes Desmosomes are the most characteristic and most numerous type of intercellular junctions seen in epithelial cells. Ultrastructurally desmosomes (Fig. 10.7) are present as a circular or ovoid area of 0.2–0.5 microns in which plasma membranes of adjacent cells remain in juxtaposition to each other with a distance of 25–30 nm. This space between the plasma membrane contains an electron dense lamina called intercellular contact layer. This layer is composed of protein particles of 5 nm diameter which are arranged in a row. On the cytoplasmic side, plasma membrane of each of the adjoining cells

show a thickening called attachment plaque and this structure contain the proteins desmoplakin, plakoglobin and plakophilin. The tonofilaments present in cytoplasm of each cell run into attachment plaque and loop out again. The tonofilaments are not attached to the plasma membrane. This arrangement of tonofilaments helps to dissipate physical forces from attachment site throughout the cell. There are a separate group of smaller filaments containing protein cadherins (desmogleins and desmocollin) attaches the tonofilaments to plasma membrane, penetrate the cell membrane. These filaments are called as traversing filaments and they traverse the intercellular region to extend into the intercellular contact layer. The traversing filaments from both cells come and attach to the intercellular contact layer retaining the attachment between the cells.

Fig. 10.7: Desmosomes

Differences between lining and masticatory mucosa Masticatory mucosa/keratinizcd mucosa Lining mucosa/nonkeratinized mucosa Tough and tightly bound to underlying• structures

Loosely attached to the underlying structures

•

Stretchable to adapt to the contraction and relaxation of underlying muscles.

Non stretchable

Relatively low rate of mitotic cell division •

Relatively higher rate of mitotic cell division

Turnover rate is slow

•

Turnover rate is relatively faster

Lamina propria is dense

•

Lamina dense

propria

is

less

Submucosa may or may not be present.• Some regions show mucoperiosteal attachment.

Distinct submucosa present which vary thickness

Epithelium connective tissue interface is• very irregular with long and narrow rete ridges, interdigitating with connective tissue papillae.

Rete ridges are short and irregular

Covering epithelium is keratinized stratified• squamous epithelium

Covering epithelium is nonkeratinized stratified squamous epithelium

Epithelial thickness is less

Epithelium is thicker

•

Four distinct layers are seen in epithelium:• stratum basale, stratum spinosum, stratum granulosum, stratum corneum

is in

Only three layers are seen: Stratum basale, stratum intermedium, stratum superficiale

Differences in various layers of epithelium Stratum basale Hemidesmosomes anchoring it to the• basal lamina is more in number and larger Stratum spinosum

Hemi desmosomes are fewer and smaller

Cells are polygonal with prickly• appearance

Cells are roughly rounded and prickly appearance is not distinct

Number of desmosomes are more

•

Number of desmosomes is lesser than that of masticatory mucosa

Percentage of cell membrane• occupied by desmosomes is more

Percentage of cell membrane occupied by desmosomes is less

Intercellular prominent

Intercellular prominent

space

are

more•

space

are

less

Cytokeratin present are 1, 6, 10, 16 •

Cytokeratin present are 3, 14, 19

Relative size of the cells in this layer• is less

Cells are larger

Adjacent cell surfaces closely applied

less•

Adjacent cell surfaces are more closely applied

Tonofilaments are in bundles and• more organized

Tonofilaments are in unbundled form and less organized

Odland bodies are ovoid with• alternating electron dense and lucent areas

Odland bodies are round in shape with central electron dense core and radiating lines

Stratum granulosum with distinct keratohyaline granules are seen

No layer called stratum granulosum is found, no keratohyaline granules. Superficial layer is composed of flattened cells Surface cells contain nucleus and cytoplasmic organelles

are

Superficial layer is composed of keratin flakes Superficial cells do not have nucleus or cytoplasmic organelles

Tight Junctions Tight junctions are characterized by fusion between adjacent plasma

membranes without any intervening space and act as diffusion barriers.

Gap Junctions The junctions that allow cytoplasmic compartments of adjacent cells to communicate are special adaptation of mucous membrane channels and are called gap junctions. In gap junctions, adjacent cell membranes run parallel to each other with a gap of 2–5 microns. In these areas some channels are present that allow communication between the cells.

Hemidesmosomes These are specific type of attachments seen between basal cells and basement membrane. These attachments are called as hemidesmosomes because the structure is equivalent to half ot a desmosome. The hemidesmosomes have one attachment plaque in the basal plasma membrane of basal cells. The traversing filaments extending from this attachment plaque enter into the basal lamina to provide attachment between epithelium and connective tissue (Fig. 10.8).

Fig. 10.8: Ultrastructure of basal lamina

Basement Membrane and Basal Lamina Complex The epithelium-connective tissue interface is irregular and the epithelium is separated from connective tissue by a distinct homogeneous structureless layer of 1–2 microns thickness called basement membrane. The basement membrane acts as a barrier which controls the movement of various materials from epithelium to connective tissue and vice versa. The term basement membrane is given based on light microscopic structure and it does not appear distinct in hematoxylin and eosin stained sections. Special stains like periodic acid-Schiff stain (PAS) or silver stains can be used to demonstrate it. Electron microscopically, the basement membrane is composed of two distinct layers: lamina lucida and lamina densa (Fig. 10.8). Therefore based on electron microscopic structure the term basal lamina is used to denote this structure.

The layer of basal lamina adjacent to the basal cell is around 45 nm thick and appears electron lucent. This layer is called as lamina lucida. Lamina lucida contain laminin and bullous pemphigoid antigen. Beneath the lamina lucida an electron dense layer of 55 nm thickness is seen which is termed as lamina densa. Type IV collagen fibers are found in lamina densa which show a chicken-wire pattern. This layer also contains proteoglycans such as heparan sulfate and chondroitin sulfate. The proteoglycans control the passage of ions across basement membrane. Smaller diameter collagen fibers (type VII) are found, beneath lamina densa, forming loops with both ends attached to the lamina densa. These fibers are called anchoring fibrils. Collagen fibers from connective tissue pass through these and loop around to form a strong attachment between epithelium and connective tissue. Both layers of basal lamina and the anchoring fibrils are together called as basal lamina complex. The basal lamina is of epithelial origin while the anchoring fibrils are of connective tissue origin.

Functions of Basement Membrane Structural attachment, i.e. providing attachment between epithelium and connective tissue. Compartmentalization: Basement membrane isolates the epithelium from connective tissue. Filtration: Transport of materials to and from the connective tissue is regulated by basement membrane. Tissue scaffolding: Basement membrane act as a scaffold during regeneration of epithelium. Polarity induction: Epithelial cells gets organized into normal layered arrangement only if they are supported by a basement membrane.

Nonkeratinocytes In the oral epithelium, both keratinized and nonkeratinized, 90% of the cells are keratinocytes which have the capability of producing keratin. Another 10% of the cells belong to a group called nonkeratinocytes. They are melanocytes, Langerhans’ cells, Merkel cells and inflammatory cells. These cells do not produce keratin and except for Merkel cells do not possess

desmosomal junctions or tonofilaments. Melanocytes: Melanocytes are dendritic cells scattered among the basal cells of epithelium and these are the melanin producing cells. The origin of these cells is from neural crest cells which migrate to ectoderm by 8–11 weeks of intrauterine life and have the capacity to replicate throughout postnatal life, though at a much slower rate than keratinocytes. These cells have a cell body containing the nucleus located at basal region and multiple long processes extending between the keratinocytes of stratum spinosum. The melanocytes neither contain tonofilaments nor possess desmosomal attachment. Because of absence of desmosomal attachment, the cell tend to shrink against the nucleus during tissue processing creating a clear halo around. Hence, these cells appear as clear cells in between the basal cells. Since they are located in basal layer as clear cells, melanocytes are called low level clear cells. The melanocytes contain characteristic electron dense cytoplasmic organelles called melano-somes that contain melanin pigments. Production of melanin depends on melanocyte stimulating hormone. The variation in pigmentation seen in different individuals depends on the activity of melanocytes and not on number of melanocytes.  The melanocytes help to impart color to skin and mucosa and also protect against u-v light.  Melanocytes can be demonstrated using special stains like silver stain and also by DOPA reaction. Langerhans’ cells: Langerhans’ cells are dendritic cells present in the epithelium of skin and mucosa. These cells have a cell body harboring the nucleus and long processes extending between the prickle cell layers. Langerhans’ cells do not have desmosomal attachment and tonofilaments. These cells also appear as clear cells in histological section because of shrinkage of cells. Because of their location in upper layer of epithelium compared to melanocytes, Langerhans’ cells are called high level clear cells. These cells cannot be differentiated by routine H and E stain. They can be demonstrated by histochemical, immuno-fluorescent or immunohistochemical techniques which reveal the cell surface antigen or ATPase reaction.  Election microscopically, Langerhans’ cells show a characteristic racquet or flask or rod shaped cytoplasmic organelle called Birbeck granules or

Langerhans’ granules.  The origin of Langerhans’ cells is from bone marrow and they are immuno-competent cells. They trap the antigens entering the mucosa, process it and present it to the immune system. They are referred to as antigen presenting cells. Merkel cells: These are modified keratinocytes located in the basal layer of oral epithelium. In contrast to other nonkeratinocytes these Merkel cells are nondendritic cells which form occasional desmosomal attachment with neighbouring epithelial cells and contain some tonofilaments. Because of few desmosomal attachments these cells do not appear as clear cells in histological sections. Electron microscopically these cells show cytoplasmic granules with dense core resembling neurosecretory granules. Presence of these granules and the close association of these cells with nerve endings suggest the possible role of sensory function of Merkel cells. The Merkel cells are considered as pressure sensitive cells responding to touch and may be demonstrated using PAS stain.  There is a controversy regarding the origin of Merkel cells. One opinion is that these cells could be of neural crest origin, while few others consider that they are formed by the division of keratinocyte like cells. Inflammatory cells: Inflammatory cells like lymphocytes are also present in the epithelium. These cells are of bone marrow origin. Since these cells move from connective tissue to epithelium and also back, they can be seen at different levels of epithelium. Lymphocytes appear as round cells with nucleus occupying the major part of the cell with little cytoplasm. They can also be demonstrated by immuno histochemical techniques that demonstrate the surface markers (OKT-3) of these cells. Lymphocytes perform defense function.

STRUCTURAL VARIATIONS OF ORAL MUCOSA Oral mucosa is composed of epithelium (either keratinized or nonkeratinized) and connective tissue. Yet, as an adaptation to the function to be performed, different parts of oral mucosa show some structural variations.

Lining Mucosa Lining mucosa includes mucosa lining the cheeks, lips, alveolar mucosa, vestibule, floor of the mouth, ventral aspect of tongue, soft palate, etc. The lining mucosa is nonkeratinized mucosa.

Characteristic Features of Lip and Cheek Mucosa Lip and cheek are lined by nonkeratinized mucosa. Epithelial ridges seen at the interface between the epithelium and connective tissue are small and irregular and interdigitate with few, short irregular connective tissue papillae. Lamina propria is thick and has less dense collagen fibers. Lip and cheek mucosa has a distinct submucosa that contains mixed salivary glands, fat cells, etc. Mucosa is stretchable and is well adapted to contraction and relaxation of underlying musculature. Vertical band of collagen fibers with elastic fibers are found extending from lamina propria to fascia covering the underlying muscle which provides attachment between the mucosa and muscle. Thus the folding of mucosa is prevented while muscle relaxation and avoids mucosa being caught between the teeth.

Vestibular Mucosa and Alveolar Mucosa Vestibular mucosa lines the vestibule; a V-shaped sulcus separating the alveolar mucosa from cheek and lip. Vestibular mucosa is in continuation with alveolar mucosa which lines the alveolar bone. Alveolar mucosa appears reddish and extends up to mucogingival junction which separates it from gingival mucosa. In contrast to cheek and lip mucosa which is tightly bound to the underlying muscle, this alveolar and vestibular mucosa is loosely attached to the underlying structures. This permits the easy movement of lip and cheek. Median and lateral labial frena are seen as folds of mucous membrane containing loose connective tissue. Alveolar mucosa is thin with a thin nonkeratinized epithelium lining it. The epithelial connective tissue junction is relatively flat with small rete ridges and connective tissue papillae which may even be absent at times. Alveolar mucosa is loosely attached to underlying bone by a loose connective tissue that also contains minor salivary gland.

Ventral Surface of Tongue and Floor of the Mouth Floor of the mouth is a small horseshoe shaped region beneath the movable part of the tongue. Mucosa lining the floor of the mouth and ventral surface of the tongue share many common features. Mucosa is thin with a nonkeratinized epithelium. The epithelial rete ridges and connective tissue papillae are short. Connective tissue shows rich blood supply which is particularly prominent in floor of the mouth. Submucosa of floor of the mouth contains adipose tissue and minor salivary glands. In the ventral aspect of tongue, submucosa may be very thin or even absent where the mucosa will be tightly bound to the underlying musculature. The thin epithelial lining and rich blood supply permit the rapid absorption of medicines administered sublingually.

Soft Palate Soft palate is lined by nonkeratinized stratified squamous epithelium. Epithelium may show presence of few taste buds. Lamina propria is highly vascular because of which soft palate appears reddish clinically. The epithelium connective tissue interface is irregular with thick and short rete ridges and connective tissue papillae. A distinct layer of elastic fibers are found forming a lamina between lamina propria and submucosa. The submucosa is composed of diffuse loose connective tissue containing numerous minor salivary glands.

Vermilion Border of Lip (Transitional Zone) This zone is the transitional zone between the skin covering the external surface of the lip and the labial mucosa lining the inner aspect (Fig. 10.9). The skin is composed of keratinized stratified squamous epithelium with all appendages like hair follicles, sweat glands and sebaceous glands. The labial mucosa is lined by nonkeratinized stratified squamous epithelium. The connective tissue beneath the labial mucosa shows minor salivary glands. The central most region of lip shows orbicularis oris muscle. The transitional zone has a thin lining epithelium with mild keratinization on the surface. There are many long connective tissue papillae reaching high into epithelium, carrying many capillary loops. This makes it more reddish compared to labial mucosa. Underlying connective tissue is characteristically devoid of glands which

causes the mucosa to dry up.

Fig. 10.9: Vermilion border of lip

Gingiva Gingiva is the part of the oral mucosa that covers the alveolar process and surrounds the neck of the tooth. The gingiva is relatively tightly bound to the buccal and lingual plates of alveolar process and extends from the dentogingival junction to the alveolar mucosa.

Macroscopic Structure of Gingiva Gingiva is pink in colour with some degree of melanin pigmentation. Anatomically gingiva can be divided into three parts; marginal gingiva, interdental papilla and attached gingiva (Fig. 10.10). Marginal gingiva: It is the unattached portion of gingiva that forms the border which surrounds the teeth in a collar like fashion. Marginal gingiva follows a scalloped line on the facial and lingual surface of the teeth. The contour depends on shape and alignment of teeth. Marginal gingiva forms the soft tissue wall of the gingival sulcus and is separated from attached gingiva by a free gingival groove which runs parallel to the terminal edge of the gingiva at a distance of 0.5–1.5 mm almost at the level of bottom of gingival sulcus.

Gingival sulcus: It is a shallow crevice or V-shaped space present around the tooth bounded by tooth surface on one side and marginal gingiva on other side. The depth of this sulcus varies from 0.5 to 3 mm with an average of 1.8 mm. More than 3 mm is considered as pathological and is called as gingival or periodontal pocket. Interdental papilla: The part of the gingiva that fills the interdental space between two adjacent teeth is called interdental papilla. Interdental papilla appears pyramidal or triangular from the facial and lingual aspect with its lateral borders and tip formed by a continuation of marginal gingiva of adjacent teeth. Three dimensionally the anterior interdental gingiva is described to have a pyramidal shape with facial and lingual gingiva tapering towards the interdental area. In the posterior region interdental gingiva has a ‘tent’ shape. This shape is formed because the interdental papillae present on lingual and buccal sides of each interproximal space are connected by a depressed central area. The facial and lingual portions of papillae forms the high points and a concave or valley like area fits below the contact area. This valley like area is called ‘col’ which is lined by nonkeratinized epithelium. This col is considered as a weak point in gingiva and is more prone to periodontal diseases. When the adjacent teeth are not in contact, the interdental gingiva appears smooth with round surface, firmly adherent to interdental bone.

Fig. 10.10: Parts of gingiva

Attached gingiva: The firm, resilient immobile portion of the gingiva which is tightly bound to the alveolar bone is called attached gingiva. This extends from free gingival groove to mucogingival junction by which it is separated from alveolar mucosa. On the palatal aspect attached gingiva blends with palatal mucosa. The width of attached gingiva varies in different regions. In maxilla, it ranges from 3.5–4.5 mm, while in mandible it is 3.3–3.9 mm. The surface of the attached gingiva is irregular with elevations and depression. The orange peel appearance created by these elevations and depression is described as stippling. These are considered as functional adaptations to mechanical stress and may be caused by traction on mucosa by underlying fibrous attachment to the bone. Loss of stippling is one of the initial signs of gingival inflammation. The pattern and extent of stippling varies in different regions of the mouth; being less prominent on lingual than on facial surface. Males tend to have more stippling than females. Gingiva may show slight vertical depression between the alveolar bone eminences of adjacent teeth. These are called interdental grooves.

Microscopic Structure of Gingiva Structurally gingiva is composed of stratified squamous epithelium and connective tissue. Covering epithelium shows structural variations in different regions. Accordingly it can be categorized as epithelium covering oral region of gingiva (outer portion), sulcular epithelium and junctional epithelium (Fig. 10.11). Oral region: Epithelium lining the oral region of gingiva is keratinized or parakeratinized stratified squamous epithelium which has four distinct layers, i.e. stratum basale, stratum spinosum, stratum granulosum and stratum corneum. Microscopically a shallow ‘V’ shaped notch on the surface corresponding to a heavy epithelial ridge which represent the free gingival groove. In the region of attached gingiva stippling is reflected by alternate rounded protuberances and depressions on the surface. The depressions correspond to the center of heavy epithelial ridges. The epithelium connective tissue interface is irregular with numerous long narrow rete ridges interdigitating with long connective tissue papillae. This extensive interdigitation increases the strength to withstand the

masticatory stresses. This long branching rete ridges help in microscopic identification of gingiva from other parts of oral mucosa. Gingival epithelium is parakeratinized in 75% of population. Sulcular epithelium: Sulcular epithelium lines the gingival sulcus and it extends from the coronal limit of junctional epithelium to the crest of gingival margin. Sulcular epithelium is composed of thin layer of nonkeratinized epithelium. The junction between epithelium and connective tissue is flat without rete ridge formation. Lack of keratinization is thought to be due to inflammation of connective tissue.

Fig. 10.11: Structural variation of gingival epithelium

Junctional epithelium: The part of the gingival epithelium that is attached to the cervical part of the tooth and therefore forming a junction between the tooth and gingiva is called junctional epithelium. Junctional epithelium is stratified squamous epithelium which appears as a triangular strip with 15–30 cell layer thickness at the cervical portion (floor of sulcus) and 3–4 cell layer thickness at the apical margin. Junctional epithelium is composed of flattened cells which are arranged parallel to the tooth surface. The cells have lesser number of desmosomal junctions and more intercellular spaces helping in migration of polymorpho nuclear leucocytes (PMNLs) into the epithelium and to the sulcus. The epitheliumconnective tissue interface is flat. One of the most important feature which makes it differ from other epithelium is the presence of basal lamina on both sides, i.e. at the junction of epithelium and connective tissue and also on

the surface adjacent to the tooth. This basal lamina on the surface is attached to the tooth by hemidesmosomes. The junctional epithelium also shows high turnover rate. The cells from the basal layer migrate to within 2–3 layers of junctional epithelium and join a migratory root in a coronal direction and finally exfoliate at the gingival sulcus.

Gingival Connective Tissue The connective tissue beneath the gingival epithelium is lamina proprina with papillary and reticular layer. The connective tissue of gingiva consists of dense collagenous tissue arranged in bundles of fibers which play a very important role in maintaining the integrity of the supporting apparatus of the tooth. These fiber groups are referred to as the secondary fibers of periodontal ligament or gingival ligament or gingival fibers of periodontal ligament (Refer Fig. 8.3). In addition to collagen fibers, oxytalan fibers and elastic fibers are also present in gingival connective tissue.

The Gingival Fibers Include Dento-gingival fibers: These fibers extend from the cervical portion of the cementum to the lamina propria of gingiva. Dento-periosteal fibers extending from cervical part of cementum to the periosteum of the alveolar crest and the vestibular and oral surface of the alveolar bone. Alveologingival fibers extend from the crest of the alveolar bone to the lamina propria of gingiva. Circular fibers: These fibers are arranged in the gingival connective tissue, encircling the neck of tooth like a collar. These fibers are also known as the marginal ligament and they play an important role in maintaining a tightly fitting gingival collar. Trans-septal fibers which are also called interdental ligament is also found in gingival connective tissue as accessory fibers extending inter proximally between adjacent teeth, from cementum of one tooth to cementum of adjacent tooth over the interdental bony alveolar crest. Lamina propria of oral gingival epithelium is firmly attached to periosteum of the alveolar bone by course collagen bundles. This type of attachment is

called mucoperiosteum. Subucosa is absent in gingiva therefore no large blood vessels or minor salivary glands are observed in gingiva. The lamina propria of sulcular and junctional epithelium is different from that of oral gingival epithelium. Connective tissue in this region is delicate with presence of inflammatory cells.

Palate The palate forms the roof of the oral cavity and is divided into immovable hard palate anteriorly and the movable soft palate posteriorly. The hard palate has a hard bony support while soft palate has only fibrous tissue. The mucosa covering the hard palate differs in microscopic and macroscopic structure in different regions.

Macroscopic Structure of Hard Palate Palate can be divided into different zones (Fig. 10.12). Gingival zone: It consists of peripheral portion of hard palate found adjacent to teeth. Midpalatine raphae: A narrow zone in the midline of hard palate extending from incisive papilla posteriorly. This zone appears depressed compared to adjacent areas. Incisive papilla: Incisive papilla is an oval prominence seen at the extreme anterior region of palate immediately behind the maxillary central incisors covering the oral opening of incisive canal. Anterolateral region between raphae and gingiva containing much of fat tissue in submucosa. The fatty zone meets the glandular zone as an arc, the lateral arm of which generally terminate in the region of first molar. Posterolateral region between raphae and gingiva containing mainly minor salivary glands in sub mucosa. Palatine rugae: Radiating outwards from the palatine raphae in the anterior region of hard palate are irregular transverse palatine ridges referred to as palatine rugae. These ridges may have a role in suckling in infants and also may be helping in backward movement of food during mastication. Fovia palatina: It is an elongated depression of few millimeters depth in post

part of palate on either side of midline.

Fig. 10.12: Palate

Microscopic Structure of Hard Palate The hard palate is covered by keratinized mucosa. The keratinized stratified squamous epithelium lining the mucosa has four different layers, stratum basale, stratum spinosum, stratum granulosum and stratum corneum. As a functional adaptation, to bear with masticatory stress, the cells show more dense tonofilaments, increased number and length of desmosomes, etc. The epithelium connective tissue interface is irregular with many long regular epithelial ridges interdigitating with connective tissue papillae. The lamina propria is dense throughout the hard palate and is thicker in anterior region than in posterior region. In the region of rugae the connective tissue core is dense with interwoven collagen fibers. Incisive or palatine papilla is also composed of dense connective tissue. This contains the remnants of nasopalatine duct which is lined by pseudostratified squamous epithelium. Small islands of hyaline cartilage may be seen around the duct opening.

Structure of submucosa varies in different regions of palate. Submucosa is absent in the peripheral zone of palate adjacent to the teeth, i.e. the gingival zone and in the mid palatine raphae. In these regions, the lamina propria is tightly bound to the periosteum of bone which is referred to as mucoperiosteal attachment. In between the gingival zone and mid palatine raphae, the palate has distinct submucosa. The submucosa is thicker in the posterior region than anterior region. The submucosa in the anterior part of the hard palate is filled with adipose tissue and in posterior region with mucous glands. Therefore anterolateral part of hard palate is referred to as fatty zone and posterolateral part the glandular zone. In spite of thick submucosa in certain regions, the mucosa of the hard palate is tightly fixed to the underlying bone and is immobile. This is achieved by dense vertical band of connective tissue which attaches mucosa firmly to the periosteum of palatal bone. These dense bands of connective tissue are at right angle to surface and divide the submucosa into compartments. The wedge-shaped area where the alveolar process joins to the horizontal plate of hard palate contains loose connective tissue which carry large vessels and nerves. The thickness of this loose connective tissue gradually increases from anterior region of palate to posterior region (structure of soft palate— refer page 114).

Tongue Tongue is a muscular organ situated in the floor of the mouth which play important role in speech, mastication, deglutition, taste sensation, etc.

Macroscopic Features Dorsum of the tongue is convex in all directions (Fig. 10.13). A V-shaped sulcus divides the dorsal aspect of the tongue into anterior 2/3rd, body or oral part and posterior 1/3rd, base or pharyngeal part. A small pit is seen where the two arms of ‘V’ meet. It is called foramen caecum representing the opening of thyroglossal duct. Anterior 2/3rds of the tongue is also called papillary part because the mucosa has numerous papillae which give it a velvety appearance. The most numerous papillae are fine pointed, coneshaped filiform papillae that are widely distributed on the dorsal surface.

These papillae make the surface of the tongue rough and help in crushing the food particles while pressing against hard palate. Numerous fungiform papillae are also seen distributed between the filiform papillae on the dorsal aspect mainly on the tip and lateral margins. Fungiform papillae are seen as red round, projections. Anterior to sulcus terminalis, 8–12 large papillae called circumvallate papillae are seen. Circumvallate papillae are partly submerged and do not project above the surface of tongue and are surrounded by a V-shaped sulcus. Margins of the papillae may project above the surface. In the posterior region of anterior 2/3rds of tongue, on the lateral margin foliate papillae are seen which consists of series of folds forming clefts. These foliate papillae are rudimentary in humans.

Fig. 10.13: Macroscopic structure of tongue

Posterior 1/3rd of the tongue has an irregular surface with round projection, the lingual follicles containing lymphoid component. Therefore posterior 1/3rd of the tongue is also called lymphoid region. The mucous membrane lining the posterior 1/3rd does not show papillae and is relatively smoother. Inferior surface or ventral aspect of the tongue is covered with smooth mucous membrane. Papillae are not seen on this aspect of tongue. The inferior surface is attached to the floor of the mouth by a loose lingual frenum. On either side of lingual frenum, prominent lingual veins are seen. Lateral part of inferior surface shows the presence of two folds called plica fimbriata which runs forward and medially to the tip of the tongue.

Microscopic Structure of Tongue The tongue is lined by stratified squamous epithelium which varies in structure and thickness in different regions. The mucosa lining the ventral aspect is nonkeratinized and is tightly bound to the underlying musculature. The epithelium is thin with many but short rete ridges. Lamina propria is relatively thin and loosely arranged. The mucosa lining the dorsal aspect of tongue is referred to as specialized mucosa because of the presence of special sense organs, i.e. taste buds. The epithelium lining is mostly keratinized. Thickness of epithelium varies with respect to papillae. Lamina propria is compact and tightly attached to the underlying muscle. Minor salivary glands are seen in the anteroventral and posterior regions. On the dorsal surface the mucosa evaginates to form papillae.

Papillae of the Tongue Filiform papillae Filiform (hair-like) papillae are seen as hair like or thread like projections on the dorsal aspect of the tongue. Filiform papilla in a histological section is seen as cone-shaped structure (Fig. 10.14a) lined by stratified squamous epithelium with thick keratin on the surface. Central core of connective tissue supports the blood vessels. Taste buds are not seen in these papillae. The mucosa between the filiform paillae is nonkeratinized and stretchable permitting the mucosa to adapt to the changes in shape of tongue. Fungiform papillae Fungiform (fungus like) papillae are mushroomshaped structure (Fig. 10.14b) projecting above the surface of the tongue and located between the filiform papillae. The epithelium covering the fungiform papillae is thin nonkeratinized stratified squamous epithelium. The superficial surface of the papillae contains few taste buds. The supporting connective tissue shows collagen fibers, fibroblasts and rich capillary network. Fungiform papillae appear reddish in color because of the capillary network visible through relatively thin nonkeratinized epithelium.

Fig. 10.14a: Filiform papillae

Fig. 10.14b: Fungiform papillae

Circumvallate papillae The circumvallate (walled) papillae are seen in the anterior two-thirds of tongue just anterior to sulcus terminals. These are 10–12 in number. The superficial surface of these papillae is at the level of surface of tongue and a V-shaped sulcus is present all around the papillae separating them from the adjacent portion of tongue (Fig. 10.14c). The lining epithelium is keratinized stratified squamous epithelium at the superficial surface and nonkeratinized on the lateral surface of circumvallate papillae. Taste buds are seen only on the lateral surface. Central portion is occupied by the connective tissue. The

characteristic feature of this papilla is presence of serous minor salivary glands (von Ebner’s gland) in the connective tissue beneath it. These glands secrete watery saliva into the V-shaped trough around the papillae to flush out the food debris.

Taste Buds Taste buds are specialized sense organs that can perceive the taste sensation. They are mainly located in papillae of tongue, i.e. superficial surface of fungiform papillae, lateral walls of circumvallate papillae and in the cleft walls of foliate papillae. In addition, taste buds are also seen in posterior part of palate, uvula, epiglottis, pharyngeal region, etc. Taste buds (Figs 10.15a and b) are barrelshaped structures composed of 30–50 spindle-shaped, modified epithelial cells that extend from basement membrane to epithelial surface. The taste buds measure around 50–80 microns in height and 30–50 microns in diameter. At the epithelial surface the tapered end of all cells end in a small opening of 2–5 microns called taste pore through with the cells communicate to exterior. Based on the morphological features 4 different types of cells can be seen in taste buds. Type I cells (dark cells): They are long narrow cells which make up the major population (60%) of cells. The base of the cells rests on basement membrane and apex end as a long finger like microvilli in the taste pore. These cells have dark nucleus, rich cytoplasmic organelles and large dense cored vesicles in apical cytoplasm.

Fig. 10.14c: Circumvallate papilla

Figs 10.15a and b: Taste buds

Type II cells (light cells): They are more or less regularly oval shaped cells with electron-lucent cytoplasm having few organelles and large round or oval light stained nuclei. Around 30–40% of the cells of taste bud belong to this group. These cells also extend from basement membrane to taste pore where they end in short microvilli. Type III cells (intermediate cells): These cells are some what similar to type II cells in morphology. They make up only 5–15% of cells. These cells end in a narrow club-shaped projection in the taste pore. In contrast to the type II

cells, type III cells have numerous dense covered vesicles concentrated in the basal region. Type IV cells (basal cells): These cells rest on basement membrane, but do not extend to the taste pore. These cells have been considered as undifferentiated precursor cells which can give rise to all three different types of cells. Difference in opinion exists about taste receptor cells. Some authors consider the type III cells are taste receptors cells because of the electron dense vesicles at the basal region and their close proximity to the nerve endings. They consider other cells as supporting cells. But others are of the opinion that the types I, II and III cells are transitional form of a single cell and all the types could act as chemoreceptor transduction cells because synapses have been observed on all three cells. The nerve fibers enter the taste buds by penetrating the basal lamina, within the taste bud they undergo extensive branching and contact with the taste receptor cells. Taste buds of fungiform papillae of tip of tongue have receptors for sweet, and that of lateral borders of tongue have receptors for salty taste. Bitter taste is perceived by taste receptors of circumvallate papillae and the sour taste by foliate papillae (physiology of taste, refer page 297–298).

Dentogingival Junction The junction between the tooth and gingiva called dentogingival junction. The junctional epithelium has an important role in this. The epithelium, i.e. the junctional epithelium that is attached to the tooth to form a dentogingival junction is called attachment epithelium and the mode by which this epithelium is attached to the tooth is called epithelial attachment (for details refer page 117). Formation of dentogingival junction: Once the enamel formation is completed the ameloblasts secrete proteinaceous material on to the surface of newly formed enamel which is structurally similar to basal lamina. This structure is called primary enamel cuticle. Once the enamel organ transforms into reduced enamel epithelium (REE) it gets attached to the surface of enamel with the help of this basal lamina through hemidesmosomes. During the process of eruption, the connective tissue between the REE and oral epithelium degenerate, followed by proliferation of oral epithelium and

REE. These layers ultimately fuse together to form a solid plug of epithelium (Fig. 10.16). The central cells of this plug degenerate forming a canal through which the tooth emerges into the oral cavity. As the tooth move to the occlusal plane, the epithelium covering the enamel surface shortens. Even after the tooth reaches the occlusal plane 1/3rd of the tooth is still covered by epithelium. Once the tip of the cusp emerges into the oral cavity the part of reduced enamel epithelium attached to the tooth is called primary attachment epithelium and is in continuation with oral epithelium. The reduced enamel epithelium gradually shortens to expose the crown of the tooth completely and is slowly replaced by the oral epithelium. The attachment epithelium derived from oral epithelium is referred to as secondary attachment epithelium. The actual movement of the tooth to occlusal plane is called active eruption and the exposure of the crown by the apical migration of the covering epithelium without actual movement of the tooth is called passive eruption.

Shift of Dentogingival Junction Once the dentogingival junction is established, the attachment epithelium shows a gradual migration an apical direction, exposing more tooth surface into the oral cavity. This shift can be discussed under four stages (Fig. 10.17).

Fig. 10.16: Passive eruption and formation of dento-gingival junction

First Stage: During this stage, the attachment epithelium is completely attached to enamel with its apical end at the cemento-enamel junction. The bottom of gingival sulcus is located on enamel surface. This level of attachment is seen between 20 and 30 years. In this stage, the clinical crown of the tooth is shorter than anatomic crown. Second Stage: In this stage, the attachment epithelium migrates apically and is attached partly onto enamel and partly onto cemental surface and apical end is on cementum. The bottom of gingival sulcus is on enamel itself. This stage is seen at 40 years. Even in this stage, the clinical crown of the tooth is shorter than anatomic crown. Third Stage: As the apical migration of attachment epithelium progresses gradually, in 3rd stage it becomes completely attached on to the cementum surface with bottom of gingival sulcus at cemento-enamel junction. At this stage the complete anatomic crown is exposed to oral cavity.

Fourth Stage: In this stage, the attachment epithelium still migrates apically on the surface of cementum and the bottom of gingival sulcus is located on the cemental surface exposing even a part of root. At this stage clinical crown is longer than anatomic crown. First two stages are physiological while the 3rd, 4th may be physiological or pathological. (Age changes—refer page 324)

Fig. 10.17: Shift of dento-gingival junction

Clinical Considerations 1. Clinical conditions resulting in alteration in structure of oral mucosa –

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Oral cavity is the mirror of general health of a person. Various local and systemic disease conditions such as nutritional deficiency, metabolic disturbances, anemia, endocrine disturbances, present with oral mucosal changes. Pale pink colour of oral mucosa may be altered in different clinical conditions. Mucosa may appear reddish in case of inflammatory conditions, pale in anemias and oral submucous fibrosis. Patchy areas of brownish pigmentations may be noted in conditions involving melanocytes. Mucosa which is soft in texture and stretchable normally becomes stiff and non-stretchable in oral submucous fibrosis and

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scleroderma due to excessive fibrosis of connective tissue. A number of dermatological disorders manifest oral mucosal lesions such as fluid filled vesicles, ulcers, erosions, red patches, etc. Examples are: Oral lichen planus, pemphigus, pemphigoid, etc. Loss of papillae of tongue resulting in bald appearance is a characteristic feature in anemias. Use of tobacco may cause oral mucosal cancer or potentially malignant disorders, which may present as white patches or plaques, ulcers, ulceroproliferative growth, involving various parts of oral mucosa. Histological alterations noted in different layers of epithelium in various disease conditions. Identification of these features helps in diagnosing the lesions accurately and providing appropriate treatment. Following are some of the important changes: 1.

Hyperkeratosis—increase in thickness of keratin layer

2.

Acanthosis and atrophy—acanthosis refers to abnormal thickening of spinous cell layer while atrophy is thinning of epithelium.

3.

Acantholysis—destruction of desmosomal junctions resulting in loss of intercellular adhesion and is a characteristic feature in blistering diseases such as Pemphigus and viral infections.

4.

Basillar hyperplasia—increased cell proliferation in the basal cell layer resulting in multiple layers of basal cells

5.

Basal cell degeneration-destruction of basal cells and is a characteristic feature in lichen planus

6.

Loss of stratification—refers to loss of arrangement of epithelial cells in different layers with progressive level of differentiation and this can be a feature of epithelial dysplasia

7.

Potentially malignant disorders and oral mucosal cancer present with a number of cellular and architectural changes in epithelium which is collectively referred to as features of epithelial dysplasia.

2. Clinical considerations related to structural variations The volume and texture of submucosa in different part of the mucosa has particular clinical significance. In masticatory mucosa, where there is no or little submucosa, the mucosa is not stretchable and is firmly attached to the underlying bone. Therefore injections in these regions will be painful as the solution cannot be dispersed easily. Any wound in these region do not gape open. Wounds in these region do not require suturing and the wound healing in these regions is by secondary intention. In contrast injections in lining mucosa is less painful. Wounds in the lining mucosa gape open, requires suturing and wound healing is by primary intention.

11 Salivary Glands

Introduction Classification of salivary glands Gross morphology Microscopic structure Clinical considerations

S

alivary glands are exocrine glands that synthesize and secrete saliva that reaches the oral cavity through a ductal system.

Classification of Salivary Glands Based on size • •

Major: Parotid, submandibular and sublingual glands Minor: Group of small glands located in the oral mucosa.

Based on location • •

Extraoral: Three pairs of major glands located outside the oral cavity Intraoral: Groups of minor glands widely distributed in the oral mucosa.

Based on nature of secretion • •

Serous glands: Parotid and von Ebner’s glands Mucous: Sublingual gland and all minor salivary glands except von Ebner’s gland

•

Mixed: Submandibular glands.

Salivary Glands Gross Morphology There are three pairs of major salivary glands which produce around 95% of total salivary volume and numerous minor salivary glands producing relatively less amount of saliva. Major salivary glands include parotid glands, submandibular glands and sublingual glands Parotid glands: (Par-otid = near to the ear). These are the largest of all salivary glands. These are a pair of pyramidal-shaped gland, located on either side of the face, enclosed by a dense fibrous capsule. The parotid gland is located subcutaneously, below and in front of the ear in the space between the ramus of the mandible and the styloid process of the temporal bone. It secretes approximately 25–30% of total output of saliva. The main parotid duct (Stensen’s duct) leaves the mesial angle of gland to traverse over the masseter muscle and turn to enter the buccinator muscle to open into the vestibule opposite to maxillary second molar. Submandibular glands: These are another pair of major mixed salivary glands which are irregular and has a shape comparable to walnut. These glands are located in the anterior part of digastric triangle (submandibular region). Wharton’s duct, the major duct of submandibular gland, starts from the anterior end of the gland, follows a tortuous course and opens in the oral cavity at the sublingual papilla situated at the side ot lingual fossa. 60% of total saliva is secreted by these glands. Sublingual glands: These are the smallest of all major salivary glands. These glands lie immediately below the oral mucosal lining of floor of the mouth. These glands releases the secretion through a major duct called Bartholin’s duct or often through a series of smaller ducts called duct of Rivinus. The Bartholin’s duct opens to oral cavity along with submandibular duct. Ducts of Rivinus open directly to the floor of the mouth along the submandibular folds. These glands secrete mucous saliva which makes up 5% of total saliva.

Minor Salivary Glands Minor salivary glands are small groups of salivary secretory units, distributed

almost throughout the oral mucosa except anterior part of the hard palate, gingiva and anterior 2/3rd of dorsal aspect of tongue. Around 600–1000 collections of minor salivary glands are named according to the location and together contribute around 5% of total saliva secreted. They release the secretions into the oral cavity through small ducts. Labial glands: These are minor salivary glands present in the labial mucosa. Although they are described as mixed, ultrastructurally only mucous cells are seen. Buccal glands: These are present in buccal mucosa which are purely serous or mixed. Palatal/palatine glands: Minor salivary glands scattered in the posterior region of hard palate and soft palate are called palatal glands. They produce mucous secretions. Glossopalatine glands: These are pure mucous glands localized to the region of isthmus in the glossopalatine/tonsillar fold. Lingual Glands: Mainly three categories of lingual minor salivary gland are seen. van Ebner’s glands: These are the minor salivary glands which are physiologically most important. They are the only serous minor salivary glands and are located below the sulcus of the circumvallate and foliate papillae of the tongue. These are purely serous in nature and their ducts open into the trough surrounding the circumvallate papillae and to the grooves between foliate papillae. The secretions of these glands wash out the trough of the papillae and ready the taste receptors for new stimulus. These glands also produce several protective and digestive enzymes such as lactoperoxidase, lysozyme, amylase and lipase, etc. Glands of Blandin-Nuhn: These are mucous/mixed glands located on ventral aspect of tongue anteriorly near the tip of the tongue. Weber glands: These are pure mucous glands located posteriorly on either side of tongue.

Development of Salivary Glands The development of salivary glands begins early in intrauterine life; Parotid gland at 4–6 weeks, submandibular gland at 6 weeks and sublingual and

minor salivary glands at 8–12 weeks. Although the parenchymal components attain maturity by 2nd month of gestation, up to two years of life the development continues with further increase in secretory units. As in case of tooth, salivary gland development begins with proliferation of oral ectoderm in to underlying ectomesenchyme. This epithelial proliferation, which is a result of epithelial mesenchymal interaction, initially form an epithelial thickening. This on further proliferation form a cord of epithelial cells with a bud at the free end, surrounded by condensed ectomesenchyme. The proliferation eventually creates, highly branched epithelial cords with bulbous ends. Degeneration or apoptotic death of central cells of the branching cords results in formation of lumen. The inner cells of the terminal bud like structure differentiate into secretory cells, i.e. serous, or mucous depending on the gland and peripheral cells give rise to myoepithelial cells. Thus the cords become ducts, and the bulbous ends become secretory acini. The condensed ectomesenchyme in due course, form connective tissue component of the gland, i.e. capsule enclosing and septa extending between the glandular tissue. The epithelial buds of parotid glands are located on the inner part of the cheek, near the labial commissures of the primitive mouth. These buds grow posteriorly toward the otic placodes of the ears, near the developing facial nerve, where further development continues. Submandibular glands develop bilaterally from epithelial buds in the sulcus surrounding the sublingual folds on the floor of the primitive mouth. Solid cords branch from the buds and grow posteriorly, lateral to the developing tongue. Epithelial buds of sublingual gland develop in the sulcus surrounding the sublingual folds on the floor of the mouth, lateral to the developing submandibular gland. Similarly minor salivary glands also develop as buddings from oral ectoderm of corresponding region.

STRUCTURE OF SALIVARY GLAND Irrespective of the size and location, all the salivary glands are composed of parenchymal components which includes the secretory units and the ductal systems. All the major glands also show a second structural component, i.e. connective tissue that forms a capsule around the salivary gland, which also

extend between parenchymal components, dividing the gland into lobes and lobules. In contrast, minor salivary glands do not have a distinct connective tissue component.

Parenchymal Components of Salivary Gland Parenchymal components of the salivary gland include acini (secretory unit) and the ductal system. These components are compared to a ‘bunch of grapes’ with acini representing the fruits and ductal system representing stalks (Fig. 11.1). Secretory units of salivary glands are composed of serous and/or mucous secretory cells arranged to form round or tubular configuration around a central lumen, which is termed as an acinus (acinus—singular and acini— plural). In addition to secretory cells, these acini also have myoepithelial cells which cradle each acinus.

Fig. 11.1: Parenchymal components of salivary glands

Acinus (acinus is a Latin word for berry or grape): Acinus is the basic functional unit of salivary gland. It comprises round or tubular collection of secretory cells which synthesizes and secretes saliva and therefore called the terminal secretory unit of a salivary gland. Serous salivary glands are predominantly composed of numerous serous acini and mucous gland has mostly mucous acini. A mixed salivary gland is composed of varying proportions of both serous and mucous acini along with few mixed acini.

Serous Secretory Cell and Acinus Serous secretory cells are specialized cells which synthesize, store and secrete the serous saliva which is thin and watery, rich in both nonenzymatic and enzymatic proteins and containing small amount of carbohydrates. These serous cells are the main secretory cell type in parotid and submandibular gland and some are also present in mixed acini of sublingual gland. Among minor salivary glands, serous cells are found only in von Ebner’s gland.

Light Microscopic Structure A serous cell, under the light microscope appears as a pyramidal cell with a broad base resting on a basement membrane and a narrow apex bordering the lumen. Nucleus of these cells is round and placed at the basal one-third of the cells. In H and E sections, apical part of the cytoplasm appears granular and stain distinctly eosinophilic because of the secretory granules while the cytoplasm in the basal portion stains with hematoxylin because of the abundant rough endoplasmic reticulum and the free ribosomes. 8–12 serous cells are arranged around a small lumen to form a serous acinus which is roughly round in shape (Fig. 11.2a). The central lumen extends between the secretory cells as intercellular canaliculi. The serous acinus is smaller in size than a mucous acinus with less number of cells and has relatively smaller lumen. Electron microscopic structure of serous cells Electron microscope can reveal all the details of cytoplasmic components of the cells and specialization of plasma membrane (Fig. 11.2b). Electron microscopic picture of the serous cells show all the organelles required for the synthesis and therefore indicate that these cells are specialized and active in protein synthesis.

Shape of the Cell and Specialization of Plasma Membrane The plasma membrane bounding the cells shows various structural specializations. Basal plasma membrane is irregular with multiple foldings. Some of these foldings extend laterally beyond the boundary of the cell and inter digitate with those of the adjacent cells. This specialization increases surface area of the cell by 60 times for diffusion of water and minerals required for the saliva from tissue fluid. The basal plasma membrane is supported by a basement membrane. Plasma membrane at the lateral region and apical portion of the cell also shows foldings called microvilli which increase the surface area of secretion. The cells on the lateral region have complex interrelationship with adjacent cells. Near the apical portion, adjacent cells are connected by intercellular junctions like tight junctions, intermediate junctions and desmosomal junctions. These junctions help to hold the cells together and prevent leakage of material from the lumen. Between the adjacent cells there is well defined intercellular space or canaliculus that extends from the lumen which is sealed off by intercellular junctions.

Cytoplasmic Components Cytoplasm of a secretory cell shows abundant cytoplasmic organelles required for synthesis and storage of proteinaceous materials. Large number of rough endoplasmic reticulum (RER) arranged parallelly is found in the cytoplasm basal and lateral to the nucleus. A prominent Golgi apparatus consisting of several stacks are seen lateral and apical to the nucleus. The Golgi apparatus are functionally connected to RER through a series of budding vesicles at the end of RER. Mitochondria, the energy source of various synthetic and transportation activities, are also abundant and are dispersed in the basal and lateral region.

Synthesis of Saliva The organic component of the saliva is synthe-sized by the secretory cells utilizing the substrate provided by the nutrients that reach the cell. Water and electrolytes required for the saliva reaches the cell from circulation and from tissue fluid.

Fig. 11.2a: Light microscopic structure of serous acinus

Fig. 11.2b: Electron microscopic structure of serous cell

Protein synthesis begins when the messenger RNA carries the message from the nucleus to the cytoplasm. The ribosomes translate the message and initiates protein synthesis by adding amino acids in required sequence. Thus forms a pre-protein with a signal sequence—an extension of 16–30 amino acids—attached to it. With the help of this signal sequence, protein synthesized, enters the RER. In the RER, the signal sequence is removed by proteolytic enzymes, and the protein assumes a helical structure. After structural modification, protein is transferred to Golgi complex through a series of budding vesicles which attach to cis or convex face of Golgi bodies. Glycosylation began in the RER continues in Golgi complex. Carbohydrates added to amino acids (like asparagine, serine, and threonine) are galactose, mannose, fructose, glucosamine, galactosamine and sialic acid. Then the secretory products are packed into secretory granules which bud off from the trans or concave face of Golgi complex. These secretory granules are termed as pre secretory granules or condensing vacuoles or immature granules. The limiting membranes of these granules are irregular indicating further addition of molecules. The secretory granules are then stored in the apical cytoplasm till the secretory unit is stimulated and the stored products are expelled out. Secretion of saliva The secretion of the stored protein is by a process called exocytosis. When the cell receives appropriate stimuli the secretory granules move towards the apical and lateral plasma membrane with the help of microfilaments and fuses to the plasma membrane. After fusion the plasma membrane of the joined region opens up releasing the content in to the lumen without disruption of plasma membrane continuity. This process is repeated till all the granules are emptied. The added fraction of membrane is retrieved by formation of endocytic vesicles, which may be destroyed by lysozymes or may be utilized for packing of secretory materials. The salivary glands are categorized as merocrine glands because of this mode of secretion. Decision for protein synthesis is taken in the nucleus ↓ Messenger RNA in ribosomes carry the message to the cytoplasm through ribosomes ↓

Ribosomes translate the message and initiate protein synthesis by adding amino acids in required sequence. Thus forms a pre-protein with a signal sequence attached to it ↓ With the help of signal sequence (an extension of 16–30 amino acids), protein synthesized enters the RER where the signal sequence is removed by proteolytic cleavage and protein assumes a helical structure ↓ Protein synthesized is transferred to Golgi complex, through cis or convex face ↓ Structural modification in the Golgi complex by addition of carbohydrates ↓ Packing of secretory product into secretory granules which are released through trans face of Golgi complex. These secretory granules are called pre-secretory granules, immature granules or condensing vacuoles ↓ Further addition of molecules resulting in maturation of secretory granules ↓ Storage of secretory granules in the apical cytoplasm ↓ Secretion of material by a process of exocytosis

Mucous Secretory Cells and Mucous Acinus Mucous cells are the specialized cells that synthesize, store and secrete mucinous secretion which is ropey and thick, rich in carbohydrates and containing small amount of non-enzymatic proteins. Since the major part of the secretion is glycoproteins, the secretion from these cells mainly helps in lubrication. The mucous cells are the predominant secretory cells in the sublingual glands and majority of minor salivary glands. Submandibular glands also have some mucous secretory cells.

Light Microscopic Structure of Mucous Cells Mucous cells are also pyramidal in shape similar to serous cells but are larger and have a relatively broader luminal surface. The broad base of the cell, rests on a basement membrane and apex borders the lumen. Nucleus of these

cells is flattened and placed at the basal part of the cells, pressed against the base, along with thin rim of cytoplasm. Apical part of the cytoplasm is filled with secretory granules which is rich in carbohydrates. In a hematoxylin and eosin stained section apical portion of mucous secretory cell appears empty, except for fine strands of cytoplasm, because the secretory droplets contain heavily glycosylated proteins (mucins) that do not take up the stain. Special stains such as mucicarmine or periodic acid Schiff stain or Alcian blue reveal the secretory products. The cytoplasm in the basal portion may show a basophilic staining due to abundant rough endoplasmic reticulum. These mucous cells are arranged around a relatively larger lumen to form a mucous acinus which is tubular in shape (Fig. 11.3a). The mucous acinus is larger in size than a serous acinus with more number of cells.

Electron Microscopic Structure of Mucous Cells Electron microscopic examination reveals various cytoplasmic components of the cells and specialization of plasma membrane (Fig. 11.3b). The mucous cells show all the synthetic organelles and therefore indicate that these cells are specialized and active in protein and carbohydrate synthesis.

Specialization of Plasma Membrane The plasma membrane surrounding the cells shows various structural specializations similar to that of serous cells. Basal plasma membrane is irregular with multiple foldings which may extend laterally even beyond the boundary of the cell to interdigitate with those of the adjacent cells. But the basal plasma membrane extensions are much less extensive than serous cell. This specialization increases surface area of the cell for diffusion of water and minerals required for the saliva from tissue fluid. The basal plasma membrane is supported by a basement membrane. Plasma membrane at the lateral region and apical portion of the cell also shows foldings called microvilli which increase the surface area of secretion. The cells on the lateral region have complex interrelationship with adjacent cells. The mucous cells posses apical junctional complex similar to the serous cells. Near the apical portion, adjacent cells are connected by intercellular junctions like tight junctions, intermediate junctions and desmosomal junctions. These junctions help to hold the cells together and prevent leakage of material from the lumen. Between the adjacent cells there are intercellular

spaces or canaliculi that extends from the lumen, which is sealed off by intercellular junctions. The intercellular canaliculi are relatively less distinct in mucous acini. If serous demilunes are present in relation to a mucous acini, the intercellular canaliculi extends from the lumen, between the cells to the serous cells and serve as a delivery route for serous secretion.

Cytoplasmic Components Cytoplasm of a secretory cell shows cytoplasmic organelles required for synthesis and storage of proteinaceous materials. Organelles are not so abundant as in serous cells. All the synthetic cytoplasmic organelles are mainly located in the peri-nuclear area and are relatively less in number and are chiefly restricted to the basal region of cells. Rough endoplasmic reticulum and mitochondria are dispersed in the basal and lateral region of nucleus. But mitochondria are few in number and rough endoplasmic reticulum is less extensive. A prominent Golgi apparatus consisting of several stacks (10–12 sacules) are seen lateral and apical to the nucleus, compressed between RER and secretory droplets. One of the major differences observed from that of serous cell is, considerably greater Golgi apparatus which indicates a greater carbohydrate metabolism. In the Golgi complex carbohydrate component is added to protein to synthesize glycoprotein of mucin. Golgi complex is also involved in proteolytic processing steps and trimming of oligosaccharides. The diluted protein received from rough endoplasmic reticulum is concentrated in the Golgi complex, which is a required step for efficient intracellular storage.

Fig. 11.3a: Light microscopic structure of mucous acinus

Fig. 11.3b: Electron microscope structure of mucous cell

The mechanism of release of stored secretory products in the apical

cytoplasm may be variable. The droplet may fuse with the plasma membrane and release the content in the same way as that of exocytosis in serous acini. Alternatively the mucin droplet may be discharged with the limiting membrane intact. This is achieved by the fragmentation of plasma membrane which occur after the fusion of secretory droplet. During rapid discharge the entire mucin along with some cytoplasmic components are spilled through this break into the lumen.

Mixed Acinus In mixed salivary glands both serous and mucous acini are seen, which vary in proportion based on the type of gland, i.e. predominantly serous or predominantly mucous. Along with these, there are few mixed acini (Fig. 11.4). The basic secretory unit of a mixed acinus is a tubular mucous acinus. At the blind end this acinus serous cells are arranged to form a crescent shaped structure called demilune of Gianuzzi (demilune-half moon). The secretions of the serous cells reaches the lumen through the intercellular canaliculi present between the mucous cells.

Myoepithelial Cells Myoepithelial cells are contractile cells found to be embracing/enveloping the secretory end piece and the first portion of the ductal system, the intercalated ducts. These cells are epithelial in origin, but exhibit contractile function like muscles, hence the name myoepithelial cells. The myoepithelial cells are ectodermal in origin. But it is not clear whether it is from intercalated duct reserve cells or from neural crest cells. Myoepithelial cells are situated between the basal plasma membrane of parenchymal cells and basement membrane supporting the secretory unit or duct. Shape varies depending on location. Cells associated with acini are dentritic cells with a cell body containing the nucleus and 4–8 cytoplasmic processes extending from the cell body, each with two or more secondary branches. Therefore these cells are compared to an ‘Octopus sitting on a rock’. The cell body is located in a region where basal region of 2–3 secretory cells come together. The cells processes run parallel to the long axis of acinus and cradle it like a basket. Therefore, these cells were called as ‘basket cells’.

Fig. 11.4: Light microscopic structure of mixed acinus

Good to Know Demilunes in mixed ocini-Fixation Artifacts? Yamashina et al published a scientific article “The serous demilune of rat sublingual gland is an artificial structure produced by conventional fixation” (HistolCytol 62: 347–354) claiming that demilunes are ‘created’ due to fixation artefacts. These researchers proposed this concept based on their findings in a study conducted using rapid freezing of the salivary gland tissue in liquid nitrogen, followed by rapid freeze substitution with osmium tetroxide in cold acetone. They demonstrated that both mucous and serous cells, aligned in the same row to surround the lumen of the secretory acinus. Sections from the same specimen fixed using conventional methods showed swollen mucous cells with enlarged secretory granules and typical serous demilunes at the periphery with slender cytoplasmic processes interposed between the mucous cells. The process of demilune formation was explained, to be caused by expansion of mucinogen of secretory granules, during routine fixation. This expansion increases the volume of the mucous cells and displaces the serous cells from their original position, thus creating the ‘demilune effect’. However Tandler raised many queries about this concept and

claimed that demilunes are real, basic units of salivary gland structure [Tandler B (2014) Are Demilunes in Mixed Salivary Glands Real or Fixation Artifacts? A Critique. J CytolHistol 5: 218.] The myoepithelial cells are attached to parenchymal cells by desmosomal junctions. The cell body contains the nucleus and few cytoplasmic organelles such as rough endoplasmic reticulum, mitochondria Golgi complex, etc. which are restricted to the perinuclear cytoplasm. The processes are filled with fine microfilaments (actin and myosin) and some dense bodies therefore resemble the smooth muscles. In addition cytoplasm of these cells also exhibit cytokeratin intermediate filaments, confirming epithelial origin. Myoepithelial cells are not readily identifiable in routine hematoxylin and eosin stained sections. However, to detect these cells histochemical tests that can demonstrate ATPase reaction or electron microscopic examination can be used. Demonstration of the cytoplasmic filaments by histochemical or immunofluorescent technique can also be used in identification of myoepithelial cells. Myoepithelial cells associated with intercalated ducts are spindle shaped with few processes, running parallel to the ducts which seldom divide. Functions of Myoepithelial Cells Since the myoepithelial cells have a contractile property, they help to squeeze and expel the secretory material from lumen to the ducts. They also prevent the back flow of saliva from the duct back to the lumen. Myoepithelial cells support the acinus by bracing it like basket. Thus, prevent over distention and subsequent disruption by accumulation of secretory product. Contraction of myoepithelial cells facilitates rapid expulsion of secretory material in mucous cells by causing rupture of plasma membrane. Contraction of myoepithelial cells surrounding the intercalated ducts shorten and widen the duct and thereby reduces peripheral resistance and helps to maintain patency. Extension of the processes of these cells on to proximal surface of associated secretory acini may serve to align the lumen of acini and duct during contraction, which facilitate flow of saliva. Myoepithelial cells may have a role in maintaining cell polarity and structural

organization of secretory end piece. The proteinase inhibitors and antiangiogenesis factor secreted by myoepithelial cells have tumour suppressor property and protect the glandular tissue from invasive epithelial neoplasms. Myoepithelial cells may have a possible role in modifying the concentration of saliva by decreasing the surface area of secretory apparatus exposed to interstitial tissue.

Ductal System The ductal system of salivary gland is composed of network of ducts where the smaller ducts join to form larger caliber ducts. The intralobular ducts, i.e. the intercalated and striated ducts join together to form interlobular ducts. The interlobular ducts join together to form a lobar duct which drain a lobe of the gland. The lobar ducts join to form interlobar duct which runs in the connective tissue between the lobes and is continued as terminal excretory duct. In the ductal system, microscopically three structurally different sequential segments can be identified: Intercalated duct and striated duct which are intralobular and excretory duct which is interlobular and inter lobar in location (Fig. 11.5). Intercalated ducts: These are the smallest diameter duct at the first portion of ductal system which is seen as a continuation of lumen of secretory acini. The intercalated ducts carry the saliva from the lumen to the striated duct. These ducts are inconspicuous and vary in length in different glands: Being least prominent in mucus-secreting salivary glands (sublingual glands) and particularly long and prominent in serous glands (parotid gland).  The intercalated ducts are intralobular ducts found among the secretory acini. Light microscopically these ducts are seen as small diameter structures lined by a single layer of cuboidal cells with faintly eosinophilic cytoplasm and centrally located nucleus. Ultrastructurally the lining cells of the intercalated duct show some resemblance to secretory cells. The cytoplasm contains basally located rough endoplasmic reticulum, mitochondria, Golgi complex, few secretory granules, etc. The intercalated ducts are supported by basement membrane and adjacent cells are attached to each other by intercellular junctions. Myoepithelial cells are found between basal plasma membrane of lining cells and supporting basement membrane. A few

undifferentiated mesenchymal cells are also present.  The intercalated ducts not only act as a conduit for saliva, but also contribute some materials to it. Striated duct: The intercalated ducts are continuous with the striated ducts which carry the saliva to the excretory duct. They have a comparatively larger lumen lined by a single layer of columnar cells, well supported by connective tissue. These cells have a centrally placed round nucleus and abundant eosinophilic cytoplasm. The basal portion of the cells present a striated appearance, thereby the name striated duct. Electron microscopically the lining cells of striated ducts show some similarity to cells involved in water electrolyte balance (Fig. 11.6). These cells show numerous infoldings at the basal part of plasma membrane which helps to increase the surface area of basal plasma membrane. Many large mitochondria are seen in the cytoplasm which are arranged within these infolding with long axis parallel to the infoldings and to each other. These basal infoldings along with mitochondria is responsible for the striated appearance under light microscope.

Fig. 11.5: Ductal system of salivary glands

Fig. 11.6: Striated duct lining cell

Excretory duct: The striated ducts are followed by larger excretory duct which constitute the principal ducts of each of the major glands. The main or terminal excretory duct which drains the saliva to the oral cavity is formed by the continued confluence of interlobular excretory duct.  Excretory ducts are the largest ducts of the salivary gland, the structure of which vary in different portions. In the initial part, near the striated duct, the excretory duct is lined by tall columnar cells with occasional basal cells. As the duct becomes larger the lining gradually becomes pseudostratified with more basal cells and few goblet cells. The ductal portion near its opening to the oral cavity is lined by stratified squamous epithelium which merges with the oral mucosal lining. Oncocytes may be present among the lining cells of the excretory duct.

Functions of Ductal System Ductal system acts as a passage through which the saliva secreted by acini reaches the oral cavity. The saliva that is secreted by acini is called primary saliva. This primary saliva undergoes modifications as it passes through various ducts and the secretion that comes out of the excretory duct is called secondary saliva.

Intercalated ducts contribute some secretory materials to saliva. They also release lysozymes and lactoferrin, two important antimicrobial components to saliva. Intercalated duct and striated duct reabsorbs some amounts of proteins. Striated ducts secretes some glycoproteins such as and kallikrein and epidermal growth factors to saliva. An important function of striated duct is to modify the electrolyte concentration of the saliva. Saliva that enters the striated duct is hypertonic or isotonic with more concentration of Na+, Cl– ions and less concentration of K+ and HCO3–. The structure of striated duct is such that it has Na+ pumping ability. The striated duct lining cells actively reabsorb the Na+ ions from the lumen and secrete K+ actively in exchange of Na+. The Na+ ions reaching the cell is pumped out to the tissue fluid, creating a concentration gradient which leads to further Na+ reabsorption from the lumen. Therefore the Na+ concentration of saliva becomes greatly reduced, whereas K+ concentration becomes increased making the saliva hypotonic. Along with Na+, Cl– are also reabsorbed passively as it follows the same chemical gradient. As an exchange for Cl–, HCO3– ions are secreted into saliva. Therefore the saliva coming out of striated duct has less of Na+ and Ch concentration and more of K+ and bicarbonate ions. Since the ducts cannot alter the water level, alteration in ions make the saliva hypotonic. However, when secretion is very rapid, the saliva remain isotonic to hypertonic, because the process of reabsorption and secondary secretion systems cannot keep up with the rate of primary secretion. Excretory duct releases mucin into the saliva because of the goblet cells in the lining epithelium. Excretory ducts also assist the striated duct in changing the tonicity of saliva.

Connective Tissue Components The connective tissue component of salivary gland forms a well formed capsule surrounding the glandular structure. The capsule is more distinct and extensive in parotid. Connective tissue septa extends from the capsule in between the parenchymal components and divide the gland into larger

compartments called lobes and smaller compartments called lobules. The connective tissue supports the ducts and carries blood vessels, lymphatics and nerves that supply the gland. The thickness of connective tissue is higher in relation to excretory duct. The connective tissue is composed of parallely arranged collagen fibers, with fibroblasts, fat cells, defense cells like macrophages, mast cells and plasma cells and few lymphocytes, etc. The ground substance of connective tissue is composed of proteoglycans and glycoproteins. Extensive capillary network is observed in the connective tissue which ensure adequate supply of water and electrolytes for saliva. Vascular channels that enter the duct along the excretory duct, follow the branching ducts to reach the lobes and lobules.

Nerve Supply Salivary secretion is mediated by innervating nerves. The salivary glands are supplied by parasympathetic and sympathetic arms of the autonomic nervous system, which travel to the glands by separate routes. Parasympathetic innervation to the salivary glands is carried via cranial nerves; parotid gland from the glossopharyngeal nerve (CNIX) via the otic ganglion from which the auriculotemporal nerve carries parasympathetic fibres; the submandibular and sublingual glands from the facial nerve (CN VII) via the submandibular ganglion. Direct sympathetic innervation of the salivary glands takes place via preganglionic nerves in the thoracic segments T1–T3 which synapse in the superior cervical ganglion. Fibres from this ganglion travel along the external carotid artery to reach the glands. Once in the glands, the nerves follow the course of blood vessels and undergo extensive branching reach up to the adjacent region of acini. The axons from each type of nerve intermingle and travel together in association with Schwann cells, forming Schwann-axon bundles. The nerve endings, maintain two types of neuro-effector relationships with salivary parenchymal and myoepithelial cells: Hypolemmal or intraepithelial type (within the parenchymal basement membrane): In this type myelinated axons that split off from the nerve bundle penetrate the basement membrane of the acinus to reach very close to secretory cells. The distance between the secretory cell and nerve ending is only 10–20 nm. The axons show varicosities which are considered as

neuroeffector site. These varicosities contain chemical neurotransmitters such as nor epinephrine and acetyl choline stored in small vesicles. Afferent nerves are found to form a hypolemmal association with the epithelial cells of main salivary ducts. Epilemmal or subepithelial or interstitial type (outside the parenchymal basement membrane): In this, the axons remain in the connective tissue and do not penetrate the basal lamina. Here the distance between the secretory cell and axon is more and is around 100–200 nm. The neuro-transmitters from the varicosities of nerve axons have to diffuses through the basal lamina to reach the secretory cells. Salivary blood vessels receive epilemmal innervations by both sympathetic and parasympathetic axons. The relative frequencies of either type of nerve differ greatly between glands and species. The classical transmitters for parasympathetic axons is acetylcholine and substance P while in sympathetic axon is noradrenaline. At least four types of influence can be exerted on salivary parenchymal cells by the nerves: hydrokinetic (water mobilizing), proteokinetic (protein secreting), synthetic (inducing synthesis), and trophic (maintaining normal functional size and state). Both parasympathetic and sympathetic stimuli result in an increase in salivary gland secretions. However, increased activity of the sympathetic nervous system can also inhibits saliva secretion, via vasoconstriction, thereby decreasing the volume of fluid in salivary secretions, producing an enzyme rich mucous saliva. To sum up, parasympathetic stimulation results in secretion of large amount of watery saliva with low organic components while sympathetic stimulation produces relatively less quantity of thick, enzyme rich saliva. (Age changes—refer page 324, functions of saliva— refer page 292) Difference between serous and mucous acini Serous acini Mucous acini Circular or round in shape

Ovoid or tubular in shape

Smaller in size

Larger

Composed of less number of cells

More number of cells

Has small lumen

Wider lumen

Cells are pyramidal in shape

Cells are shape

pyramidal/columnar

in

Nucleus is round and placed at Nucleus is flattened and pressed basal 1/3rd of the cell against basal plasma membrane of the cell Apical cytoplasm appears eosinophilic because of zymogen granules

Apical cytoplasm appears empty in H and E sections because of mucin

Clinical Considerations A number of disease conditions can involve salivary glands which include a. Developmental defects like aplasia (lack of development) hypoplasia (under development) b. Infections that may be caused by virus or bacteria referred to as sialadenitis; causing pain and swelling of salivary gland. Mumps is a common viral sialadenitis primarily affecting parotid gland. c. Sialolithiasis is a condition characterized by intermittent swelling and pain particularly while eating, caused by blocking of salivary flow by sialolith (stone in the salivary duct). d. Sjogren’s syndrome is an autoimmune disorder affecting the salivary gland resulting in marked reduction in salivary secretion resulting in xerostomia or dry mouth. e. Cysts of salivary gland—mucocele is an example for cysts involving saliary gland, frequently, the minor salivary glands of the lower lip. f. Benign or malignant tumors—a number of benign and malignant tumors develop in the salivary gland tissue with abnormal proliferation of ductal, acinar or myoepithelial cells, each one causing swelling and other manifestations, e.g. of benign tumours are pleomorphic adenoma

and monomorphic adenoma. Malignant tumours are adenoid cystic carcinoma, muco-epidermoid carcinoma, acinic cell carcinoma, etc.

12 Temporomandibular Joint

Introduction Anatomy and histology of TMJ Ligaments of TMJ Movements of TMJ Clinical considerations

T

emporomandibular joint (TMJ) is the joint or articulation between the movable mandible and fixed temporal bone of the cranium. It is a ginglimo diarthrodial synovial joint. The joint is capable of a combination of sliding and a hinge movement or rotation and both right and left joints move together. Articulation is achieved by two joints between the condyles of mandible and glenoid fossa.

ANATOMY AND HISTOLOGY OF TMJ TMJ comprises of two bony structures and interposed fibrous disc, enclosed in a fibrous capsule (Fig. 12.1).

1. Bones Forming Articulation or Articulating Surfaces Bony element of the joint is made up of the condyle below and articular surface of glenoid fossa above.

Condyle

Condyle is a large solid oblong structure which is wider medio-laterally (20 mm) than anteroposteriorly (10 mm). It is noticeably convex capsule when viewed from the side, but only slightly convex when viewed from the front. The long axis of each condyle inclines slightly backward and medially. Articulating surface is convex and is located on the superior and anterior surface of the head of the condyle. The anterior border of the articulating surface is distinctly marked. A triangular depression beneath this border marks the insertion of the lower fibers of lateral pterygoid muscle. The medial and lateral poles of the condyle are also distinct.

Fig. 12.1: Temporomandibular joint

The Articular Surface of Temporal Bone The articular surface of temporal bone consists of concave posterior part called articular fossa and a convex anterior part called articular tubercle or eminence. Articular fossa (glenoid fossa or mandibular fossa) is an ovoid depression in the temporal bone just anterior to auditory canal. The boundaries of the fossa can be readily determined. The boundaries are, anteriorly articular eminence, medially the spine of sphenoid, laterally root of zygomatic process, posteriorly the tympanic plates of petrous portion of temporal bone. Articular eminence is the bony prominence located immediately anterior to the articular fossa.

Histology of Articulating Surfaces

The bony surface of the head of the condyle is made up of a dense compact bone with cancellous bone in the center. The cancellous bone contains red cellular marrow which may be replaced by fatty marrow in older individuals. As age advances, the trabeculae of cancellous bone increases in thickness, thereby reducing the marrow spaces. The trabeculae are found to be radiating from the neck of the condyle and end at the cortex at right angle. The articular fossa is lined by a thin layer of compact bone. The articular eminence has a core of cancellous bone covered by a layer of compact bone. Articular fibrous covering: The articular surface of the condyle and the articular fossa are composed of four distinct layers. The most superficial zone is called articular zone and is composed of fibrous tissue in contrast to the hyaline cartilage covering of articular surfaces of other synovial joints. This fibrous layer consists of few fibroblasts scattered in a dense largely avascular layer of type I collagen fibers which are arranged in bundles oriented nearly parallel to the articular surface. These connective tissue may contain few cartilage cells which increase with age. The second zone is proliferative zone which is highly cellular and composed of undifferentiated mesenchymal tissue. This layer is responsible for proliferation of articular capsule in response to functional demand. The proliferative zone also plays an important role in remodeling and repair of articular surfaces. Third zone is fibrocartilagenous zone made up of bundles of collagen fibers arranged in a crossing pattern and some in radiating pattern. This layer provides resistance against compressive or lateral forces. The fourth zone is calcified zone made up of chondrocytes and chondroblasts distributed throughout the articular cartilage. This zone provides an active site for remodeling activity as endosteal bone growth proceeds. Articular fibrous covering layer is fairly of even thickness and may be particularly thick on the articular surfaces which oppose one another, i.e. in the anterosuperior surface of condyle and on inferoposterior surface of articular eminence of temporal bone.

2. Articular Capsule Articular capsule is a dense collagenous sheet of tissue or a sac that encloses the joint space. The articular capsule is circumferentially attached to the rim of glenoid fossa and articular eminence above and to the neck of the condyle

below. The anterior portion of the capsule is attached above to the ascending slope of the articular eminence and below to the anterior margin of condyle. The posterior portion is attached above to the squamotympanic fissure and below to the posterior margin of ramus of mandible, adjacent to neck of mandible. Anterolateral aspect of the capsule may be thickened to form temporomandibular ligament. Posterior fibers of the capsule blend with articular disc as they traverse from temporal bone to mandible.

Histology of Articular Capsule Fibrous capsule is composed of two layers; Outer fibrous layer of dense connective tissue and an inner layer termed as synovial membrane. Synovial membrane lines the inner aspect of fibrous capsule and therefore forms the lining of joint cavity. The inner surface of the synovial membrane is thrown into folds giving rise to villi like processes projecting into the joint cavity. Histology of the synovial membrane varies in different regions. The synovial membrane consists of two layers: inner cellular intimal layer resting on a highly vascular subintimal layer (Fig. 12.2). The subintimal layer is composed of loose connective tissue containing blood vessels, scattered fibroblasts, macrophages, mast cells etc. Some elastic fibers are also present along with collagen fibers which prevent the folding of membrane which might otherwise become entrapped in between articular surfaces. Intimal layer consists of few layers (1–4) of synovial cells, distributed in an amorphous intercellular matrix. These cells are not attached to each other and do not form a continuous layer. Three types of cells are mainly observed in the intimal layer. They are: Type A cells (macrophage like cells): These cells have irregular outline with plasma membrane invaginations. Cytoplasm is rich in mitochondria, Golgi apparatus and lysosomes while rough endoplasmic reticulum (RER) is less distinct. Type A cells have phagocytic properties. Type B cells (fibroblast like cells or secretory—S cells): These cells are rich in RER and involved in synthesis of hyaluronic acid which is found in synovial fluid. Third type of cells have cellular morphology between the other two types.

Joint cavity contains approximately 1 ml of synovial fluid which is formed by diffusion of plasma from the rich capillaries of subintimal layer, to which proteins and hyaluronic acid secreted by fibroblast like cells are added. Synovial fluid also contains few synovial cells and defense cells.

Fig. 12.2: Synovial membrane

Functions of Synovial Membrane Lubrication: Helps in lubrication of the joint by providing a fluid environment for the joint. Nutritive: Provides nutrition to the avascular fibrous tissue of joint, i.e. articular fibrous covering and the center part of the articular disc. Regulatory: Controls the movement of nutrients, electrolytes and other materials to the synovial fluid Secretory: The intimal cells secrete proteins and hyaluronic acid to the

synovial fluid. Phagocytic: Type B cells are phagocytic and therefore help in debriding.

3. Articular Disc Articular disc or the meniscus is a tough biconcave pad of dense fibrous connective tissue, located between the condyle and articular surface of temporal bone, i.e. the glenoid fossa and articular eminence. The disc is thinnest at the center (about 1 mm) and thicker towards the periphery (2–3 mm). Varying thickness of the disc has lead to the description of four distinct regions namely anterior band, intermediate zone, posterior band and bilaminar region. The shape confirms to the articular surfaces to which it is opposed. The upper contour of the disc is concave in the anterior region to fit under articular eminence and convex posteriorly and loosely rest against articular fossa. The lower surface of the disc is concave in both directions thus adapting to the upper surface of mandibular condyle. The medial and lateral margins of the disc blend with the capsule. In the anterior region disc is divided into two lamellae, the upper one running forward to fuse with capsule and periosteum in the anterior slope of articular eminence while the lower one runs down to attach to the front of neck of the condyle. The region of disc between upper and lower lamellae merges with the capsule or with lateral pterygoid muscle. Posteriorly also the disc is divided into two lamellae, upper lamellae consisting of fibrous and elastic tissue fusing with capsule and inserting into the squamotympanic fissure. The lower lamella is nonelastic as it is composed of only collagen and turns down to blend with periosteum of neck of condyle. Between the lamellae, loose highly vascular connective tissue is found which is called bilaminar zone. The articular disc divides the joint space into upper compartment called temporo-discal which is between disc and temporal fossa and a lower compartment called condylo-discal situated between disc and condyle. Lower joint allows the rotational movement of head of the condyle which is also called hinge movement. Upper joint space allows a translatory movement anteriorly along the slopes of the articular eminence to produce an anterior and inferior movement of the jaw.

Histolosy of Articular Disc Articular disc is composed of dense fibrous tissue with tightly packed interlacing collagen fibers. The fibroblasts are elongated with long processes. A few elastic fibers may be present. As an age change, in older persons articular disc may show cartilage cells. These cartilage cells may be increasing the resilience and resistance of fibrous tissue. The center portion of the disc is devoid of blood vessels and nerves while periphery is highly vascular. Periphery of articular disc is attached to the fibrous capsule. Anterior part of the disc fuses with the capsule while posteriorly it is loosely attached. Being loosely attached posteriorly the disc moves with head of the condyle but only about half as far.

Functions of Articular Disc Divide the joint cavity into two compartments, therefore allowing different types of the mandibular movements. Upper joint cavity mediates a translatory or gliding movement while the lower joint cavity allows rotation. Since it is a soft tissue component between two hard tissue components of the joint, it prevents rubbing or friction between these components, therefore reducing physical wear. Articular disc acts as a cushion against heavy load and therefore helps in shock absorption Stabilizes the condyle by filling up the space between the articulating surfaces. The proprioceptive nerve fibers present in the anterior and posterior portion of disc help to regulate movements of condyle. Assists in lubricating mechanism (synovial fluid) Articular disc being loosely attached posteriorly, the disc moves with head of the condyle but only about half as far and prevents, undue forward movement of condyle. Helps in distribution of weight across the joint by increasing the area of contact which may thus prevent wear.

LIGAMENTS OF TMJ 1. Capsular Ligament Capsular ligament or articular capsule is a fibrous sac that encloses the joint cavity. It is attached to the articular margins of the temporal bone superiorly and to the neck of condyle inferiorly. An articular disc intervenes between the two articular surfaces and is attached peripherally to the inner surface of the capsule. The capsule is thin and loose between the temporal bone and articular disc, but between the disc and mandible it is thicker and stronger. It is lined by synovial membrane. Anteriorly the tendon of lateral pterygoid muscle is inserted into it.

2. Temporomandibular Ligament Temporomandibular ligament or lateral ligament is a strong fan-shaped ligament functioning to reinforce the lateral wall of the articular capsule and thus act to limit the lateral and posterior movements of the joint. It is attached superiorly to the articular tubercle and a segment of the zygomatic process and runs posteroinferiorly to attach to the condyle and posterolateral aspect of the neck of mandible.

3. Accessory Ligaments There are two accessory ligaments which do not contribute to support of the temporomandibular joint, but facilitate and limit the movements. Accessory ligaments include: Sphenomandibular ligament extends from the spine of the sphenoid to the lingula and lower margin of the mandibular foramen. It represents the unossified intermediate part of the sheath of the Meckel’s cartilage of the first pharyngeal arch. Over movement of the mandible is limited by this ligament. Stylomandibular ligament extends from the lateral border of the styloid process to the posterior border of the ramus of the mandible above its angle. It is the thickened part of the investing layer of the deep fascia of the neck. It separates the parotid gland from the submandibular salivary gland. This ligament participates in limiting the protrusive movement.

These ligaments are thought to play a significant role during protrusion and depression of the jaw.

MOVEMENTS OF TMJ Temporomandibular joint exhibits two types of movements; namely rotation (hinge movement) and translation (gliding movement). The upper compartment of the joint shows anteroposterior gliding movement during which condyle and articular disc move as a single unit against the glenoid fossa. The lower compartment shows a hinge movement during which condyle moves against the articular disc and glenoid fossa which together act as a single unit. The mandibular movements are determined by: Condylar guidance, which is the mandibular guidance generated by condyle and articular disc traversing the contour of glenoid fossa. The glenoid fossa along with articular eminence forms an S-shaped path along which the condyle moves. This shape of the glenoid fossa, which determines the path of movement of condyle, is called condylar guidance. Incisal guidance: It is defined as the influence of the, contacting surfaces of the mandibular and maxillary anterior teeth during mandibular movements. When the mandible is moved forward, the incisal edge of lower anterior teeth slide along the lingual surface of maxillary anterior teeth. Therefore the lingual surface of the maxillary anterior teeth guides the mandible during protrusive movement and is called incisal guidance. Neuromuscular factors: The movement of the jaw is determined to a greater extend by the muscles of mastication which coordinate together to move the mandible in a symmetric manner.

Types of Mandibular Movements 1. Rotation Mandible can rotate in three directions around three axis. Rotation around transverse or hinge axis: In this case, mandible rotates

around a horizontal axis extending from right side to left side condyle. This type of movement is seen during protrusive movement. Rotation around sagittal axis: This type of movement is seen in association with lateral movement. Here the mandible rotates around an imaginary axis running along the mid sagittal plane. During this movement, condyle on the working side (the side to which mandible is moved) moves laterally and upward while condyle of balancing or nonworking side moves medially and downward along the medial slope of glenoid fossa. Rotation around vertical axis: Mandible rotates around a vertical axis that runs through the condyle and posterior border of ramus of mandible. This type of movement occurs when the jaw is moved to the side (lateral movement). When a person opens the mouth to about 20–25 mm, the mandible moves around the horizontal axis. This kind of movements are observed while crushing the food or taking the food. After a certain degree of mouth opening, i.e. beyond 13° rotation of condyle in TMJ, the condyle begins to glide forward and downward along the anterior slope of glenoid fossa. This type of movement occurs while incising or grasping food.

2. Lateral Jaw Movements Lateral jaw movements can be of two types, i.e. lateral rotation and Bennett movement. Lateral rotation occurs when the mandible move away from mid sagittal plane. This can occur on right or left side and take place while chewing the food. During this lateral rotation condyles on either side do not share the common path of movement. The condyle on working side (the side to which the mandible moves) move laterally and upward, downward, forward, backward or outward. While the condyle on balancing or non-working side move forward downward and medially. Bennett movement: It is defined as the bodily lateral movement or lateral shift of mandible resulting in movements of the condyles along the lateral inclines along the mandibular fossae in lateral jaw movements. During this, lateral translation of the condyle occurs and the mandible shift by 1-A mm towards the working side. This movement of mandible is called

Bennett movement and is recorded in the region of translating condyle of the nonworking side. This lateral shift of condyle occurs along with or before lateral rotation. Bennett movement can be classified based on the time of shift in relation to the forward movement of non-working condyle. Immediate shift: This movement of mandible occurs before the forward movement of non-working condyle. Average movement is 0.75 mm. Precurrent side shift: This occurs during the first 2–3 mm of forward movement of non working condyle. During this mandible shift rapidly in initial stage (2–3 mm lateral shift) followed by less rapid shift. Progressive side shift or Bennett side shift movement. This lateral shift is gradual and occurs often 2–3 mm of forward movement of nonworking condyle.

Clinical Considerations 1. TMJ ankylosis: It is the stiffening (immobility) or fixation (fusion) of the joint which leads to chronic, painless limitation of the movements of the joint. This can be a true bony fusion or due to enlargement of the coronoid process, depressed fracture of the zygomatic arch, scarring from surgery, irradiation, infection, etc. Ankylosis of TMJ may result in restricted jaw movements, inadequate masticatory (chewing) function, restricted mouth opening, inhibited facial and physical growth, impaired speech, etc. 2. Luxation and subluxation: Luxation refers to complete dislocation of TMJ with head of the condyle moves anteriorly over the articular eminence into such a position that it cannot be returned voluntarily to its normal position. Luxation can be caused due to traumatic injury or is a result of yawning or opening mouth too wide for dental procedures, etc. Subluxation refers to partial or incomplete dislocation of TMJ, where the condyle may lie well anterior to the articular eminence. Such anterior positioning is normal for many people 3. TMJ pain dysfunction syndrome/Myofacial pain dysfunction syndrome: It is a psycho-physiologic disorder that involves the masticatory muscles and is characterized by dull, aching and radiating pain that is

exacerbated by mandibular function, tenderness on muscle palpation and limited movement of joint. This condition may be caused due to bilateral loss of posterior teeth, excessive alveolar bone resorption in patients with complete dentures, malocclusion, improperly occluding restorations, stress, etc. Patients may experience pain, muscle tenderness, limitation of mouth opening, clicking or popping sound while opening the mouth. 4. Degenerative disease: As other joints of the body, TMJ is prone to degenerative joint disease (arthritis and arthrosis). Arthritis is characterized by inflammation while arthrosis, by the presence of low and no inflammation. Osteoarthritis of TMJ results from wear and degeneration caused by normal use or parafunctional use of the joint. Rheumatoid arthritis, an autoimmune joint disease, can also affect the TMJs. Degenerative joint diseases may lead to defects in the shape of the tissues of the joint, limitation of jaw movements, and joint pain.

13 Maxillary Sinus Dr Usha Balan Introduction Anatomy and maxillary sinus Microscopic features of maxillary sinus Clinical considerations

P

aranasal sinuses are air filled spaces or pneumatic spaces situated as bilateral pairs in the frontobasal region of skull communicating with the nasal cavity. Various paranasal sinuses are maxillary, frontal, sphenoid and ethmoidal sinuses (Fig. 13.1). Maxillary sinus is a paranasal air sinus located in the body of the maxilla which communicates with the middle meatus of the nose. It is the largest of all the sinuses and also called Antrum of Highmore or Maxillary antrum. Development—Refer page 14, embyology.

Anatomy Maxillary sinus is the largest of the paranasal sinuses and is pyramidal in shape. It has a volume of approximately 15 ml (34 × 33 × 23 mm). Maxillary sinus has four sides and a base. The base faces medially towards the nasal wall and apex points laterally towards the body of the zygomatic bone. Anterior side is towards the facial surface of the body of maxilla, while the posterior side is towards the infratemporal surface of maxilla. Inferior side is bordered by the alveolar and zygomatic processes of maxilla and the superior side is bordered by orbital surface of maxilla. The base of the sinus is thinnest of all the walls. The floor may extend between the roots of

maxillary teeth.

Fig. 13.1: Paranasal sinuses

The maxillary sinus communicates with the nasal cavity through the ostium, which is located at the level of middle nasal meatus in the lower part of the hiatus semilunaris. A second opening is often present at the posterior end of the hiatus in middle meatus. Both the openings are above the level of the floor of sinus.

Microscopic Features Maxillary sinus is lined by respiratory epithelium composed of pseudostratified ciliated columnar epithelium (Fig. 13.2). Epithelial lining is made up of columnar cells of varying sizes arranged in a single layer on a basement membiane. The nuclei of the cells are placed at different levels giving the erroneous appearance of stratification. The cells on the superficial aspect have got cilia which help in the movement of the mucous secretions. Along with these ciliated columnar cells, nonciliated columnar cells, basal cells and goblet cells are also present. Goblet cells (Fig. 13.3) are unicellular secretory organs which are goblet shaped with a basally placed nucleus and apical cytoplasm filled with secretory products. Various cellular organelles like smooth and rough endoplasmic reticulum, and Golgi bodies are also located in the basal region. The secretions are rich in mucopolysaccharides and are finally secreted by exocytosis on the surface of epithelium. Since the secretory material is mucopolysaccharides, in a hematoxylin and eosin

stained section, the goblet cells appear empty. The epithelium is separated from subepithelial connective tissue by a basal lamina. Subepithelial connective tissue layer has collagen fibers and fibroblasts, protective cells such as lymphocytes, plasma cells and eosinophils. Minor salivary glands including both serous and mucous glands are distributed in the connective tissue. This layer is attached to the periosteum lining the bony wall of the maxillary sinus.

Functions of the Maxillary Sinus Conditioning of inspired air a.

b.

Warming up of the inspired air is possible, because of the rich arterial supply and it helps to maintain an even and tolerable temperature. Humidification of the inspired air is achieved and the possibility of irritation to the respiratory mucosa by dry air is prevented.

Fig. 13.2: Histology of maxillary sinus lining

Fig. 13.3: Pseudostratified ciliated epithelium with goblet cells

Reduction of weight of facial skeleton: Maxillary sinus lightens the weight of skull as it is filled with air. Increases the craniofacial resistance to trauma. Phonetic resonance and auditory feed back: The maxillary sinus may act as a resonating box for the voice. Sinuses also affect the conductance of voice to ones own ear. Increases the area for olfaction Production of lysozymes which has bactericidal property. Filtration: Because of the presence of mucous blanket and cilia, the maxillary sinus mechanically trap the micro-organisms and dust particles present in inspired air and helps in filtration of inspired air. Insulation: The temperature of inspired air can vary. The maxillary sinus may insulate the orbit from intranasal temperature variations. Growth and development of facial skeleton: Expansion of sinuses particularly maxillary sinus helps in rapid growth of facial skeleton.

Clinical Considerations 1. Oro-antrai communication/fistula is the connection that is established between oral cavity and maxillary sinus. This condition commonly arises as a result of complication of extraction of maxillary first and second molar especially when the bony wall separating sinus from root is very thin. Palatal root of maxillary first molar is found in very close

proximity to sinus and therefore any surgical manipulation or chronic periapical inflammation related to this tooth can erode the bone, establishing a communication between oral cavity and maxillary sinus. The communication might get epithelialized and establishes permanent connection between maxillary sinus and the oral cavity. 2. Developmental defects: such as agenesis (absence), hypoplasia (small sinus), supernumerary (extra) sinus may involve maxillary sinus. 3. Infection/Inflammation: Maxillary sinus is prone to infection and inflammation due to various causes and this condition is referred to as maxillary sinusitis. 4. Sinusitis and toothache: Infection from maxillary teeth may spread to the sinus and may be one of the possi ble cause for sinusitis. Likewise sinusitis may lead to toothache. Swelling and the concentration of mucus fluids resulting from sinusitis can build-up of pressure inside the sinus cavity and over the upper jaw bones. The nerves innervating the roots of the maxillary molar teeth which are in close proximity to the sinus may be affected by this pressure and the patient experiences a pain much similar to toothache. This is called a sinus toothache. The intensity of pain depends on the extend of sinus infection and swelling along with the proximity of the root endings to the infected sinus.

Section 3

Oral and Dental Anatomy 14. Introduction to Dental Anatomy 15. Deciduous Maxillary Anterior Teeth 16. Deciduous Mandibular Anterior Teeth 17. Deciduous Maxillary Molars 18. Deciduous Mandibular Molars 19. Comparison between Deciduous and Permanent Dentition 20. Permanent Maxillary Central Incisors 21. Permanent Maxillary Lateral Incisors 22. Permanent Mandibular Central Incisors 23. Permanent Mandibular Lateral Incisors 24. Permanent Maxillary Canines 25. Permanent Mandibular Canines 26. Permanent Maxillary First Premolars 27. Permanent Maxillary Second Premolars 28. Permanent Mandibular First Premolars 29. Permanent Mandibular Second Premolars 30. Permanent Maxillary First Molars 31. Permanent Maxillary Second Molars

32. Permanent Maxillary Third Molars 33. Permanent Mandibular First Molars 34. Permanent Mandibular Second Molars 35. Permanent Mandibular Third Molars 36. Occlusion

14 Introduction to Dental Anatomy Dr Rajeesh Mohammed PK and Dr Girish KL Human dentition Tooth and supporting structures Types of dentition and teeth Chronology and sequence of eruption Tooth numbering systems Terminologies used in dental morphology

H

uman face which plays a major role in visual recognition of an individual is constituted by many a feature. Of these, the teeth along with a pleasing smile always leave a striking impression. The feature of each tooth is of utmost importance for a dental student. This section aims in describing all the features that are required to enable the student in easier identification of normal features of the tooth. Dental anatomy/morphology is the field of anatomy dedicated to the study of the anatomical and morphological characteristics of the teeth. Dental morphology can be considered to be a subdivision of oral anatomy, which deals with the study of all the structures in and around the oral cavity. It is important to study dental morphology as it forms the background knowledge of all the subjects associated with dentistry. A thorough knowledge of dental morphology is therefore of a great importance and can be considered to be the first step in becoming a successful dentist.

Objectives of Dental Anatomy To describe the detailed morphology of individual tooth so that the normal features can be differentiated from abnormal. To use of appropriate dental terminology so as to communicate with other people in the dental field. To describe the detailed morphology of individual tooth. To describe the eruption sequence of the primary and permanent teeth. This knowledge helps in determining if a child has missing or impacted teeth, or some abnormality in growth and development. To describe the intra-arch and inter-arch relationship of the teeth and their effect on the health of the supporting structures. To impart proper restorative and esthetic treatment. As the process of evolution continued, the human beings have become specialized to enable them to cope up with the changing situations and different lifestyles. Those features which were of use were retained or modified and those which were not required were discarded. Same is the case with human dentition also, which have become modified to enable us to make the most out of our resources, that is, to chew food more efficiently and to extract nutrients more quickly and thoroughly. The dentition of the present day human being, when compared to the prehistoric man has undergone a lot of changes. These changes can be attributed to the change in lifestyle which includes use of cooked and refined food substances. The shape of an animal’s teeth is related to its diet. In carnivorous animals the teeth are more pointed and sharp, while in herbivorous animals the teeth have a broad occlusal surface. The dentitions of human beings are a combination of teeth that are seen in case of carnivorous and herbivorous animals. The size, shape, number, construction, location and lifespan of teeth reflect their function and their evolution history. We retain many of the early patterns from the ancient past. The order of eruption, the interdigitation of the teeth, the regional specializations of teeth into classes, and the replacement of deciduous teeth with permanent successors are among a few of those patterns. In humans two sets of teeth are present: primary and permanent teeth (called diphyodont), composed of different kinds of teeth (called heterodont),

inserted into sockets and connected to the bone by a suspensory ligament (called thecodont), teeth are arranged in opposing arches: maxillary (or upper teeth) and mandibular (or lower teeth) and can be divided along the mid sagittal plane, into left and right halves. Teeth fossilize more consistently than any other part of a mammal, and indeed many species of extinct mammals are known only from their teeth. Teeth are unique in their own way; they show features which may be similar or dissimilar to teeth in a person’s oral cavity, and also show characteristic features when compared to another individuals teeth. These features along with their positioning in the jaws makes teeth as a source of identification of victims in case of disasters, by the forensic odontologists.

Tooth (Latin: Dentes) The hardest calcified tissue present in the human body, which is normally present in the oral cavity, helping in mastication (chewing), phonation (speech), esthetics (appearance), self protection and attack (mainly in animals). Teeth are among the most distinctive features of mammal species. Teeth are very important part of the human body. Besides being an important member of the digestive system, the dentition also has a vital role to play in the facial appearance of the person. Teeth are often the focus of attention in a human face and therefore their health and appearance is of utmost importance for the psychological well being.

FUNCTIONS OF TEETH Mastication •

Teeth helps to tear, grind, and chew food in the first step of digestion, enabling salivary enzymes in the mouth to further break down food.

Appearance •

Speech

Teeth plays an important role in a person’s appearance. They support the tissues around the mouth and provide an appealing look to the face.

•

Teeth along with the lips and tongue, plays an important role in forming a clear and understandable speech. The role of teeth is of paramount importance, as speech plays a huge part in development of one’s personality and social acceptance.

Growth of jaws •

Teeth play a role in the growth of the jaws in some periods of life.

Self protection and attack •

Primarily in animals.

PARTS OF A TOOTH The tooth can be divided into (Fig. 14.1): Crown Root Cervix/neck

Fig. 14.1: Parts of a tooth

Crown Portion of the tooth that is covered by enamel is called crown. Clinical crown This term is used to describe the portion of the tooth that is visible in the oral cavity. Anatomic crown The entire portion of the crown that is covered by enamel is called anatomic crown. In case of an erupting tooth the clinical crown is shorter than anatomic crown, as the full anatomic crown is not yet exposed to the oral cavity. In contrast, in a tooth with gingival recession and root exposure, the clinical crown is longer than anatomic crown, because this include full anatomic crown and part of the root that is exposed to oral cavity.

Root The portion of the tooth which is covered by cementum is called root and is embedded within the alveolar bone; may be single or multiple (double or tripleroots).

Single-rooted Permanent Teeth All the anterior teeth Mandibular premolars Maxillary second premolar

Multi-rooted Permanent Teeth Two-rooted teeth Maxillary first premolars (one buccal and one lingual). Mandibular molars (one mesial and one distal).

Three-rooted teeth Maxillary molars (two buccal [1 mesiobuccal, 1 disto-buccal], and one palatal).

Single-rooted Deciduous Teeth All the anterior teeth

Multi-rooted Deciduous Teeth Two-rooted teeth Mandibular molars (one mesial and one distal)

Three-rooted teeth Maxillary molars (two buccal [1 mesiobuccal, 1 disto-buccal], and one palatal). Variations frequently occur. In single-rooted teeth, roots generally present a conical shape, narrow down towards the tip. The tip of the root is referred to as root apex. In multi-rooted teeth the root begins at the cervix as undivided portion and then divides at various levels. Clinical root is the portion of the root that is embedded in the jaw bone and covered by the gingival (gum) tissue and not exposed to the oral cavity. Anatomical root is the entire portion of the root covered by cementum.

Cervix or Neck The constricted portion of the tooth, where the anatomic crown and the root meets, i.e. junction between the enamel and cementum is referred to as cervix of the tooth.

STRUCTURE OF THE TOOTH AND SUPPORTING STRUCTURES The tooth is made up of three hard tissue components and one soft tissue component (Fig. 14.2). Hard tissue components are enamel, dentin and cementum. Soft tissue component is pulp.

Enamel The enamel is the outermost layer and covers the anatomic crown. It is the hardest and most highly mineralized tissue of the body. Enamel is translucent in nature and the color varies from light yellow to grayish white. Enamel is a nonliving tissue and is incapable of remodeling and repair. Specialized cells called ameloblasts forms enamel and the process of enamel formation is called amelogenesis.

Dentin Dentin is a hard, connective tissue which makes up the bulk of the tooth. It is covered by enamel on the crown portion and cementum on the root portion. It is located between enamel or cementum and the pulp chamber. Dentin is yellowish in color. Unlike enamel, dentin is a living tissue and responds to stimulus and the exposed dentin is often sensitive to cold, hot, air and touch. The hardness of dentin is lesser than enamel. The process of dentin formation is called dentinogenesis and odontoblasts are the specialized cells that form dentin.

Cementum The cementum covers the root portion of the tooth. It overlies the radicular dentin and joins the enamel at the cemento-enamel junction (CEJ). Cementum is yellowish in color and is softer in consistency than enamel. Formation of cementum takes place by the specialized cells called cementoblasts. Primary function of cementum is to anchor the tooth to the bony socket by providing a media for attachment of the periodontal ligament fibers.

Pulp Pulp is the only soft tissue component of tooth and occupies the central portion of the tooth. Pulp is a mesenchymal connective tissue that supports the dentin and is surrounded by dentin on all sides except at the apical foramen and accessory pulp canal openings, where it is in communication with periodontal soft tissue. The pulp consists of connective tissue, nerves and blood vessels, which enter the pulp through a small opening at the apex called apical foramen. It consists of cells (odontoblast, fibroblast,

undifferentiated mesenchymal cells, macrophages, immunocompetent cells), fibers and intercellular substance.

Fig. 14.2: Section of a tooth with supporting structures

Supporting Structures of Teeth Teeth are suspended in alveolar sockets of maxilla and mandible with the help of periodontal ligament. Periodontal ligament comprises connective tissue with bundles of collagen fibers attached on one side to cementum of tooth and other side to alveolar bone.

JUNCTIONS BETWEEN HARD TISSUES OF THE TOOTH Cemento-enamel Junction (CEJ) The cemento-enamel junction can be described as the junction between the enamel covering the crown and the cementum covering the root. This junction is located at the cervix of the tooth.

Dentino-enamel Junction (DEJ)

The junction between coronal dentin and the enamel of the tooth is referred to as dentinoenamel junction and is scalloped in nature. If this junction is weak, the enamel is separated from the dentin easily and the enamel is chipped off or lost.

Cemento-dentinal Junction (CDJ) Cemento-dentinal junction is the junction between radicular dentin and the cementum covering the root.

ARRANGEMENT OF TEETH (THE JAWS OR ARCHES AND QUADRANTS) The teeth are arranged in the jaw bones which follow a U-shaped arch form. The upper jaw is the maxilla and lower jaw is mandible. Therefore, the teeth in the maxillary arch are referred to as maxillary or upper teeth, while those in the mandibular arch are referred to as mandibular or lower teeth. The maxillary and mandibular arches can be divided along the mid sagittal plane into right and left halves. Accordingly, in humans, teeth are arranged in four quadrants, namely right maxillary, left maxillary, right mandibular, and left mandibular.

TYPES OF DENTITION AND TEETH In humans, varieties of teeth can be observed which can be grouped into different classes, based on form and function. In each class there will be one or more types of teeth exhibiting specific morphological characteristics. Classes of teeth in permanent dentition are: Incisors, canines, premolars and molars. In deciduous dentition incisors, canines and molars are present. Premolars are absent in deciduous dentition. The types of teeth in each class of deciduous and permanent dentition is as follows: Permanent Incisors

Deciduous a. Incisors

•

Central incisor

•

Central incisor

•

Lateral incisor

•

Lateral incisor

Canine

b. Canine

c.

Premolars

• •

First premolar Second premolar

Molars

Premolars absent

c. Molars

•

First molar

•

First molar

•

Second molar

•

Second molar

•

Third molar

DENTITION The term dentition is used to collectively consider upper and lower teeth. In human dentition two sets of teeth can be identified: Deciduous or primary dentition and permanent dentition.

Deciduous Dentition It is also known as the primary, baby, milk or lacteal dentition. There are 20 deciduous teeth in total of which 10 are present in the upper and 10 in the lower jaw. The deciduous dentition consists of 2 incisors, 1 canine (0 premolar) and 2 molars in one quadrant, i.e. 5 in one quadrant, 10 on one side and 20 in total (Fig. 14.3). There are no premolars in deciduous dentition. The first deciduous tooth erupts into oral cavity by the age of six months and the last one by two and a half to three years. The child has only deciduous teeth till the age of six years until the first permanent tooth erupts. Dental formula of a human deciduous dentition on left/right side

Permanent Dentition Permanent teeth are the second set of teeth formed in humans that replace the deciduous teeth in normal conditions. They are also called secondary dentition. There are 32 permanent teeth in total, 16 on either arch and 8 in each quadrant. The deciduous anteriors are replaced by the corresponding permanent anterior tooth and the deciduous molars are replaced by the premolars. The permanent teeth that replace the deciduous predecessors are called as successor or succedaneous teeth. The permanent molars erupt distal to the space occupied by the deciduous dentition. The permanent molars are called non-successor teeth ornon-succedaneous teeth as they do not have deciduous predecessors. The first permanent tooth usually erupts in the mouth at around six years of age and the last one usually erupts at around 18 years of age, but this can vary greatly between individuals. The permanent dentition consists of 2 incisors, 1 canine, 2 premolars and 3 molars in one quadrant, i.e. 8 in one quadrant, 16 on one arch and 32 in total (Fig. 14.4).

Fig. 14.3: Deciduous dentition

Fig. 14.4: Permanent dentition

Dental formula of a human permanent dentition on left/right side

The dentition will be in a transition period with both deciduous teeth and permanent teeth from the age of 6 years to 12 years, until the last deciduous tooth is exfoliated and replaced by permanent tooth. The period where there is presence of both deciduous and permanent dentition is called mixed dentition period and does not constitute a third stage of dentition.

Chronology of Human Dentition and Sequence of

Eruption The word chronology means a record of events in the order of their occurrence. Chronology of dentition denotes the time at which various events related to milestones of tooth development such as initiation, first evidence of calcification, crown completion, eruption and root completion occur (Table 14.1). Both deciduous and permanent teeth develop in the jaw bones and make their appearance in the oral cavity at different times. Root formation is completed only approximately two years after the tooth erupts into oral cavity. The order in which teeth appear in the oral cavity is described as sequence of eruption. The knowledge of chronology and sequence of eruption help a clinician to assess the dental age and detect the defect in developmental process. Table 14.1: Chronology of the human dentition

Sequence of Eruption of Deciduous Teeth First deciduous tooth that appear in oral cavity is central incisor followed by laterals, first molars, canines and then the second molars. Mandibular teeth erupt slightly earlier than maxillary teeth. Both right and left teeth erupt nearly at the same time.

Sequence of Eruption of Permanent Teeth First permanent tooth that erupt into the oral cavity is first molars at the age of 6 years followed by mandibular central and lateral incisors, maxillary central incisors, maxillary lateral incisors, mandibular canine, maxillary and mandibular first premolars, maxillary and mandibular second premolars, maxillary canine, maxillary and mandibular second molars and maxillary and mandibular third molars. Third molars erupt only by 17 to 21 years.

POSITION OF TEETH WITH RESPECT TO THE MIDLINE Anterior Teeth The term anterior teeth is used to describe those teeth which are closer to the midline; consists of 12 teeth in the front, facing the lips, 6 in each arch and includes 4 incisors and 2 canines (Figs 14.3 and 14.4).

Posterior Teeth These are teeth, which are further away from the midline. In permanent dentition, the posterior teeth consist of 10 teeth behind the anterior teeth, facing the cheek, which includes 2 premolars and 3 molars in one quadrant; 10 teeth on one side and 20 posterior teeth in total. In deciduous dentition, the posterior teeth consist of 8 teeth behind the anterior teeth, facing the cheek and include 2 molars in one quadrant; 4 teeth in one side and 8 in total (Figs 14.3 and 14.4).

SURFACES OF TEETH (Fig. 14.5)

Facial Surface This term is used to describe the surface of a tooth that “faced” toward the lips or cheeks. When there is a requirement to be more specific, terms like labial and buccal are used.

Labial Surface The surface of the tooth that is facing the lip. This term is used while describing anterior teeth.

Buccal Surface The surface of the tooth that is facing the inner cheek. This term is used while describing posterior teeth.

Lingual and Palatal Surfaces Linsual Surface The surface of the tooth that is closest to the tongue.

Palatal Surface The surface of the tooth that is closest to the palate. Although the term lingual is used generally to describe the inner surface of both maxillary and mandibular teeth facing the oral cavity proper, the term palatal is more appropriate in case of maxillary teeth because it is closer to the palate. The use of the term lingual may be restricted to the mandibular teeth.

Proximal Surface This term is used to describe sides of the tooth or the surfaces that lie next to an adjacent tooth. All teeth have two proximal surfaces, the mesial and the distal.

Mesial Surface Mesial surface is the surface of the tooth that is oriented toward or closer to

the midline of the dental arch.

Distal Surface Distal surface is the surface of the tooth that is oriented away from the midline of the dental arch.

Fig. 14.5: Surfaces of teeth

Except for the maxillary and mandibular central incisors, the mesial surface of the tooth contacts the distal surface of the adjacent tooth. The mesial surface of central incisor contacts the mesial surface of the adjacent central incisor. The distal surface of third molars does not contact any other tooth.

Occlusal and Incisal Surfaces This term is used to refer to the cutting or chewing surface of the tooth.

Occlusal Surface Occlusal surface is the broad chewing surface of posterior teeth.

Incisal Surface Incisal surface is the narrow cutting surface of anterior teeth. Since the incisal

or cutting surface of a newly erupted anterior tooth is narrow and resemble a ridge the term incisal ridge is preferred. As the teeth undergo physiological wearing, the ridge becomes flat and form a sharp angle with the labial surface and is then referred to as incisal edge.

TOOTH NUMBERING SYSTEMS Dental notations are the name given for the systems that are used to identify a tooth in relation to one another, to the midline and to the arches and helps in the documentation of these data. Tooth numbering provides the dentists with an essential shortcut in clinical record-keeping. Different dental notation systems are used by dentists worldwide for associating information to a specific tooth. Since tooth numbering systems are used like shortcuts, they are easier and save time. It allows everyone in the oral health team to efficiently share information amongst them and further provides those outside the team with clear and precise information about their work. The three most common systems are the FDI World Dental Federation notation, Universal numbering system, and Zsigmondy Palmer notation. Each of these systems has their own merits and demerits and no single system is superior to the other. Orientation of the chart in all the systems is traditionally “patient’s view”, i.e. patient’s right corresponds to notation-chart right. The designations “left” and “right” on the chart correspond to the patient’s left and right, respectively. Requirements of ideal tooth numbering system Simple to understand and teach Easy to pronounce in conversation and dictation Readily communicable in print Easy to translate into computer input Easily adaptable to standard charts used in general practice

Zsigmondy Palmer Notation/Grid System The Palmer notation is the commonly used system by dentists to associate information to a specific tooth. It was originally termed as “Zsigmondy

system” after the Austrian dentist Adolf Zsigmondy who developed the idea, using a Zsigmondy cross to record quadrants of tooth positions. This was modified later by Palmer and the system came to be known as Zsigmondy Palmer notation. In this system, the dentition is divided into quadrants and symbols (brackets) are used to designate, in which quadrant the tooth is found. The symbol ″⌋″ represents upper right quadrant, the symbol ″⌊″ represents upper left quadrant, the symbol ″⌈″ represents lower left quadrant and the symbol ″⌉″ represents lower right quadrant. A number is placed within these brackets, which denotes the position of the tooth from the midline. Permanent teeth are numbered 1 to 8, with the lowest digit assigned to the teeth closer to the midline (1 is permanent central incisor and 8 is third molar). The deciduous teeth are indicated by a letter A to E, with the starting letter ′A′ assigned to the teeth closer to the midline (′A′ is deciduous central incisor and ′E′ is deciduous second molar). Hence, the left and right maxillary central incisor would have the same number, ′1′, but the right one would have the symbol, ″⌋″, along with it, while the left one would have, ″⌊″. Even though the Palmer notation is used commonly, it has the drawback of not being able to record them using the conventional keyboard input and word processing software. Palmer notation is simple, easy to use and most often used by clinicians. Since the quadrant symbols are same for deciduous and permanent dentition, it becomes easy for use with basic understanding about different quadrants. However, the major drawback of this system is inability to record them using the conventional keyboard input and word processing software. Furthermore, using this system in verbal communication is not possible. Deciduous dentition—Palmer system

Permanent dentition—Palmer system

Universal Numbering System The universal numbering was first suggested by Parreidt in 1882 and uses numbers 1–32 for permanent teeth and uppercase letters A through T for primary teeth. The numbering starts from the upper right third molar and ends with the lower right third molar in a clockwise direction. Tooth number 1 is the patient’s upper right third molar and the numbering continues toward the front and across to the third molar tooth back on the upper left side (number 16). The tooth numbering continues by descending to the lower left third molar (number 17) and the numbering continues toward the front and across to the right third molar tooth (number 32). All teeth should be numbered, including those teeth that have been removed for any reason or have not erupted yet (e.g. wisdom teeth). In this system each tooth is assigned a unique number or alphabet, allowing easier use on keyboards and word processing software. Use of this system in verbal communication is possible. Nevertheless, memorizing the assigned number becomes difficult without adequate practise. Deciduous dentition—universal system

Permanent dentition—universal system

FDI World Dental Federation Notation The FDI (Federation Dentaire Internationale) World Dental Federation developed in 1971, a system to identify teeth with a number. This system is called the FDI Two-Digit Notation, also known as the ISO-3950 notation. Each tooth, deciduous or permanent is given a two digit number. The first digit indicates dentition, arch and quadrant. In permanent, the dentition is divided into quadrants which are numbered from 1 to 4, in a clockwise direction, starting from the upper right quadrant. The digit 1 is upper right quadrant, 2 is upper left, 3 is lower left and 4 is lower right quadrant. Similarly, the deciduous dentition is also divided into quadrants which are numbered from 5 to 8, in a clockwise direction, starting from the upper right quadrant. The digit 5 is upper right quadrant, 6 is upper left, 7 is lower left and 8 is lower right quadrant. The second digit indicates the position of the tooth relative to the midline (similar to Palmer notation). For permanent dentition, a number is assigned to a tooth from 1 to 8, starting from the central incisor (number 1) and moving backwards up to the third molar (number 8) in each quadrant. For deciduous dentition, a number is assigned to a tooth from 1 to 5, starting from the central incisor (number 1) and moving backwards up to the second molar (number 5) in each quadrant. The combination of these two numbers (quadrant code number and tooth code number which are pronounced separately) is the Two-Digit World Dental Federation Notation. Deciduous dentition—FDI system

Permanent dentition—FDI system

FDI system is internationally accepted and followed in many countries, ideal system for verbal communication and visual sense. Furthermore suitable for computer processing.

TRAIT CATEGORIES The human dentition is peculiar in that, it shows features which are similar or dissimilar to the adjacent and opposing tooth. Features that help to differentiate the dentition and teeth into different groups are called trait categories, which include:

Set Trait Teeth in primary and permanent dentition show some common characteristics so that they can be categorized as deciduous or permanent teeth. These features which help in differentiating permanent teeth from deciduous teeth are referred to as set trait. Example: Deciduous teeth are much smaller compared to permanent teeth(with few exceptions).

Arch Trait Common features observed in teeth of maxillary or mandibular arch that help in differentiating the maxillary (upper) teeth from the mandibular (lower) teeth are referred to as arch trait. Example: From the proximal aspect, crowns of all mandibular teeth show a lingual inclination.

Class Trait Features which help to categorize the teeth, depending on the prominent features into various classes like incisors, canines, premolars and molars. Example: Incisors have crowns compressed labiolingually for efficient cutting; canines have single pointed cusps for piercing food; premolars have two or three cusps for shearing and grinding; molars have 3–5 somewhat flattened cusps for grinding.

Type Trait Features that helps in differentiating different teeth within one class (incisors, canines, premolars and molars) into central and lateral incisors, first and second premolar, first, second and third molars. Example: Maxillary central incisor has a straight incisal edge while lateral incisor has a curved incisal edge. The term permanent maxillary central incisor denotes all the four trait categories, i.e. Permanent: Set trait Maxillary: Arch trait Incisor: Class trait Central: Type trait

DESCRIPTIVE DIVISION OF THE TOOTH 1. Division of Crown into Thirds The facial or lingual surface of the crown portion of the tooth can be divided into three portions in cervico-incisal/cervico-occlusal direction, by arbitrary horizontal lines into (Fig. 14.6a): Cervical third Middle third Incisal/occlusal third The facial or lingual surface of the crown portion of the tooth can also be divided into three portions in a mesio-distal direction by arbitrary vertical lines into (Fig. 14.6b): Mesial third

Middle third Distal third The mesial or distal surface of the crown portion of the tooth can also be divided in a cervico-occlusal/incisal direction into three parts by arbitrary horizontal lines into (Fig. 14.6c): Cervical third Middle third Incisal/occlusal third The mesial or distal surface of the crown portion of the tooth can also be divided in facio-lingual direction into three parts by arbitrary vertical lines into (Fig. 14.6d): Facial/labial third Middle third Lingual third

2. Division of Root into Thirds The root can be divided from the facial or lingual surface and the mesial or distal surface in a cervico-apical direction into three parts by arbitrary horizontal lines into (Figs 14.6a and c): Cervical third Middle third Apical third The root portion from the facial or lingual surface can be divided in a mesiodistal direction into three parts by arbitrary vertical lines into (Fig. 14.7b): Mesial third Middle third Distal third The mesial or distal surface of the root portion can be divided in facio-lingual

direction into three parts by arbitrary vertical lines into (Fig. 14.6d): Facial third Middle third Lingual third

Figs 14.6a to d: Division of teeth in thirds

Fig. 14.7: Division of the crown cervico-occlusally and root cervico-apically

LINE ANGLES AND POINT ANGLES Line angle is the junction between two surfaces that meet each other. The name of the line angle is based on the two surfaces which meet to form that line angle. Accordingly, posterior teeth have eight line angles, namely mesiobuccal, disto-buccal, mesio-lingual, disto-lingual, mesio-occlusal, distoocclusal, bucco-occlusal and linguo-occlusal. Mesioincisal and distoincisal line angles are not considered in anterior teeth and therefore anterior teeth have only six line angles; namely mesio-labial, disto-labial, mesio-lingual, disto-lingual, labio-incisal and linguo-incisal. Point angles are the point where three surfaces meet and are named by joining the name of those three surfaces which meet. Both anterior and posterior teeth have four point angles each. The point angles of anterior teeth are mesio-labio-incisal, mesio-linguo-incisal, disto-labio-incisal and distolinguo-incisal. The point angles of posterior teeth are mesio-bucco-occlusal, mesio-linguo-occlusal, disto-bucco-occlusal and disto-linguo-occlusal.

DESCRIPTIVE TERMS USED IN TOOTH MORPHOLOGY Teeth may show various anatomic landmarks in the form of elevations or depressions. These elevations and depressions help the tooth to interdigitate

with the opposing tooth and helps them in efficient functioning in mastication and also to withstand both the functional and parafunctional occlusal loads without damage. While explaining the morphology of teeth, different terminologies are used to describe these landmarks, the knowledge of which is very essential for understanding the subject.

Terminologies used While Describing the Crown A cusp is a point, peak, rounded elevation or mound on the crown portion of a tooth making up a divisional part of the occlusal surface. Each cusp has two inclines or slopes; the mesial cusp slope and the distal cusp slope which meet at different angles. All the cusps have a basic shape of gothic pyramid, having four ridges. The ridges that is associated with a cusp are: Mesial cusp ridge, distal cusp ridge, buccal cusp ridge and triangular ridge (in canines lingual ridge is seen instead of triangular ridge). They are seen on the occlusal surface of a cuspid, bicuspid, or molar tooth. Each cusp is representative of a center of calcification (a lobe) in the developing tooth. The number of cusps varies from each class and type of tooth. One cusp (cuspid) –

Deciduous and permanent canines

Two cusps (bicuspid) – –

Maxillary 1st and 2nd premolars Mandibular 1st premolars

Three cusps –

Mandibular 2nd premolars (may be even 2 cusps)

Four cusps – – – –

Permanent maxillary 2nd molars Permanent mandibular 2nd molars Deciduous maxillary 1st molar Deciduous mandibular 1st molar

Five cusps –

Permanent maxillary 1st molars (if cusp of Carabelli present)

– – –

Permanent mandibular 1st molars Deciduous maxillary 2nd molar (if cusp of Carabelli present) Deciduous mandibular 2nd molar

The cusps can be a functional cusp or nonfunctional cusp; in maxillary arch, the palatal cusp is the functional cusp and in mandibular arch, the buccal cusp is the functional cusp. A tubercle is a smaller elevation on some portion of the crown produced by an extra formation of enamel. They are variable in size and shape, but usually smaller than cusps and are considered to be deviations from the typical form. These occur on the marginal ridges of posterior teeth or on the cingulum of anterior teeth, e.g. cusp/tubercle of Carabelli. A cingulum is a bulge or elevation on the lingual surface of incisors or canines. It is the lingual lobe of an anterior tooth and makes up the bulk of the cervical third of the lingual surface. The term is derived from the Latin word for girdle because its convexity mesio-distally resembles a girdle encircling the lingual surface at the cervical third. The cingulum forms the upper border or boundary of the lingual fossa of the incisors. A ridge is any linear elevation present on the surface of the tooth and is usually named according to its location (e.g. buccal ridge, incisal ridge, marginal ridge). 1. Marginal ridges are those rounded borders of the enamel that form the mesial and distal margins of the occlusal surfaces of premolars and molars and the mesial and distal margins of the lingual surfaces of the incisors and canines. The marginal ridges of anterior teeth run vertically in a cervico-incisal direction and the marginal ridges of posterior teeth run horizontally in a bucco-lingual direction. In anterior teeth the marginal ridges on either side form the boundary of the lingual surface and in posterior teeth they form the boundary of the occlusal surface. 2. Triangular ridges are the ridges that run from the tips of the cusps of premolars and molars toward the center of the occlusal surfaces. They are so named because the slopes of each side of the ridge are inclined to resemble two sides of a triangle. They are named after the cusps, to which they belong, e.g. the triangular ridge of the buccal

cusp of the maxillary first premolar. 3. Transverse ridges are formed by two triangular ridges (buccal and lingual triangular ridge) that join transversely across the occlusal surface of the posterior teeth. 4. The oblique ridge is a ridge that runs obliquely on the occlusal surfaces of maxillary molars. It is formed by the union of the triangular ridge of the disto-buccal cusp and the distal ridge of the mesio-palatal cusp. 5. Labial ridge is a ridge that runs vertically in a cervico-incisal direction on the labial surface of canines. It starts from the cusp tip and extends to the cervical region of the tooth. Labial ridge is more prominent in maxillary canine than mandibular canine. 6. Lingual ridge is a ridge that runs vertically in a cervico-incisal direction on the lingual surface of canines. It starts from the cusp tip and extends to the cingulum dividing the lingual fossa into two. Lingual ridge is more prominent in maxillary canine than mandibular canine. 7. Linguo-incisal ridge is a ridge that runs horizontally in a mesiodistal direction on the incisal one-third of the lingual surface of the crown of the upper incisors. It forms the lower border of the lingual fossa on the incisors. 8. Buccal ridge is a ridge that runs vertically in a cervico-occlusal direction on the buccal surface of premolars. It starts from the cusp tip and extends to the cervical area of the tooth. It is more prominent on the first premolars than the second premolars. 9. Cervical ridge is a ridge that runs horizontally in a mesio-distal direction on the cervical one-third of the buccal surface of the crown. Cervical ridge is prominent in all deciduous teeth and on the permanent molars. 10. Cusp ridges are the mesial and distal slopes or inclined surfaces of the cusps which meet at different angles to form cusps of varying sharpness.

Fossa Fossa is an irregular depression or concavity found on the lingual surface of

anterior teeth or occlusal surface of posterior tooth. Fossae can be lingual fossa, central fossa, distal fossa and triangular fossa. A lingual fossa is found on the lingual surface of anterior teeth, bounded by marginal ridges, cingulum and linguoincisal ridge. The lingual fossa in canines are divided into two by lingual ridge. A central fossa is a major fossa found on the occlusal surface of molars which are formed by the converging ridges, terminating at a central point in the bottom of a depression, where there is a junction of grooves. Distal fossa is another major fossa seen on the occlusal aspect of maxillary molars which is located distal to the oblique ridge Triangular fossae are found in molars and premolars on the occlusal surfaces just inside the marginal ridges. There are two triangular fossae including mesial triangular fossa which is seen adjacent to mesial marginal ridge and distal triangular fossa which is seen adjacent to distal marginal ridge. A sulcus is a broad linear depression or valley on the surface of a posterior teeth. Sulcus is the area between ridges and cusps, the inclines of which meet at an angle to form a groove called developmental groove. A groove is a linear depression or a line present at the deepest part of the sulcus. The grooves can be developmental grooves or supplemental grooves. Developmental grooves are sharply defined, narrow and linear depression seen, separating the major portions of a tooth developing from different lobes. All posterior teeth have a distinct central developmental groove which divides the occlusal aspect into two parts. Additional developmental grooves are found extending in buccal or lingual direction separating the cusps from adjacent ones which may extend onto the buccal and lingual surfaces as buccal and lingual grooves. A supplemental groove is a small irregularly placed shallow groove which is less distinct than the developmental groove. Supplemental grooves are supplemental to a developmental groove and are not found between developmental portions of a tooth. Pits are small pinpoint depressions located at the junction of developmental grooves or at terminals of the grooves. Pits are usually found at the deepest

part of fossae, and depending on the location they are named as central pit, mesial or distal pit, lingual or palatal pit and buccal pit. Pits can be a site for initiation of caries. Fissure is a cleft or ditch formed at the bottom of a developmental groove. This also can be a site for initiation of caries. A lobe is one of the primary sections of formation in the development of the crown. Cusps and mamelons are representative of lobes. All the deciduous incisors develop from one lobe while the second deciduous molars develop from five lobes. All the permanent anterior teeth develop from four lobes: Three labially (mesial, labial and distal lobes) and one lingually (lingual lobe). The lingual lobe is represented by cingulum. Mamelons are the three rounded protuberances found on the incisal ridges of newly erupted incisor teeth (Fig. 14.8) representing the three lobes from which the labial portion of the tooth develop. Premolars also develop from four lobes, namely mesial, buccal, distal and lingual lobe. Mandibular second premolar which is three cusp type develop from five lobes: Three for buccal portion and two for lingual portion. In this tooth, the lobes are named as mesial, buccal, distal, mesio-lingual and distolingual lobes.

Fig. 14.8: Mamelons

The number of lobes from which molar develop vary depending on the number of cusps and are named same as the cusps. Maxillary molars develop from two buccal and two lingual lobes while the mandibular first molar develops from three buccal lobes and two lingual lobes. The ‘cusp of Carabelli’, an accessory cusp on the maxillary first molar, when present may be a part of the large mesio-palatal lobe or may form from a fifth lobe.

Terminologies used While Describing the Root (Fig. 14.9) Root trunk: This term is used to describe the undivided cervical portion of the root of multirooted teeth. Furcation is the division of the root of the multirooted teeth. When the root is dividing into two, it is referred to as bifurcation and when three, trifurcation. Apical and accessory foramen are small openings found at the apical region of root through which the connective tissue of pulp communicate with surrounding periodontal soft tissue.

FUNCTIONAL OR FUNDAMENTAL CURVATURES OF TEETH All the teeth show a form that is directly related to the function it has to perform. The fundamental curvatures of teeth are proximal contours creating contact points and interproximal spaces, facial and lingual contours and curvature of cervical line.

Fig. 14.9: Parts of the root

Contact Point or Contact Areas Contact areas are the places on the proximal surfaces of tooth crowns where a tooth touches the adjacent tooth in the same arch, when the teeth are in proper

alignment. The proximal contact areas are located on the mesial and distal surface of each tooth at the widest portion and at the greatest curvature. The distal contact area of one tooth touches the mesial contact area of the tooth posterior to it, with the exception of central incisors, where the mesial contact area of one central touches the mesial contact area of the other, and in third molars distal surface do not contact with any tooth, as the third molars on both the arches does not have tooth distal to it. Even though there are 32 teeth, there are only 60 contacting proximal surfaces. The contact area may be lacking or modified in some instances like in diastema, poor dental position, drifting of teeth, etc. While describing proximal contact, two terms can be used; contact point and contact area. In newly erupted teeth, the contact is limited in size and is circular or slightly oval in form and is called contact point. The teeth wear by rubbing against one another as they move slightly in their sockets during mastication and the contact point becomes more extensive and is denoted as the contact area.

Location of the Contact Areas Contact Areas from Facial View Anterior teeth (Fig. 14.10a) – –

Contact areas are closer to the incisal third of the teeth Except distal surface of canine—in middle third

Posterior teeth (Fig. 14.10b) Contact areas are located nearer to the middle third of the teeth. The more posterior the tooth, the more cervical is the location of its contact area.

Contact Areas from Occlusal or Incisal View Anterior teeth Contact areas are located approximately in the center of proximal surface in a facio-lingual direction. Posterior teeth (Fig. 14.10c) Contact areas are located more facially to the center of proximal surface in a

facio-lingual direction.

Importance of Proper Interproximal Contact Proper contact area helps to prevent food wedging during mastication and allows transmission of the masticatory forces in a sagittal plane. It also helps to stabilize the dental arch and to maintain the arch length. The shape of the interdental gingival contour is determined by interproximal contact area. Open contacts can lead to food impaction, followed by gingival inflammation, periodontal complication, etc.

Facial and Lingual Contours Contours are curvatures that are seen on the cervical or middle third on the facial or lingual aspect of all the teeth in dental arch. The facial and lingual surface posses some degree of convexity that help in protection and stimulation of the supporting tissues during mastication. Normal tooth contours deflect food away so that the passing food only stimulates (by gentle massage) rather than irritates the investing tissues. Convexities are generally located at the cervical third of the crown on the facial surface of all teeth and the lingual surface of the incisors and canines. On lingual surface of posterior teeth, the height of contour is located in the middle third of the crown. In young children, most curvatures, buccal and lingual curvatures lie beneath the gingiva and as the teeth erupt, the curvature becomes more apparent clinically. In normal adults whose tooth eruption has been completed, the gingival crest is cervical to the facial and lingual contours of all teeth.

Fig. 14.10a: Contact area of anterior teeth

Fig. 14.10b: Contact area of posterior teeth—facial view

Fig. 14.10c: Contact area of posterior teeth—occlusal view

Functions Buccal and lingual contours can deflect the food material away from gingival margin during mastication and therefore helps in maintenance of health of periodontal tissues. Cervical contours also serve to decrease the tooth bulk from its gingival third to incisal third. Height of contour or crest of curvature is the greatest bulge of the curved outlines of a tooth (Fig. 14.11). As a general rule, height of contour of facial and lingual surfaces of both anterior and posterior teeth are in the cervical third, except lingual crest of curvature of posterior teeth which is near the middle third. The crest of curvature of proximal surfaces represent the contact areas.

Interproximal Spaces Triangular or V-shaped spaces seen between adjacent teeth, cervical to their contact is called interproximal spaces. The sides of the triangle are formed by the proximal surfaces of adjacent teeth, the base is formed by the alveolar bone and the contact area of the two teeth forms the apex of the triangle. These spaces are normally filled with gingival tissues called papillary gingiva or interdental papilla. When gingival recession occurs between the teeth, the interdental papilla and bone no longer fill the entire interproximal space, and creates a void which exists cervical to the contact and is called a cervical embrasure or gingival embrasure.

Embrasures Embrasures (spillways) are triangular or V-shaped spaces seen on facial, lingual or occlusal to the contact areas (Figs 14.10b and c). They allow the passage of food around the teeth, so that food is not forced into the contact area between the teeth. These embrasures or spillways are named for their locations in relation to the contact area and are facial (buccal or labial),

lingual, incisal or occlusal. The gingival or cervical embrasure is generally termed as interproximal space. The term gingival embrasure is used if the interdental gingiva is not filling the space as in case of gingival recession.

Fig. 14.11: Crest of contour

Facial embrasures are spaces that widen out facially from the area of contact. Lingual embrasures are spaces that widen out lingually from the area of contact. Incisal or occlusal embrasures are spaces that widen out above the contact areas in an incisal or occlusal direction and are bounded by the marginal ridges. The facial, lingual and incisal or occlusal embrasures are continuous with each other.

Functions of Embrasures Makes a spillway and allow food to be forced away from contact areas and thus prevent food from being packed between them. Embrasures help to dissipate and reduce occlusal forces. They permit a slight amount of stimulation to the gingiva by fractional massage of food while at the same time protecting the gingiva from undue trauma.

Curvature of Cervical Line The cervical line of a tooth represents the cemento-enamel junction. When the teeth are in its normal alignment, the epithelial attachment, i.e. the attachment between the teeth and gingiva follows the same contour as that of cervical line. Since the height of gingival tissue of interproximal region directly depends on epithelial attachment, the curvature of cervical line has a functional significance. As a rule, the cervical line on the labial and lingual aspect of a tooth curve towards the root and on mesial and distal aspect towards the crown.

Generally cervical line on the mesial aspect shows more curvature than the distal aspect. The degree of curvature of cervical line depends on location of contact area and bucco-lingual or labio-lingual diameter of the crown. Anterior teeth exhibit greater curvature compared to posterior teeth. Highest curvature of cervical line is observed on the mesial aspect of maxillary and mandibular central incisors (about 3.5 mm). In posterior teeth the extent of curvature is only 1 mm or less on mesial aspect and very less or no curvature on distal aspect.

15 Deciduous Maxillary Anterior Teeth

Introduction Importance of deciduous teeth Description of morphology of maxillary central incisor, lateral incisor and canine

P

rimary dentition is seen in children up to the age of six years. The first primary tooth appears in the oral cavity at the age of six months and the dentition is completed by two and a half years. These teeth are called deciduous teeth because they fall off to give space for permanent successors. The term deciduous is derived from a Latin word which means ‘falls off’. Other names used to describe these teeth are ‘milk teeth’, ‘temporary teeth’ or ‘baby teeth’. The deciduous teeth are twenty in number which include two incisors, one canine and two molars in each quadrant. There are no premolars in primary dentition. The primary molars are replaced by premolars of permanent dentition.

Importance of Primary Teeth Primary teeth are important for the physical and psychological development of a child. They are necessary for efficient mastication of food, development of normal speech, esthetics, etc. Badly decayed or missing anterior teeth can cause psychological trauma to the child. Primary teeth are also essential to maintain space and arch

continuity for the eruption of permanent teeth. Premature loss of these teeth may lead to malocclusion in permanent dentition. Since the periapical infection of primary teeth can cause developmental defects like enamel hypoplasia of permanent successors, avoidance of infection is also important.

PRIMARY MAXILLARY CENTRAL INCISOR The deciduous maxillary central incisors are two in number, one on each side of the midline.

Crown Crown is mesio-distally wider than cervico-incisally, i.e. width of crown is more than the length. The opposite holds true for the corresponding permanent tooth. Mesial outline is relatively straight while distal outline is convex. Incisal edge is straight with sharp mesio-incisal angle and rounded disto-incisal angle. Cervical line is curved towards the root. Labial surface is slightly convex and smooth without any developemental grooves. Cervical third of labial surface shows a prominent cervical ridge running in a mesiodistal direction. From the lingual aspect, a significant lingual convergence of the crown can be appreciated. Cingulum and marginal ridges are well developed, making lingual fossa more distinct. Cingulum may extend beyond the cervical onethird towards the incisal region resulting in partial division of lingual fossa. Mesial and distal aspects of deciduous maxillary central incisors show marked convexity. Cervical one-third of crown is wider bucco-lingually because of well developed cingulum and a prominent cervical ridge on buccal aspect. Curvature of cervical line is greater on the mesial aspect than distal. A wider mesio-distal dimension of crown than bucco-lingual can be well appreciated from incisal aspect. Incisal edge is straight and is centered over the bulk of crown.

Root Root is cone shaped with evenly tapered sides till the blunt apex. Root is longer in proportion to the crown length. Because of lingual tapering, in cross-section root is triangular in shape with base at the labial aspect and tip

at the lingual aspect. A prominent developmental groove may be present on the mesial surface of root.

PRIMARY MAXILLARY LATERAL INCISOR Maxillary lateral incisor is located distal to the central incisor, one on each side of the maxillary arch. Morphology of maxillary lateral incisor is similar to that of central incisor. The differences are: Crown is wider cervico-incisally than mesio-distally, i.e. crown length is more than its width. Crown is smaller in all dimensions and less symmetrical. Disto-incisal angle is more rounded Root is much longer in proportion to the crown length.

PRIMARY MAXILLARY CANINE It is the third tooth from the midline in the maxillary arch. It is located distal to the lateral incisor on either side. It is also called cuspid.

Crown Primary maxillary canine is larger than central and lateral incisors. Crown length is almost equal to the width and is constricted at the cervix. Mesial and distal outlines are convex with both mesial and distal contact areas located nearly at the same level, i.e. at middle of middle one-third. Primary maxillary canine has a well developed sharp cusp. The mesial cusp slope is longer than the distal cusp slope. Labial surface shows a labial ridge extending vertically from the cervical region to the cusp tip. Lingual aspect of this tooth shows a bulky cingulum and prominent marginal ridges and lingual fossa. A distinct lingual ridge is also present which divides lingual fossa into mesio-lingual and disto-lingual fossae. From the proximal aspect a greater labio-lingual measurement especially at cervical one-third can be appreciated which is due to prominent labial

cervical ridge. Cervical line curves incisally to a greater degree on mesial aspect than on the distal. When observed from incisal aspect, crown is wider mesio-distally than labio-lingually. Cingulum is centered over the crown.

Root Root is long, slender and tapering. Length of root is twice as that of crown. Measurement table of deciduous maxillary central incisor

Measurement table of deciduous maxillary lateral incisor

Measurement table of deciduous maxillary canine

Deciduous maxillary central incisor

Deciduous maxillary lateral incisor

Deciduous maxillary canine

16 Deciduous Mandibular Anterior Teeth Description of morphology of mandibular central incisor, lateral incisor and canine.

DECIDUOUS MANDIBULAR CENTRAL INCISORS The deciduous mandibular central incisors are two in number, one on each side of the midline of mandible.

Crown Deciduous mandibular central incisors have considerable morphological resemblance to permanent counterparts but are significantly smaller in size. These teeth are the smallest incisors in the mouth. From labial aspect this tooth is smooth and relatively flat except for the cervical 1/3rd. The mesial and distal surfaces tapers evenly to a narrow cervix and contact areas mesially and distally are located at incisal 1/3rd. The incisal edge is straight and is at right angles to the long axis of tooth. Both mesioincisal and disto-incisal angles are sharp. Crown is narrower lingually and cingulum, marginal ridges and lingual fossae are less prominent. When a deciduous mandibular central incisor is examined from proximal aspect, cervical convexity of labial and lingual outlines appears to be prominent. Incisal edge is centered over the crown, from the proximal aspect. Distally the curvature of cervical line is lesser

when compared to mesial side.

Root Root is twice as long as crown and it tapers to a sharp tip. There may be developmental depression on the distal aspect of root.

DECIDUOUS MANDIBULAR LATERAL INCISORS Mandibular lateral incisors are two in number and are located distal to the central incisors, one on each side of the mandibular arch. In general this tooth resembles a deciduous mandibular central incisor. Differences are: Larger than centrals in all dimensions except labio-lingual dimension which is same in central and lateral incisors. Incisal edge is slopping distally with distal contact area located at a lower level. Mesio-incisal angle is sharp while disto-incisal angle is rounded. Cingulum, marginal ridges and lingual fossa are more prominent.

DECIDUOUS MANDIBULAR CANINE It is the third tooth from the midline in the mandibular arch. It is located distal to the lateral incisor on either side. It is also called cuspid. The mandibular primary canine has the same general form as the maxillary canine. Differences observed are: Crown is longer and narrower mesio-distally but thicker labio-lingually, but to a lesser extent when compared to maxillary canine. Cusp tip is pointed with a longer distal cusp slope than mesial slope. Labial ridge is less prominent. Labio-lingual measurement is not as great as maxillary canine because of less prominent cervical ridge and cingulum.

On lingual aspect the cingulum, marginal ridges, lingual ridge and lingual fossae are less prominent.

Root Root is shorter and tapers to a pointed tip. Measurement table of deciduous mandibular central incisor

Measurement table of deciduous mandibular lateral incisor

Measurement table of deciduous mandibular canine

Deciduous mandibular central incisor

Deciduous mandibular lateral incisor

Deciduous mandibular canine

17 Deciduous Maxillary Molars Description of morphology of deciduous maxillary first molar and deciduous maxillary second molar.

DECIDUOUS MAXILLARY MOLARS The deciduous maxillary molars are four in number, two on either side of the arch, which includes the first and second molars.

DECIDUOUS MAXILLARY FIRST MOLAR Deciduous maxillary first molar is quite different from permanent maxillary first molars. This tooth more closely resembles the maxillary first premolar, which is its succedaneous tooth.

BUCCAL ASPECT Crown Shape of Buccal Aspect Crown appears wider mesio-distally than cervico-incisally, i.e. width of crown is more than the length. Mesial half of the tooth is longer than distal half. The crown tapers considerably towards the cervix with the cervical measurement 2 mm less than that at contact area. This gives a narrower

appearance to the cervical portion of crown and root.

Outlines of Buccal Aspect Mesial outline is nearly straight with contact area in the occlusal 1/3rd. The distal outline is more convex and distal contact area is located at the middle 1/3rd. Occlusal outline is represented by cusps and cusp slopes and is scalloped without definite cusp form. The scalloped outline divides the large mesiobuccal cusp from indistinct disto-buccal cusp. Cervical outline is slightly convex towards the root.

Buccal Surface Buccal surface is smooth with a little evidence of developmental grooves. A poorly developed buccal developmental groove extending onto buccal surface is located distal to the center, separating the larger mesio-buccal cusp from disto-buccal cusp. A prominent buccal ridge is present, running from the tip of mesio-buccal cusp to a cervical direction. The cervical ridge is distinct on the buccal surface running in a mesio-distal direction and is significantly prominent on the mesial half.

Root Deciduous maxillary first molar has three roots: Two buccal and one palatal, namely mesio-buccal, disto-buccal and palatal. All three roots can be seen from this aspect and are long, slender and widely separated. The furcation is close to the cervical line. So the root trunk is very small.

PALATAL ASPECT The form of the palatal aspect is similar to that of buccal aspect. Features observed are: Crown is narrower palatally Palatal surface is slightly convex cervico-occlusally and markedly convex

mesio-distally Mesio-palatal cusp is the most prominent cusp of this tooth and is largest and sharpest. Disto-palatal cusp may or may not be present. If present is small, rounded and poorly defined. The disto-palatal cusp is separated from mesio-palatal cusp by a less defined palatal groove. When disto-palatal cusp is absent it is called three cusp type and in such types die palatal cusp occupies the entire palatal portion of occlusal aspect. Because of the small disto-palatal cusp, a portion of the disto-buccal cusp which is more developed can be seen from palatal aspect.

Root All three roots can be seen from this aspect, palatal being the longest.

MESIAL ASPECT Crown Shape of Mesial Aspect Cervical 1/3rd of crown is much wider bucco-palatally than the occlusal 1/3rd making the occlusal table narrow.

Outlines of Mesial Aspect Buccal outline is significantly convex at cervical 1/3rd representing the cervical ridge. From the crest of convexity, buccal outline is flat to the occlusal margin. Palatal outline is more gradually curved at cervical and middle 1/3rd and flat at occlusal 1/3rd. Occlusal outline is represented by cusps and marginal ridge. Two cusps can be seen from this aspect: Mesio-palatal and mesio-buccal cusp. The mesiopalatal cusp is longer and sharper than mesio-buccal cusp. Mesial marginal ridge is nearly as wide as cusp tips and may be crossed by a marginal groove.

Cervical line mesially shows more curvature occlusally.

Mesial Surface Mesial surface is relatively flat except for the region of contact area.

Root From the mesial aspect, only two roots are visible; mesio-buccal and palatal root. Lingual root is thin and long, with a sharp curve buccally above middle 1/3rd. The disto-buccal root is not visible because mesio-buccal root is broad enough to hide die disto-buccal root. Since the furcation is close to the cervical line, the root trunk is very small.

DISTAL ASPECT Crown tapers markedly towards the distal aspect and therefore is narrower distally than medially. Crown length is lesser on distal aspect than mesial. Disto-buccal cusp is more prominent than disto-palatal which is smaller and may be even absent. Cervical ridge on the buccal surface is not as prominent as on mesial aspect Cervical line is curved occlusally to a lesser extend than mesial Distal marginal ridge is more cervically oriented. All three roots can be seen from the distal aspect: Mesio-buccal, disto-buccal and palatal. Because the disto-buccal root is shorter and narrower than mesiolingual root, a portion of that root is also seen.

Root Root trunk is very small with the level of bifurcation closer to the cervical line.

OCCLUSAL ASPECT

Shape Occlusal aspect is nearly rectangular with shorter sides represented by marginal ridges. Because of palatal convergence crown is wider buccally than lingually in a mesio-distal direction. Since the crown has a distal convergence, it is wider mesially than distally in a bucco-lingual direction. The occlusal table is narrow in this tooth because of occlusal convergence from the proximal aspects.

Occlusal Surface Occlusal surface shows anatomic landmarks such as cusps, ridges, fossae, pits, grooves, etc.

Cusps Occlusal surface of primary maxillary first molar shows four cusps, namely mesio-lingual, mesio-buccal, disto-lingual and disto-buccal. Mesio-lingual cusp is largest and sharpest of all cusps, followed by mesio-buccal and distobuccal. Disto-lingual cusp is very small or may be even absent.

Ridges Triangular ridges of all cusps are found extending from cusp tip towards the center of occlusal surface. Oblique ridge: Sometimes a well-developed oblique ridge may be seen extending between mesio-palatal and disto-buccal cusps. Transverse ridge may be formed between mesio-lingual and mesio-buccal cusps. Mesial marginal ridge forms the mesial boundary of occlusal aspect and is well developed and is occlusally located than distal marginal ridge. The palatal half of the mesial marginal ridge shows a distal inclination making the palatal surface narrow. Distal marginal ridge is straight in bucco-palatal direction and is smaller, less developed and cervically located than mesial marginal ridge.

Fossae Central fossa is the major fossa located at the center of occlusal aspect. Mesial triangular fossa is a minor fossa located just inside the mesial marginal ridge and is large and deep when compared to distal triangular fossa. Distal triangular fossa is located just inside the distal marginal ridge and is less distinct.

Grooves Occlusal surface shows both developmental and supplementary grooves. These grooves show an ‘H’ pattern.

Developmental grooves Central groove: Extends from the central fossa in a mesial direction to end in the mesial triangular fossa. Distal extension of central groove: This groove is seen in teeth in which the oblique ridge is less prominent and this extends from central pit to the distolingual developmental groove. Buccal groove: Starts from the central pit and traverse in a buccal direction separating the two buccal cusps and may extend onto buccal surface. Distal developmental groove: It is present distal to the oblique ridge outlining the disto-palatal cusp. Lingual developmental groove: This groove is present only in four cusp types where a disto-palatal cusp is present and this is seen as a lingual extension of distal developmental groove between two lingual cusps.

Supplementary grooves Supplementary grooves are found radiating from mesial pit, one in a buccal direction, one in a lingual direction and a third one towards the marginal ridge which may cross the marginal ridge and extend onto the mesial side.

Pits

Central, mesial and distal pits are seen at the deeper part of respective fossae. Sometimes distal pit is absent.

DECIDUOUS MAXILLARY SECOND MOLARS The maxillary second primary molar is similar in many respects to permanent maxillary first molar having similar arrangement of pits, grooves and cusps. But it differs in being smaller and more bulbous with a narrow occlusal table. It also has prominent buccal cervical ridge, narrow cervix and divergent roots.

BUCCAL ASPECT Crown Crown of deciduous second molar is considerably larger than that of first deciduous molar with a narrow cervix. Buccal view shows two well developed cusps: mesio-buccal and disto-buccal cusps which are separated by a buccal developmental groove. Mesio-buccal cusp is larger than distobuccal cusp. Buccal surface shows a prominent cervical ridge but not as prominent as in first molar.

Root Deciduous maxillary second molar has three roots, namely mesio-buccal, distobuccal and palatal. All three roots can be seen from this aspect and are long, slender and widely separated. The trifurcation is close to the cervical line. So the root trunk is very small.

PALATAL ASPECT Crown From lingual aspect there are two major cusps visible: Mesio-lingual and disto-lingual cusps. The lingual cusps are separated by lingual groove which

extends onto lingual aspect which is gradually obliterated as it reaches the cervical 1/3rd. Mesio-lingual cusp is larger and well developed. A fifth cusp, ‘cusp or tubercle of Carabelli’ is found lingual to mesio-lingual cusp which is separated from the mesio-lingual cusp by a developmental groove. In some teeth, fifth cusp may be absent or is represented by traces of developmental groove.

Root All three roots are visible from the palatal aspect. Palatal root is thicker and larger and same length as mesio-buccal.

MESIAL ASPECT Crown From mesial aspect, deciduous maxillary second molar is similar to permanent maxillary first molar. Features observed are:

Shape of Mesial Aspect Bucco-palatal measurement of crown is more when compared to length, so it appears short from mesial aspect.

Outlines of Mesial Aspect Buccal outline is almost straight, from the cervical 1/3rd where the crest of curvature is located, to the tip of buccal cusp. Palatal outline is smooth and round from the cervical region to the palatal cusp tip. Occlusal outline is represented by cusps and marginal ridge, two cusps can be seen from this aspect: Mesio-palatal and mesio-buccal. Mesio-palatal cusp is large when compared to mesio-buccal which is shorter and sharp. Fifth cusp, ‘cusp of Carabelli’ is also seen lingual to mesio-palatal cusp. Mesial marginal ridge is at a higher level and is crossed by mesial groove. Cervical line shows only a little curvature.

Mesial Surface Mesial surface is convex cervico-occlusally and less so bucco-palatally.

Root Only two roots are seen from this aspect: Mesio-buccal and palatal. Bifurcation is around 2 or 3 mm above cervical line.

DISTAL ASPECT The morphology of this aspect resembles that of mesial aspect. Differences observed are: Distal aspect is narrower because the crown converges distally Disto-buccal and disto-palatal cusps are of almost same size Cervical line is nearly straight All three roots can be seen from the distal aspect: Mesio-buccal, disto-buccal and palatal. Because the disto-buccal root is shorter and narrower than mesiobuccal root, a portion of that root is also seen.

Root Root trunk is very small with the level of bifurcation at an apical level than other sides.

OCCLUSAL ASPECT Occlusal aspect of deciduous maxillary second molar resembles the permanent maxillary first molar to a greater extent, with similar shape, cusps, ridges, groove pattern, etc.

Shape Shape is rhomboidal. The crown is wider mesially than distally in a buccopalatal direction because of distal convergence. The occlusal table is narrow

in this tooth because of occlusal convergence from the proximal aspects.

Occlusal Surface Occlusal surface shows various anatomic landmarks such as cusps, ridges, fossae, pits and grooves.

Cusps Occlusal surface of primary maxillary second molar shows four well developed cusps, namely mesio-palatal, mesio-buccal, disto-palatal and distobuccal. Mesio-palatal cusp is largest and sharpest of all cusps and this cusp occupies a greater portion of occluso-palatal area. Second largest cusp is mesio-buccal followed by disto-buccal. Disto-palatal cusp is smallest of the four major cusps. A fifth cusp, ‘tubercle of Carabelli’ is found palatal to mesio-palatal cusp which is separated from the mesio-palatal cusp by a developmental groove.

Ridges Triangular ridges of all cusps are found extending from cusp tip towards the center of occlusal surface. Oblique ridge: A well developed oblique ridge is seen extending between mesio-palatal and disto-buccal cusps. Transverse ridge may be formed between mesio-palatal, mesio-buccal cusps. Mesial marginal ridge forms the mesial boundary of occlusal aspect and is well developed. Distal marginal ridge forms the distal boundary of occlusal aspect and is equally well developed as mesial marginal ridge.

Fossae Central fossa is the major fossa, located mesial to the oblique ridge. Distal fossa is less prominent and is located distal to the oblique ridge. Mesial triangular fossa is well defined and is situated distal to mesial marginal ridge.

Distal triangular fossa is less distinct and found just inside the distal marginal ridge.

Pits Pinpoint depressions can be seen at deepest part of the fossae where the grooves converge. Mainly three pits are seen in deciduous maxillary second molar: Central pit, mesial pit and distal pit.

Grooves Both developmental and supplementary grooves are present:

Developmental grooves Central groove extends from central pit in a mesial direction to the mesial pit. Buccal developmental groove also begins from central pit and traverse buccally separating the two buccal cusps. Distal developmental groove extends from central fossa in a distal direction across the oblique ridge to join the distal pit. Disto-lingual developmental groove is found in the distal fossa with a mesial inclination separating the two lingual cusps. Lingual developmental groove is seen as an extension of disto-lingual developmental groove which extends onto the palatal side between palatal cusps.

Supplementary grooves Supplementary grooves are present in mesial and distal triangular fossae. Measurement table of deciduous maxillary first molar

Measurement table of deciduous maxillary second molar

Deciduous maxillary second molar

Deciduous maxillary first molar

18 Deciduous Mandibular Molars Description of morphology of deciduous mandibular first molar and deciduous maxillary second molar.

DECIDUOUS MANDIBULAR MOLARS The primary mandibular molars are four in number, two on either side of the arch, which includes the first and second molars.

DECIDUOUS MANDIBULAR FIRST MOLAR This tooth is morphologically unique and does not resemble any other deciduous or permanent tooth. Its outline and form differs considerably from that of all other primary and permanent teeth. Differentiating feature is its overdeveloped mesial marginal ridge which somewhat resembles a cusp.

BUCCAL ASPECT Crown Shape of Buccal Aspect From the buccal aspect crown appears wider mesio-distally than

occlusally. Distal part of crown is shorter than mesial part. Considerable degree of cervical convergence is observed which is more from the distal aspect than mesial.

Outlines of Buccal Aspect Mesial outline is nearly straight with contact area more cervical than on distal aspect. Distal outline is convex with contact area in the middle of the crown. Occlusal outline is represented by cusps and cusp slopes. Two buccal cusps are seen from the buccal aspect: Mesio-buccal and disto-buccal. Mesiobuccal cusp is larger than disto-buccal cusp. Cervical line on buccal surface dips apically as it joins to the cervical line on mesial aspect so the mesial half of the crown appears to be longer.

Buccal Surface Buccal surface is convex in mesio-distal direction but slopes abruptly towards the occlusal surface. In the cervical region of buccal surface, a prominent cervical ridge is seen extending in a mesio-distal direction. This cervical ridge is more prominent at mesial half and referred to as ‘tubercle of Zuckercandl’. On the buccal surface, two buccal cusps are separated by a depression which may at times harbor a buccal developmental groove.

Root Mandibular first primary molar has two roots: Mesial and distal. Furcation is close to cervical line and the root trunk is short. Mesial root is wider and longer than distal root. Roots are slender widely separated and the apical third spread beyond the crown outlines.

UNGUAL ASPECT Crown Crown and root converges lingually on the mesial side, making mesial

surface visible from this aspect. Since there is no convergence from distal aspect, distal surface cannot be seen. Crown length is almost equal in both mesial and distal portions in contrast to the buccal aspect. Two lingual cusps are seen from this aspect: Mesio-lingual and disto-lingual cusps. Mesio-lingual cusp is larger, longer and sharper, while disto-lingual cusp is small and rounded. The mesio-lingual cusp in some teeth is so prominent and is almost centered over mesial root. Mesial marginal ridge is so well developed that it resembles a cusp from lingual aspect. Along with the lingual cusps part of two buccal cusps also may be seen from lingual side. Cervical line is nearly straight Lingual surface is convex mesio-distally and cervico-occlusally and the surface is traversed by a lingual groove which separates both lingual cusps.

Root Both mesial and distal roots can be seen from this aspect. Since the bifurcation is slightly more apical, the root trunk may be longer lingually.

MESIAL ASPECT Crown Shape of Mesial Aspect Crown appears to incline lingually as in case of permanent mandibular teeth. Crown length is more on the mesio-buccal part than mesio-lingual part. Cervical portion of the crown is much wider than occlusal, making the occlusal table narrow.

Outlines of Mesial Aspect Buccal outline shows an extreme curvature at cervical 1/3rd because of prominent cervical ridge. The outline is flat from crest of curvature up to the tip of mesio-buccal cusp. Buccal outline is longer than lingual outline. Lingual outline is shorter than buccal outline and it extends beyond the

confines of lingual margin or root. Occlusal outline is represented by cusps and marginal ridge. Two well developed cusps, i.e. mesio-buccal and mesio-lingual cusps are visible. Buccal cusp is well within the confines of root base, but the lingual cusp tip may be either in line with lingual margin of root or extends beyond the confines of lingual margin. Mesial marginal ridge is very prominent, concave and is longer and located more occlusally than distal marginal ridge. Cervical line on this aspect curves occlusally and is slanting occlusally from buccal to lingual surface.

Mesial Surface Mesial surface is relatively flatter.

Root Only one root is visible from this aspect, i.e. mesial root. The mesial root is flat and square with broad apex. Deep developmental depression is present, running the entire length of root.

DISTAL ASPECT The morphology of this aspect resembles that of mesial aspect. Differences observed are: The crown length on buccal and lingual aspect is uniform. Buccal cervical ridge is not so prominent. Disto-buccal and disto-lingual cusps are almost of same size and not as long or sharp as mesial cusps. Distal marginal ridge is not well developed and is cervically placed when compared to mesial marginal ridge, so more of occlusal aspect can be seen from distal aspect. Cervical line is almost straight on the distal aspect.

Root

Distal root is rounded, thinner and less broad than mesial root.

OCCLUSAL ASPECT Shape Shape of occlusal aspect is roughly rhomboidal with an obtuse disto-buccal angle and acute mesio-buccal angle, because of prominent buccal cervical ridge. The occlusal surface is wider mesio-distally than bucco-lingually. Generally the occlusal table is narrow with relatively shallow surface. The distal half of the occlusal table is wider than the mesial half. Occlusal aspect also shows a lingual convergence.

Occlusal Surface Occlusal surface shows various anatomic landmarks such as cusps, ridges, pits, fossae and grooves, etc.

Cusps Deciduous mandibular first molar has four cusps: Mesio-lingual, mesiobuccal, disto-buccal and disto-lingual cusps. Both mesio-lingual and mesiobuccal cusps are larger and well developed while distal cusps are smaller. The mesio-lingual cusp is the largest and the disto-lingual cusp is the smallest of all cusps.

Ridges Triangular ridges of all cusps are found extending from cusp tip towards the center of occlusal surface. Transverse ridge may be found between mesio-lingual, mesio-buccal cusps. Mesial marginal ridge forms the mesial boundary of occlusal aspect and is very prominent, long and occlusally placed. Distal marginal ridge forms the distal boundary of occlusal aspect. It is not well developed and is cervically placed when compared to mesial marginal ridge.

Fossae and Pits Central fossa is the major fossa located at the center of occlusal aspect. Mesial triangular fossa is located just inside the mesial marginal ridge. Distal triangular fossa is shallow and it lies inside the distal marginal ridge Pits may be seen as pinpoint depression at deepest part of fossae where grooves join.

Grooves Both developmental and supplementary grooves are seen:

Developmental grooves Central groove runs from mesial pit to central pit, separating the mesiobuccal and mesio-lingual cusps. Buccal developmental groove begins from central pit, traverse buccally between the mesiobuccal and disto-buccal cusps. This groove usually does not extend onto the buccal surface. Lingual groove extends between two lingual cusps which also may not extend onto lingual surface.

Supplementary grooves Supplementary grooves are present in both mesial and distal triangular fossae.

DECIDUOUS MANDIBULAR SECOND MOLAR It is a five-cusped tooth closely resembling the permanent mandibular first molar with same general contour and surface pattern, except that primary tooth is small in all dimensions.

BUCCAL ASPECT

Mesiodistal dimension of the crown at the cervix is much less when compared to that at contact area making the cervix narrow. Crown appears to be tilted distally on its root base. Three cusps are visible from buccal aspect: mesio-buccal, disto-buccal and distal cusp. All the buccal cusps are nearly of the same size in contrast to the permanent mandibular first molar. Buccal surface shows two grooves: A mesio-buccal groove which separates mesio-buccal and disto-buccal cusp and a disto-buccal groove which separates disto-buccal cusp from distal cusp. A well-developed cervical ridge is present on the buccal surface immediately below the cervix extending mesio-distally.

Root Mandibular second primary molar has two roots: Mesial and distal. Bifurcation is very close to the cervical line and the root trunk is short. Roots are slender widely separated and the apical third spread beyond the crown outlines.

LINGUAL ASPECT Lingual aspect is narrow when compared to buccal because of lingual convergence. Crown appears to be tilted distally with a longer mesial portion than distal. Two cusps are present on lingual aspect: Mesio-lingual and disto-lingual cusps both are of nearly equal size. The mesio-lingual and disto-lingual cusps are about the same size. Lingual surface is convex, especially at the cervical region. Lingual groove extends onto this surface separating both the lingual cusps.

Root From lingual aspect, both mesial and distal roots are seen. Bifurcation is very close to the cervical line and the root trunk is short. Roots are slender, widely separated and the apical third of the root may extend beyond the crown outlines.

MESIAL ASPECT Shape of Mesial Aspect Shape of the mesial aspect is rhomboidal with the crown tilted lingually on the root axis. A greater bucco-lingual measurement of the crown and the root can be appreciated from this aspect.

Outlines of Mesial Aspect Buccal outline shows prominent curvature at cervical 1/3rd because of cervical ridge. Thereafter outline is flat with a great lingual tilt up to the cusp tip making the occlusal aspect narrow. Buccal outline is well within the confines of root. Lingual outline extends beyond the root base. Occlusal outline is represented by cusps and marginal ridge. Two well developed cusps i.e. mesio-buccal and mesio-lingual cusps are visible. Mesio-lingual cusp is longer of the two. Buccal cusp is well within the confines of root base. Mesial marginal ridge is very prominent and is crossed by a groove. Since the mesial marginal ridge is located more occlusally, mesio-buccal and mesio-lingual cusps appear to be short. Cervical line is regular but slopes occlusally towards the lingual aspect.

Mesial Surface Mesial surface is convex except for cervical region which is flat.

Root Only one root is visible from this aspect i.e. mesial root. Mesial root is broad and flat with blunt apex.

DISTAL ASPECT Distal surface is convex except at cervical region.

Distal surface is narrower than mesial due to distal convergence. So part of mesio-buccal cusp can be seen along with disto-buccal cusp. Distal marginal ridge is short and is at a lower level so part of occlusal surface can also be seen from distal aspect. Disto-lingual cusp is well developed. Cervical line is regular as in case of mesial aspect, it tilts occlusally at the lingual part.

Root Only one root is visible from this aspect, i.e. distal root. Distal root is flat and almost as broad as mesial root but tapers at the apical 1/3rd.

OCCLUSAL ASPECT General morphology is similar to that of permanent mandibular first molar.

Shape Shape of occlusal aspect is rectangular with a lingual and distal convergence. Because of lingual convergence crown is wider buccally than lingually. Mesial half is wider in bucco-lingual direction than distal half due to the distal convergence.

Occlusal Surface Occlusal surface shows various anatomic landmarks such as cusps, ridges, pits, fossae, grooves, etc.

Cusps Deciduous mandibular second molar has five cusps: Three buccal cusps and two lingual cusps. The buccal cusps are mesio-buccal, disto-buccal and distal cusps. All the three buccal cusps are nearly of same size. Lingually there are two cusps: The mesio-lingual and disto-lingual cusps which are also nearly of same size. Mesio-lingual cusp is the most prominent cusp of this tooth.

Ridges Triangular ridges are seen extending from the tips of all five cusps towards the central part of occlusal surface. Triangular ridges of lingual cusps are longer than that of buccal cusps. Mesial marginal ridge: Forms the mesial boundary of the occlusal aspect and is better developed, more pronounced and located more occlusally than the distal marginal ridge. Distal marginal ridge: It is located at distal margin of occlusal aspect. It is shorter, less developed and more cervically placed. Cusp ridges: Forms the buccal and the lingual boundaries of the occlusal aspect

Fossae Three fossae can be seen; one major (central fossa) and two minor (mesial and distal triangular fossae). The central fossa is the largest fossa located at the center of the occlusal aspect. Mesial triangular fossa is a triangular shaped depression located distal to the mesial marginal ridge. Distal triangular fossa is less distinct and is located mesial to distal marginal ridge.

Pits Pits are present as small pinpoint depression at the deepest part of all fossae, where the developmental grooves converge. The pits are named according to the fossa in which they are located: Central pit, mesial pit and distal pit.

Grooves Both developmental and supplementary grooves are seen:

Developmental grooves Central groove: It is the major groove seen on the occlusal aspect and is

centrally located dividing occlusal surface into buccal and lingual halves. It starts from the mesial pit and runs in a mesial direction to end in the distal pit. Mesio-buccal groove starting from central groove extends between two mesio-buccal and disto-buccal cusp and extends on to buccal side. Disto-buccal groove: Between disto-buccal and distal cusp and extends on to buccal surface. Lingual groove-separates two lingual cusps and extend onto the lingual surface.

Supplementary grooves Supplemental grooves are seen in the mesial and distal triangular fossae, sometimes crossing over the marginal ridges. Differences between deciduous mandibular second molar and permanent mandibular first molar Deciduous mandibular second Permanent mandibular first molar molar Smaller and more bulbous from proximal aspect

Larger

Three buccal cusps are nearly equal in size

Mesio-buccal cusp is largest and distal cusp is smallest

Prominent cervical mesio-buccal aspect

on

Cervical ridge is not prominent on mesio-buccal aspect

Roots are thin and longer compared to crown length and are flared

Roots are thick and smaller relative to crown length and are not flared

Furcation is close to cervical line. So root trunk is small

Furcation is 3–4 mm below cervical line. So a distinct root trunk is seen.

ridge

Measurement table of deciduous mandibular first molar

Measurement table of deciduous mandibular second molar

Deciduous mandibular second molar

Deciduous mandibular first molar

19 Comparison between Deciduous and Permanent Dentition Difference between permanent and deciduous dentition General differences Features Deciduous dentition Permanent dentition Number teeth

of

20 in number, 5 in each quadrant

32 in number: 8 in each quadrant

Premolars are absent deciduous dentition

2 premolars are present in each quadrant

Type of teeth

Color

in

Only two molars are present

3 molars are present in each quadrant

Whiter in color due to more opaque enamel resulting from less mineral content

Yellowish white in color or less whiter due to translucent enamel resulting from high mineral content which reflect color of underlying dentin

Inter dental spacing

Natural spacing exists between deciduous teeth, e.g. primate space

Less or no spacing between the teeth

Orientation

Primary incisors have got upright orientation than permanent teeth

Undergo attrition to a great extent Teeth are more labially inclined

Attrition

Undergo attrition to a great extent

Undergo attrition but to lesser extent

Shape

Deciduous teeth are bulbous and have consistent shape with anomalies

Permanent teeth are less bulbous and the shape is less consistent with more anomalies

Size

Smaller in all dimensions than permanent counterparts. Deciduous molars are also smaller than permanent molars but larger than their successors, i.e. the premolars.

Larger in dimensions

In deciduous dentition second molars are larger than first molars

In permanent dentition first molars are larger than second molars

Relatively more bulbous. Crowns of most of deciduous teeth are wider in mesio-distal direction relative to their crown length

Less bulbous. Length of crown is more than width in mesio-distal direction

Contact areas are smaller

Relatively contact areas

Contour/shape

10. Contact area

more more fewer

all

larger

11. Mamelons

Absent in deciduous teeth

12. Cervical margin

Less curved than permanent teeth

of

More curved

13. Cusps

More pointed when the teeth erupt, becomes less sharper due to attrition

Less sharper

Shallow occlusal surface with less prominent fossae and ridges. Only a few grooves are seen

Deeper occlusal surface with more prominent fossae and ridges. More number of grooves are seen

15. Cervical constriction

Deciduous teeth have markedly pronounced cervical constriction

Less pronounced

16. Cervical ridge

Prominent cervical ridge is seen in all deciduous teeth mainly in molars, on the buccal aspect especially in the mesio-buccal portion

Cervical ridge is seen even in permanent molars but is less pronounced

17. Cingulum

Cingulum is more prominent

Relatively prominent

18. Surfaces

The labial and lingual surfaces are flat above the cervical ridge. Both these surfaces converges occlusally, so that the buccolingual measurement near occlusal portion is much lesser than that of cervical region

The labial and lingual surfaces are relatively convex. There is no such convergence

19. Occlusal table

Occlusal table is narrow

Occlusal table relatively broad

14. Depth occlusal surface

of

that

Present

less

is

Morphological differences of root 20. Root length

Roots of deciduous teeth are shorter and thinner

Occlusal table is wider. Longer and larger roots

21. Root ratio

Root of deciduous teeth are longer when relative to their crown length

Root is not so longer when relative to crown length

22. Root flare

Roots flare out beyond crown boundary

Do not flare out and are well within the confines of crown boundary

23. Inclination of root

Roots of deciduous anterior teeth show a labial inclination

Do not show labial inclination.

24. Level furcation roots

Furcation of root is closer to the cervix in deciduous molars

Furcation is relatively apically placed

25. Root trunk

The root trunk is very small and not distinct

The root trunk is longer and distinct

26. Apical foramen

Deciduous teeth generally have a large apical foramen

Constricted foramen

crown

of of

apical

Differences in pilpal morphology 27. Pulp chamber

Large pulp chamber relative to the crown size

Smaller relative to the crown size

28. Pulp horns

Pulp horns are at a higher level

Pulp horns are not as high as in deciduous teeth

29. Pulp canal

Pulp canal is wider relative to the

Pulp

canal

is

size of root. Less curved and apically less constricted

narrower relative to the size of root. More tortuous and apically more constricted

30. Accessory pulp canals

In deciduous teeth accessory canals are located mainly in the furcation area

Accessory canals are located mainly in the apical region of roots

31. Pulpal calcifications

Not seen

Pulpal calcifications are seen as regressive changes

Histological differences 32. Enamel cap

Enamel cap is thinner and is of more uniform thickness. Enamel cap ends in a marked ridge at cervical region

Enamel cap is thicker and is of varying thickness. Enamel cap ends in a feather edge at cervical region

33. Direction of enamel rods

Enamel rods of deciduous teeth are arranged either horizontally or slopes occlusally at neck region

They slope cervically at the neck region

Dentin thickness is limited in some areas but a relatively greater thickness of dentin is found over the pulp chamber in the occlusal fossae

Dentin is thicker and is relatively of uniform thickness

Neonatal lines of enamel and dentin are seen in all the deciduous teeth

Neonatal lines of enamel and dentin are seen only in permanent first molars

34. Dentin thickness

35. Neonatal line

36. Dentinocemental junction

May be scalloped in deciduous teeth

Relatively smooth in permanent teeth

37. Cementum thickness

Cementum is thin with less thickness of cellular cementum

Cementum is relatively thicker in permanent teeth with more thickness of cellular cementum

20 Permanent Maxillary Central Incisors

Introduction to incisors Chronology of maxillary central incisors Measurement table Morphology of maxillary central incisors Developmental variations and clinical considerations

P

ermanent incisors are eight in number; four in the maxilla and four in mandible, which include two central incisors and two lateral incisors in maxilla and mandible each. Central incisors are located on either side of the midline with their mesial surfaces in contact. Lateral incisors are situated distal to the central incisors on each side of the arch. As the name indicate the incisors function in incising or cutting food. These teeth are also important in articulation of speech and esthetics. The maxillary and mandibular incisors guide the jaw during closure.

PERMANENT MAXILLAR/CENTRAL INCISORS Maxillary central incisors are two in number and occupy either side of the midline. They are the most prominent teeth in the oral cavity with a great esthetic value and are larger in all dimensions than the lateral incisors. The morphologic characteristics of this tooth can be described from five aspects, namely labial, lingual, mesial, distal and incisal.

LABIAL ASPECT Labial aspect is the surface of the tooth facing the lip. The description of features is categorized as features of crown and of root.

Crown Shape Shape of maxillary central incisor is squarish or rectangular with a slight cervical convergence (narrower at the cervical region than incisal). Cervicoincisal length is 2 mm more than the mesiodistal width.

Outlines from the Labial Aspect Mesial outline is slightly convex with contact area (crest of curvature) at the incisal third close to the mesio-incisal angle. Distal outline is more convex than mesial outline and contact area is located at the junction between middle and incisal third. Cervical outline is semicircular with curvature towards the root. Incisal outline is represented by incisal edge which is straight and regular with sharp mesio-incisal angle and slightly rounded disto-incisal angle.

Labial Surface Labial surface of maxillary central incisor is smooth with convexity at the cervical third. Surface becomes relatively flat as the incisal edge is approached. Two shallow vertical depressions may be appreciated dividing the labial surface into three portions, each representing parts developed from three different lobes. Chronology of permanent maxillary central incisor

Root Root is conical in shape, broader at cervical 1/3rd, narrows through middle part to end in a relatively blunt apex. The apex may show a distal tilt.

MLATAL/LINGUAL ASPECT Palatal/lingual aspect is the surface of the tooth facing the palate/tongue.

Crown The lingual outline of maxillary central incisor is reverse of the labial outline. The crown and the root show convergence towards the lingual side which makes the mesial and the distal surfaces visible from this aspect. In contrast to the smooth labial surface, the lingual surface shows concavities and convexities.

Convexities There is a convex area called cingulum located at the cervical third which is placed slightly to the distal in a mesio-distal direction. Either side of the lingual aspect is bordered by linear elevations called marginal ridges. The ridge on the mesial side is called mesial marginal ridge and on the distal side is called distal marginal ridge. Mesial marginal ridge is slightly longer than

the distal marginal ridge as a result of distal location of the cingulum. The lingual surface also shows the presence of linguo-incisal ridge, which forms the incisal boundary of the lingual surface.

Concavity The major portion of the lingual aspect of the central incisor is occupied by a concavity called lingual fossa. The lingual fossa is M-shaped and is bounded superiorly by the cingulum, inferiorly by the linguo-incisal ridge and on either side by the mesial and distal marginal ridges. A deep developmental groove may be present on the lingual surface extending onto the cingulum. Cervical outline is semicircular with curvature towards the root.

Root Root is lingually converged and conical in shape with blunt and rounded apex. Cross section of the root is triangular in shape with rounded angles.

MESIAL ASPECT Mesial aspect is one of the proximal aspects that is closer to the midline of the face. Morphological characteristics of crown and roots are separately mentioned.

Crown Shape of mesial aspect: From the mesial aspect the crown appears triangular or wedge shaped with the base at the cervix and apex at the incisal edge.

Outlines of Mesial Aspect Labial outline is convex and the maximum convexity (crest of curvature) at the cervical third. Labial outline becomes relatively straight from the crest of curvature to the incisal edge. Lingual outline shows convexities and concavities: Cervical third is convex representing the cingulum followed by a concavity in the region of the lingual fossa in the middle and again followed by a convexity, the incisal ridge. Crest of convexity on lingual aspect is located at cervical third, on

cingulum. Cervical outline is curved with curvature towards the crown. The extent of curvature of cervical line is more on mesial side. The incisal edge and the root tip lies in the midline which bisects the tooth. Mesial surface: Mesial surface of central incisor is smooth and convex.

Root Root is cone shaped and tapered with a rounded apex. The root surface on the mesial aspect is relatively flat and may show longitudinal developmental depression.

DISTAL ASPECT Distal aspect is the proximal aspect that is away from the midline of the face. Distal aspect is similar to that of mesial aspect with slight differences. The crown appears to be broader from this aspect than the mesial aspect. Extent of curvature of cervical line is less on the distal aspect The root is tapered towards the rounded apex.

INCISAL ASPECT Incisal aspect is the cutting/biting surface of the tooth. The features described may be appreciated when the tooth is held in such a way that incisal edge is towards the observer, in a horizontal direction with labial aspect upwards. When the tooth is observed in this manner, root will be visible as it superimposed over the crown. The shape of the maxillary central incisor is triangular from this aspect with the base on the labial surface and the apex towards the cingulum. The mesio-distal dimension is greater than the labio-lingual dimension. The labial aspect shows a semicircular arch form and the lingual aspect is tapered and more convex at the cingulum. Slight disto-lingual twist of the incisal edge may be appreciated from this aspect because of lingual positioning of the

disto-incisal angle compared to that of mesio-lingual angle. As the incisal edge is located at the center, equal extent of the labial and lingual halves can be seen. The lingual fossa, marginal ridges and distally placed cingulum can be appreciated on the lingual aspect. Root cannot be appreciated from this aspect because it is superimposed on the crown.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS ‘Shovel shaped’ central incisors: The term shovel shaped is used to describe an incisor that has prominent mesial and distal marginal ridges and deep lingual fossa. Deep lingual pit: As a developmental variation, central incisors may show a deep lingual pit at the incisal border of cingulum. Such teeth may be prone to develop dental caries. Accessory lingual ridge: Rarely maxillary central incisors may show vertical ridges extending from cingulum to incisal edge. Talon cusp: At times in incisors, the cingulum may become very prominent to such an extent, it resembles eagle’s talon, which is referred to as talon cusp. In such cases chances of dental caries is considerably more. In addition, very prominent cingulum may interfere with occlusion and may cause trauma to tongue. Screw driver shaped central incisor: In patients affected by congenital syphilis which is a bacterial infection, central incisors may assume a screw driver shape. In this condition, due to the absence of middle lobe, mesial and distal outlines of central incisors converge incisally making incisal 1/3rd narrower than cervical 1/3rd. In addition these teeth may also show a notching of incisal edge.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE

Labial Aspect Straight incisal edge with sharp mesio incisal angle and rounded disto-incisal angle. Relatively straight mesial outline and slightly curved distal outline. Mesial contact area is more incisally placed (incisal 1/3rd, closer to mesioincisal angle) than distal (at junction of incisal and middle 1/3rd). Distal tilt of root.

Lingual Aspect Distal placement of cingulum. Longer mesial marginal ridge than distal marginal ridge.

Proximal Aspect Cervical line more curved on mesial aspect than distal.

Incisal Aspect Disto-lingual twist of incisal edge. Permanent maxillary right central incisor

21 Permanent Maxillary Lateral Incisors

Introduction to incisors Chronology and measurement table Morphology of maxillary lateral incisor Differences between maxillary central and lateral incisors

L

ateral incisors are the second teeth from the midline located on either side of the dental arch, distal to the central incisors. Lateral incisors bear close resemblance to central incisors and support them in functions. When compared to central incisors, these teeth are smaller and appears relatively long and narrow. The morphologic characteristics can be described from five aspects, namely labial, lingual, mesial, distal, and incisal aspects. Further, features on each aspect (except incisal) is described under two subheadings, i.e. crown and root.

LABIAL ASPECT Crown Shape of Labial Aspect Shape of maxillary lateral incisor is rectangular with a slight cervical convergence. Crown is smaller in all dimensions and is less symmetrical than that of central incisors.

Outlines of the Labial Aspect Mesial outline is slightly convex with contact area located at the junction of middle and incisal 1/3rd. Distal outline is more convex from cervix to disto-incisal angle with contact area at the middle of middle 1/3rd. Incisal outline is represented by incisal edge which is rounded or slightly curved with rounded incisal angles. Mesio-incisal angle is rounded in contrast to that of central incisors. Disto-incisal angle of maxillary lateral incisor is more rounded compared to mesio-incisal angle. More rounded disto-incisal angle along with convex outline gives a semicircular shape to the distal outline of the tooth. Cervical line is semicircular, curved towards the root.

Labial Surface Labial surface is smooth similar to that of central incisors, but is more convex with less prominent labial depressions.

Root Root tapers evenly from cervix to apex; apex is distally curved. Like crown, root is 2 mm narrower than central incisor but with same length giving a long and narrow appearance. Chronology of permanent maxillary lateral incisor

LINGUAL ASPECT Crown The lingual outline of maxillary lateral incisor is reverse of the labial outline. The crown and the root are narrower on the lingual side because of lingual convergence. Therefore, the mesial and the distal surfaces are visible from this aspect. In contrast to the smooth labial surface, the lingual surface shows concavities and convexities.

Concavities The lingual aspect of the lateral incisor shows a concavity called lingual fossa which is more pronounced than that of central incisors. The lingual fossa is inverted ‘V’ shaped and is bounded superiorly by the cingulum, interiorly by the linguo-incisal ridge and on either side by the mesial and distal marginal ridges. A deep developmental groove may be present on the lingual surface extending onto the cingulum. Cervical outline is semicircular with curvature towards the root.

Convexities Cingulum is seen as a convexity at the cervical third. Unlike the central

incisors, the cingulum is narrower and is located at the center in a mesiodistal direction. Mesial and distal sides of the lingual aspect are bordered by linear elevations called marginal ridges. In most of the lateral incisors, mesial and distal marginal ridges are more prominent than in central incisors. Incisally the lingual surface is bounded by a prominent linguo-incisal ridge.

Root Root is narrower lingually, conical in shape with blunt and distally tilted apex.

MESIAL ASPECT The mesial aspect of lateral incisor closely resembles the central incisors but is smaller in all dimensions, than central incisors.

Crown Shape of mesial aspect: From the mesial aspect the crown appears triangular or wedge shaped with the base at the cervical portion and apex at the incisal edge.

Outlines of Mesial Aspect Labial outline is convex with crest of curvature at the cervical third, near the cervical line. The outline becomes relatively straight from the crest of curvature to the incisal edge. Lingual outline shows convexities and concavities and the crest of curvature is located at cingulum. Cervical third of lingual outline is convex in the region of cingulum and concave at the lingual fossa. A slight convexity is again found at the incisal ridge. Crest of convexity on lingual aspect is located at cervical third, on cingulum. Cervical line is curved and the curvature is marked on the mesial side, towards the incisal edge. The incisal edge, is thicker than that of central incisor and lies either in line with or slightly labial to the root axis plane.

Mesial Surface Mesial surface is smooth and convex.

Root Root is cone shaped and tapering to a blunt apex.

DISTAL ASPECT Distal aspect of maxillary lateral incisor resembles the mesial aspect. The differences observed are: The width of crown appears thicker on distal side. Curvature of cervical line is less. Root may show a developmental groove.

INCISAL ASPECT Incisal aspect generally resembles that of central incisor except for its smaller size. Labial and lingual outlines are more rounded or convex, giving ovoid or round shape to the incisal aspect in contrast to the triangular shape of the central incisor. The cingulum is more prominent and is centered in a mesiodistal direction. Differences between central and lateral incisors (type traits) Central incisor Lateral incisor Larger in all dimensions

Smaller in all dimensions, than central incisor

Labial surface is smooth and relatively flatter except the cervical 1/3rd

Labial surface is smooth but is more convex with less prominent labial depressions

Contact areas are more incisally placed with mesial contact area at

Contact areas are more cervically placed with mesial contact area at

incisal third close to incisal angle and distal contact area at the junction between incisal and middle third

junction between incisal and middle third and distal contact area at the middle of middle 1/3rd

Incisal edge is straight

Incisal edge is rounded or curved

Mesio-incisal angle is sharp and disto-incisal angle is slightly rounded

Both the incisal angles are rounded: Disto-incisal angle is more rounded than mesio-incisal angle

Mesial outline of labial aspect is straight and distal outline is slightly rounded

Both mesial and distal outlines of labial aspect are convex and distal outline is distinctly convex

Shallow lingual fossa and is Mshaped

More deep lingual fossa with developmental groove and is inverted V-shaped

Cingulum is slightly off to distal, making mesial marginal ridge longer

Cingulum is narrower than that of central incisor and is at the center and marginal ridges are of equal length

Marginal ridges, cingulum and lingual fossa are relatively less prominent

Marginal ridges, cingulum and lingual fossa are more prominent

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS Permanent maxillary lateral incisors are less symmetrical, and may often show variations in the form and size. Peg shaped laterals: This is a common developmental variation observed in which maxillary laterals present with a characteristic conical shape. Peg

shaped lateral may also be observed as a developmental malformation caused due to congenital syphilis. Missing laterals: This is one of the commonest tooth that may be congenitally absent. ‘Shovel shaped’ lateral incisors: The term shovel shaped is used to describe a lateral incisor that has prominent mesial and distal marginal ridges and deep lingual fossa. Deep lingual pit: As central incisors, even lateral incisors may show a deep lingual pit at the incisal border of cingulum. Such teeth may be prone to develop dental caries. Accessory lingual ridge: Rarely maxillary lateral incisors may show vertical ridges extending from cingulum to incisal edge. Talon cusp: At times in lateral incisors, the cingulum may become very prominent to such an extent, it resembles eagle’s talon, which is referred to as talon cusp. In such cases chances of dental caries is considerable more, In addition, very prominent cingulum may interfere with occlusion and may cause trauma to tongue. Palatal gingival groove: Lateral incisors may show a deep groove extending from cingulum to the root surface.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Labial Aspect Curved incisal edge with relatively less rounded mesio-incisal angle and more rounded disto-incisal angle. Relatively more curved distal outline than mesial outline. Mesial contact area is more incisally placed (at junction of incisal and middle 1/3rd) than distal (middle of middle 1/3rd). Distal tilt of root.

Proximal Aspect Cervical line more curved on mesial aspect than distal. Permanent maxillary right lateral incisor

22 Permanent Mandibular Central Incisors

Introduction to incisors Chronology and mandibular central incisor Measurement table Morphology of mandibular central incisor Differences between maxillary and mandibular incisors

M

andibular incisors are four in number; two central incisors and two lateral incisors. Both mandibular central and lateral incisors have similar morphology. These teeth assist maxillary incisors in functions like cutting food, esthetics, speech and also in guiding the mandible while closing.

PERMANENT MANDIBULAR CENTRAL INCISORS Mandibular central incisors are two in number located on either side of the midline of mandibular arch, with their mesial surfaces in contact. They are the smallest teeth in permanent dentition. The morphologic characteristics of this tooth may be described from five aspects, namely labial, lingual, mesial, distal and incisal. Further, features on each aspect (except incisal) is described under two subheadings, i.e. crown and root.

Chronology of permanent mandibular central Incisor

LABIAL ASPECT Crown Shape of Labial Aspect Mandibular central incisors have a narrow long appearance from the labial aspect. The crown is nearly bilaterally symmetrical.

Outlines of Labial Aspect Mesial and distal outlines are relatively straight and evenly taper from contact areas to a narrow cervix. Mesial and distal contact areas are nearly at the same level and are located in the incisal 1/3rd close to the incisal angles. Incisal outline is represented by incisal edge which is straight and perpendicular to the long axis of tooth. Both mesio-incisal and disto-incisal angles are sharp. Cervical outline is convex and is curved towards the root.

Labial Surface Labial surface is smooth without any developmental lines. Surface is flat at

incisal 1/3rd, but slightly convex at cervical and middle 1/3rd.

Root Root is conical and it tapers to apex which may show a distal tilt.

LINGUAL ASPECT Crown Mandibular central incisor shows a lingual taper and therefore part of mesial and distal surfaces are visible from this aspect. At cervical 1/3rd, cingulum is present as a convexity, which is centered on the lingual aspect. Confluent with cingulum on either side, marginal ridges are seen. Between the marginal ridges, the lingual fossa is present as a slight concavity. In this tooth cingulum, marginal ridges and lingual fossa are not distinct.

Root Root is narrower lingually and conical in shape.

MESIAL ASPECT Crown Shape of Mesial Aspect Mandibular central incisor is wedge-shaped from proximal aspect with the base located at the cervix and apex at incisal edge.

Outlines of Mesial Aspect Labial outline is slightly convex at cervical 1/3rd, where crest of convexity is located from which the outline slopes rapidly to incisal edge. Lingual outline is relatively straight at cervical 1/3rd, in the region of less prominent cingulum, then becomes slightly concave till it joins to the incisal

edge. Both labial and lingual outlines show less curvature than maxillary incisors. Incisal outline is represented by incisal edge which is located lingual to the midline of the tooth (arch trait of mandibular teeth). A cervical line shows a deep curvature on the mesial aspect which extends up to the cervical third (only feature that help in identification of side).

Mesial Surface Mesial surface is smooth and flat except for slight convexity at incisal 1/3rd, where contact area is located.

Root Root outlines are straight up to middle 1/3rd, from where tapering start. Root tip is located in the midline. Development depression may be seen on the surface of the root. Differences between maxillary and mandibular incisors Maxillary incisors (arch traits) Mandibular incisors (arch traits) Maxillary incisors are larger (both crown and root) and less symmetrical

Mandibular incisors are smaller and relatively more symmetrical

Central and lateral incisors vary in size and in morphology

Both central and lateral incisors are nearly of same size and have somewhat similar morphology

Mesial and distal sides are not as flat as in mandibular incisors, rather is convex with contact areas located at different levels

Mesial and distal sides are relatively flat with contact areas located nearly at the same level, closer to the incisal edge

Incisal angles rounded

relatively

Incisal angles are relatively sharper

such

Less prominent anatomic landmarks.

Anatomic

are

landmarks

as

cingulum, marginal ridges and lingual fossa are more prominent. Lingual fossa often shows a pit

Lingual fossa do not show pits or grooves, making lingual surface relatively smooth

When observed from proximal Labial surface is inclined lingually so aspect, incisal edge is located in that incisal edge is located lingual to line with root axis plane the midline Crown is wider mesio-distally than labio-lingually

Crown is wider labio-lingually than mesio-distally

Since the difference between the crown length and width is only a little the crown of maxillary incisors has a squarish shape

The crown length is much more relative to the mesio-distal measurement giving the crown of mandibular incisors a thin long appearance

Since the upper incisors have a labial position in normal occlusion, attrition leads to lingual inclination of incisal edge

Since the lower incisors have a lingual position in normal occlusion, attrition leads to labial inclination of incisal edge

DISTAL ASPECT Morphology resembles mesial aspect except: Less curvature of cervical line More distinct developmental depression on the root.

INCISAL ASPECT From this aspect labial and lingual surfaces of the tooth are visible. Since the incisal edge is located lingual to midline, more of labial surface can be appreciated. The incisal edge is straight and perpendicular to the labio-lingual root axis plane. Bilateral symmetry of mandibular central incisor can be

better appreciated from this aspect. Crown shows a greater labio-lingual measurement than mesio-distal. The labial surface is broader and shows a considerable lingual inclination. Although the cervical 1/3rd of labial surface is convex, and relatively flatter middle and incisal 1/3rd as incisal edge is approached. The lingual surface shows a convexity at the cingulum which is centered in a mesio-distal direction. Middle and incisal 1/3rd are concave because of the lingual fossa.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS Developmental variations are relatively rare in mandibular incisors Bifurcated root: Rarely mandibular incisors may present with two roots, labial and lingual roots. Gemination: This is a developmental variation that occur due to an attempted division of one tooth germ into two, resulting in incomplete division. Affected tooth may present with a deep groove on the surface. Fusion: At times mandibular central incisor may appear large with adjacent lateral incisor missing. This results from fusion of two adjacent tooth germs resulting in formation of a large tooth instead of two.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE As permanent mandibular central incisor is bilaterally symmetrical, it may be difficult to differentiate the side. However, curvature of cervical line can be a feature that is helpful. On mesial aspect cervical line significantly more curved than on distal aspect and may extend up to cervical 1/3rd. Permanent mandibular right central incisor

23 Permanent Mandibular Lateral Incisors

Introduction Chronology and mandibular lateral incisor Measurement table Morphology of mandibular lateral incisor Differences between mandibular central and lateral incisors

M

andibular lateral incisors are two in number, located distal to the central incisors. They are larger and less symmetrical than central incisors. The morphologic characteristics of this tooth may be described from five aspects, namely labial, lingual, mesial, distal and incisal. Further, features on each aspect (except incisal) is described under two subheadings, i.e. crown and root.

LABIAL ASPECT Crown Shape of Labial Aspect Mandibular lateral incisors have a narrow long appearance from the labial aspect similar to that of central incisors. In contrast to central incisor, the crown is larger and not bilaterally symmetrical and is tilted distally.

Chronology of permanent mandibular lateral incisor

Outlines of Labial Aspect Mesial and distal outlines are relatively straight and taper from contact areas to a narrow cervix. Distal outline appears shorter and shows more convergence towards the cervix giving an impression that a small fraction is added to distal part of a symmetrical tooth. Crown shows a distal tilt on the root base. Mesial and distal contact areas are not located at the same level. Mesial contact area is close to mesio-incisal angle while distal contact area is located at incisal 1/3rd, at a more cervical location. Incisal outline is represented by incisal edge which shows a slope to distal direction. Mesio-incisal angle is sharp and disto-incisal angle is slightly rounded. Cervical outline is convex and is curved towards the root.

Labial Surface Labial surface is smooth without any developmental lines. Surface is flat at incisal 1/3rd, but slightly convex at cervical and middle 1/3rd.

Root Root is conical and it tapers to apex which may show a distal tilt.

LINGUAL ASPECT Crown Morphologic features are similar to that of central incisor. Tooth shows a lingual taper and therefore a part of mesial and distal surfaces are visible from this aspect. At cervical 1/3rd, cingulum is present as a convexity, which is slightly distally placed. Confluent with cingulum on either side marginal ridges are seen. Distal placement of the cingulum makes mesial marginal ridge longer than that of distal marginal ridge. Between the marginal ridges lingual fossa is present as a slight concavity. Marginal ridges, cingulum and lingual fossa may be slightly more prominent than in central incisors but not as prominent as that of maxillary incisors.

Root Root is narrower lingually and is conical in shape.

MESIAL ASPECT Crown Mesial aspect is similar to that of central incisors. Except for the difference in size, no morphological difference is appreciated.

Shape of Mesial Aspect Mandibular lateral incisor is wedge shaped from proximal aspects with the base located at the cervical region and apex at incisal edge.

Outlines of Mesial Aspect Labial outline is slightly convex at cervical 1/3rd where crest of convexity is located from which the outline slopes rapidly to incisal edge. Lingual outline is relatively straight at cervical 1/3rd in the region of less prominent cingulum, and then becomes slightly concave till it joins to the

incisal edge. Both labial and lingual outlines show less curvature than maxillary incisor. Incisal outline is represented by incisal edge which is located lingual to the midline of the tooth. A cervical line shows curvature in an incisal direction and is deep on the mesial aspect.

Mesial Surface Mesial surface is smooth and flat except for slight convexity at incisal 1/3rd where contact area is located.

Root Root is conical with an apical taper. Root tip is located in the midline. Developmental depression may be seen on the surface.

DISTAL ASPECT Morphology resembles that of mesial aspect. While comparing the mesial and distal aspects of a mandibular lateral incisor following differences can be observed: The lingual inclination of the crown appears to be more on distal aspect because of disto-lingual inclination of the incisal ridge. Differences between permanent mandibular central and lateral incisors Mandibular central incisors (type Mandibular lateral incisors (type traits) traits) Smallest tooth in permanent dentition

Larger than central incisors

Bilaterally symmetrical

Not bilaterally symmetrical

Crown is straight without any distal tilt

Crown is tipped distally on the root

Incisal edge is straight, perpendicular to the long axis of tooth

Incisal edge is inclined distally

Contact areas are nearly at the same level, close to incisal edge

Contact areas are at different levels with more cervically located distal contact

Both mesio-incisal angle and distoincisal angles are sharp

Mesio-incisal angle is sharp and disto-incisal angle is rounded

Cingulum is placed at the center with mesial and distal marginal ridge of equal length

Cingulum is distally placed with longer mesial marginal ridge

Incisal edge is straight when viewed from incisal aspect and do not show disto-lingual twist

Incisal edge is curved in a distolingual direction

On the distal aspect, the degree of cervical curvature is less.

INCISAL ASPECT Crown shows a greater labio-lingual measurement than mesio-distal. In contrast to central incisor, lateral incisor does not appear bilaterally symmetrical. From the incisal aspect labial and lingual surfaces of tooth can be visible. Since the incisal edge is located lingual to midline more of labial surface can be appreciated. Incisal edge is not straight in a mesio-distal direction, but slightly curved disto-lingually corresponding to the curvature of mandibular arch. The labial surface is broader, inclines lingually. Distal placement of cingulum, marginal ridges and lingual fossa can be better appreciated from this aspect.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS

Developmental variations are relatively rare in mandibular incisors: Bifurcated root: Rarely mandibular incisors may present with two roots, labial and lingual roots. Gemination: This is a developmental variation that occur due to an attempted division of one tooth germ in to two, resulting in incomplete division. Affected tooth may present with a deep groove on the surface. Fusion: At times mandibular lateral and central incisors may fuse together resulting in formation of a large tooth instead of two.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Labial Aspect Incisal edge sloping distally. Crown tipped distally on root. Contact areas are at different level with more cervically located distal contact. Sharp mesio-incisal angle and rounded disto-incisal angle.

Lingual Aspect Distal placement of cingulum. Longer mesial marginal ridge than distal marginal ridge.

Proximal Aspect Cervical line more curved on mesial aspect than distal.

Incisal Aspect Disto-lingual twist of incisal edge. Permanent mandibular right lateral incisor

24 Peermanent Maxillary Canines

Introduction Chronology of maxillary canine Measurement table Morphology of maxillary canine Developmental variations and clinical considerations

C

anines are four in number; two in maxillary arches and two in mandibular arches. They are the third teeth from midline situated between lateral incisor and first premolar, at the corners of mouth. Their name is derived from the Latin word for dog (canus) because these teeth resemble dogs’ teeth. These teeth are also called cuspids. Canines are the longest teeth in the dental arch and usually the last one to be lost. The selfcleansing property of these teeth resulting from their particular shape and efficient anchorage to the jaw help in their longer retention. Canines help to cut or shear the food. It also functions with the incisors to support the lips and facial muscles. These teeth have significant importance in facial expression. Canines are important teeth in occlusion and also in protecting the posterior teeth when the jaw moves laterally. Class traits of canines Permanent canines are the longest teeth in the human dentition with long, thick root to ensure proper anchorage to alveolar bone.

Incisal edge is characterized by a pointed cusp, which is formed by mesial and distal slope meeting at an angle. Mesial slope is shorter than distal slope. Canines do not have mamelons but may have a notch on either cusp slopes. Have a distinct labial ridge, extending in a vertical direction from cervical region to the cusp tip. Canines have greater labio-lingual measurement than mesio-distal. A lingual ridge is present, which divides the lingual fossa into mesial and distal fossae.

PERMANENT MAXILLARY CANINES Maxillary canines are two in number located on either side of the dental arch, distal to the lateral incisors. The morphologic characteristics can be described from five aspects, namely labial, lingual, mesial, distal and incisal. Further, features on each aspect (except incisal) is described under two subheadings, i.e. crown and root.

LABIAL ASPECT Crown Shape of Labial Aspect The crown is roughly pentagonal in shape and is narrower by 1 mm than central incisors. Chronology of permanent maxillary canine

Outlines of Labial Aspect Mesial outline may be convex or slightly concave from cervix to the contact area which is located at the junction of middle and incisal 1/3rd. Distal outline is concave from cervix to the contact area. Contact area is more cervically located when compared to the mesial contact area and is at the middle of middle 1/3rd. Cervical outline is curved in an apical direction. Incisal edge is represented by the cusp and cusp slopes. In contrast to broadly curved incisal edge in incisors, incisal edge in canines is divided into two parts, i.e. mesial slope and distal slope meeting at an angle to form a point called cusp. Distal slope of the cusp is longer than mesial cusp slope. The junction between distal outline and distal cusp slope is rounded, while the junction between mesial outline and mesial cusp slope is more angular. Cusp tip is in line with root axis line and it is more mesially placed.

Labial Surface Labial surface is convex. The middle labial lobe is well developed forming a prominent labial ridge running cervico-incisally up to the cusp. On either side of labial ridge shallow depressions are seen dividing the labial aspect into mesial, middle and distal lobes.

Root

Maxillary canines have the longest root. Root is slender and conical with bluntly pointed apex which bends distally.

LINGOAL/PALATAL ASPECT Crown The crown outline on the lingual aspect is similar to that of the labial aspect. Canines show a significant lingual tapering because of which both crown and root are narrower lingually. As in other anterior teeth, lingual aspect of canines also shows convexities and concavities.

Convexities The cingulum is seen as a convexity at cervical 1/3rd and is very prominent resembling a cusp. In a mesio-distal direction, cingulum is centered over the tooth. Mesial and distal sides of the lingual aspect are bordered by linear elevations called marginal ridges. Both the mesial and distal marginal ridges are prominent: Distal marginal ridge being more elevated than the mesial marginal ridge. In addition, canines have a distinct lingual ridge running in a cervico-incisal direction from cingulum to the cusp tip. Cingulum, marginal ridges and lingual ridge are confluent with each other with a little evidence of developmental grooves.

Concavities The lingual aspect of canines shows a concavity called lingual fossa, which is more pronounced than those of other anterior teeth. The lingual fossa is divided into mesial and distal lingual fossae by the lingual ridge.

Root Root is narrower lingually and much of mesial and distal surfaces are seen from this aspect.

MESIAL ASPECT

Crown Shape of Mesial Aspect The crown is wedge shaped with base at cervical 1/3rd and apex at cusp tip. The entire crown appears bulkier from this aspect because of prominent labial and lingual ridges. A greater labio-lingual measurement of this tooth can be appreciated from this aspect.

Outlines of Mesial Aspect Labial outline is convex with the crest of convexity at cervical 1/3rd which is not very close to the cervical line unlike that of incisors. The outline becomes less convex as it proceeds incisally and becomes more or less straight as it approaches the cusp tip. The extent of convexity of labial outline is more than other anterior teeth. Lingual outline is convex in the cervical 1/3rd at cingulum where the crest of curvature is located. The outline straightens and becomes concave at middle 1/3rd and again convex in the region of incisal ridge. Cervical line is curving towards the crown with a greater curvature on mesial side than distal. Cusp tip is placed labial to root axis line in most of the canines while in some it may be in line with the root axis line.

Mesial Surface Mesial surface is convex on all aspects except for a shallow concavity between contact area and cervix.

Root Root is conical with an apical taper. Mesial surface of root has a deep developmental depression running cervico-apically.

DISTAL ASPECT

General morphology is similar to that of mesial aspect. Differences observed are: Curvature of cervical line is less Distal marginal ridge is heavier and regular Developmental depression on distal surface of the root is more pronounced than on the mesial side.

INCISAL ASPECT From the incisal aspect a greater labio-lingual measurement than mesio-distal can be well appreciated. The crown is not symmetrical. The mesial half of the tooth appears more convex and bulkier labio-lingually than distal half so that the distal half appear as though it is pulled and stretched to make contact with adjacent tooth. Cusp tip is located labial to midline in a labio-lingual direction and mesial in a mesio-distal direction with the mesial and distal cusp slopes almost in straight line mesio-distally. Although parts of labial and lingual surfaces are visible from this aspect more of lingual surface is seen because of the labial placement of cusp tip. Labially, the labial ridge can be seen. The labial surface is markedly convex near cervical region, becoming broader and flatter at middle and incisal 1/3rd. Lingual outline forms a shorter arc than labial outline because of lingual taper. Lingual surface presence a prominent cingulum which is at center, marginal ridges, lingual ridge and lingual fossae.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS Permanent maxillary canines show wide variation in size from very small to large. Root division: Root may be bifurcated into labial and lingual roots. Talon cusp: At times in canines the cingulum may become very prominent to

such an extent, it resembles eagle’s talon, which is referred to as talon cusp. In such cases chances of dental caries is considerable more. In addition, very prominent cingulum may interfere with occlusion and may cause trauma to tongue.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Labial Aspect Distal slope of the cusp is longer than mesial slope. Relatively more curved distal outline than mesial outline. Mesial contact area is more incisally placed (at junction of incisal and middle 1/3rd) than distal (middle of middle 1/3rd). Distal tilt of root.

Proximal Aspect Cervical line more curved on mesial aspect than distal. Distal surface of the crown often shows a concavity below contact area.

Incisal Aspect The mesial half of the tooth appears more convex and bulkier labio-lingually than distal half. The distal half of the crown appears stretched with greater mesiodistal width than the mesial half. Permanent maxillary right canine

25 Permanent Mandibular Canines

Introduction Chronology of mandibular canines Measurement table Morphology of mandibular canine Differences between maxillary and mandibular canines Developmental variations and clinical considerations

M

andibular canines are the third teeth from the midline situated on either side of mandibular arch, between lateral incisor and first premolar. They bear close resemblance to maxillary canines and assist them in function. The morphologic characteristics are described from five aspects, namely labial, lingual, mesial, distal, and incisal. Further, features on each aspect (except incisal) is described under two subheadings, i.e. crown and root.

LABIAL ASPECT Crown Shape of Labial Aspect The crown of mandibular canine appears narrower and longer when

compared to bulky maxillary canine. The long thin appearance is created by lesser mesio-distal measurement, incisally located contact areas and nearly straight mesial and distal outlines. Crown is tilted distally on the root base.

Outlines of Labial Aspect Mesial outline is slightly convex and is in line with mesial root outline. Mesial contact area is located close to mesio-incisal angle. Distal outline is somewhat parallel to mesial outline and contact area is located at the junction of incisal and middle 1/3rd (more cervical location than on mesial side). Incisal outline is represented by the cusp with its ridges. Cusp has mesial and distal cusp ridges meeting at an obtuse angle making it less sharp. Mesial cusp ridge is nearly horizontal and is noticeably shorter than longer distal cusp ridge which slopes in an apical direction. Cervical line is curved towards the root.

Labial Surface Shows a distinct ridge extending from cervical 1/3rd to the cusp tip which is named as labial ridge and is less prominent than in maxillary canine.

Root Root is conical in shape with an apical taper ending in a blunt apex.

LINGUAL ASPECT Crown Both crown and root are narrower on the lingual side because of lingual convergence. Lingual aspect of mandibular canine resembles that of maxillary canine and shows convexities and concavities. At cervical 1/3rd, cingulum is present as a convexity, which is slightly distally placed. Confluent with cingulum on either side marginal ridges are seen. Distal placement of cingulum makes the mesial marginal ridge longer but distal

marginal ridge is bulkier. The lingual fossa is divided into mesial and distal lingual fossae by the lingual ridge that runs from cingulum to cusp tip. In contrast to the maxillary canine in mandibular canine various anatomic landmarks such as cingulum, marginal ridges and lingual fossae are less prominent. Therefore lingual aspect appears flatter and smoother. Chronology of permanent mandibular canine

Root Root is conical in shape with an apical taper ending in a blunt apex and is narrower on lingual aspect throughout its length.

MESIAL ASPECT Crown Shape of Mesial Aspect From the mesial aspect mandibular canine is wedge-shaped with the base located at the cervix and apex at incisal edge. A greater bulk in labio-lingual direction at cervical 1/3rd can be well appreciated. But the incisal portion appears thinner and cusp appears more pointed due to less prominent lingual ridge.

Outlines of Mesial Aspect Labial outline is curved although the curvature is lesser than that of maxillary canine. Crest of curvature of labial outline is more close to the cervix. From the crest of convexity the labial outline shows a lingual inclination up to the cusp tip. Lingual outline is relatively straight at cervical 1/3rd in the region of less prominent cingulum, and then becomes slightly concave till it joins to the incisal edge. Lingual outline also shows less curvature than maxillary canine. Cervical line is curved in an incisal direction and is to a deeper degree than in maxillary canines. The cusp tip is located lingual to the root axis line in most of the specimens and in few it may be even centered over root axis line.

Mesial Surface Mesial surface is convex on all aspects except for a shallow concavity between contact area and cervix. Differences between maxillary and mandibular canines (arch traits) Maxillary canine Mandibular canine Crown is wider with convex mesial and distal outlines

Crown is long and narrow with relatively straight mesial and distal outlines

Cusp is sharp

Less sharp

Contact areas are at different levels and is more cervically located

Nearly at same level and is more incisally placed

Mesial slope is shorter than distal slope

Mesial slope is much shorter than distal slope

Labial ridge is prominent

Labial ridge is less prominent

Cingulum, lingual ridge, lingual fossae, and marginal ridges are prominent

Relatively smooth lingual surface with less prominent cingulum, lingual ridge, lingual fossae and marginal ridges

Cingulum is centered

Distally placed

Cusp tip is located labial to midline

Lingual to midline

Both the cusp slopes are in straight line

Distal cusp slope is lingually placed

10. Crown appears less symmetrical when viewed from incisal aspect

More symmetrical

Root Root outlines are straight up to middle 1/3rd, from where tapering start. Root tip is more pointed. Deep developmental depression may be seen on root surface.

DISTAL ASPECT Distal aspect resembles mesial aspect except for: Lesser curvature of cervical line Lingual placement of disto-incisal angle due to disto-lingual twist of crown More prominent distal marginal ridge Deeper developmental depression on root.

Incisal Aspect Greater labio-lingual measurement compared to mesio-distal can be appreciated from this aspect. When the tooth is viewed from this aspect the labial and lingual surfaces and cusp can be seen. Labial contour is wider than

lingual contour because of considerable lingual convergence. When viewed from incisal aspect crest of contour of labial outline is more mesially located. Distal half of the crown appears more flat compared to convex mesial portion. Crest of contour of lingual outline is located over cingulum which may be distally located making mesial marginal ridge longer. Cusp tip is located more mesially in a mesio-distal direction while lingual to the center in a labio-lingual direction. The cusp ridges are lingual to the cusp which is more so in case of distal cusp ridge. The disto-incisal line angle is located more lingually due to disto-lingual twist of the crown to follow the dental arch.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS Permanent maxillary canines show wide variation in size from very small to large. Root division: Root may be bifurcated into labial and lingual roots.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Labial Aspect Crown is tilted distally on the root base. Distal slope of the cusp is longer than mesial slope. Mesial contact area is more incisally placed, located close to mesio-incisal angle than distal (at junction of incisal and middle 1/3rd). Distal tilt of root.

Lingual Aspect Cingulum is distally placed. Mesial marginal ridge is longer than distal marginal ridge.

Proximal Aspect Cervical line more curved on mesial aspect than distal. Distal marginal ridge is more prominent. Developmental depression on distal surface of root is more prominent.

Incisal Aspect The distal half of the crown appears more flat compared to convex mesial half. The disto-incisal line angle is located more lingually due to disto-lingual twist of the crown to follow the dental arch. Permanent mandibular right canine

26 Permanent Maxillary First Premolars

Introduction Chronology of maxillary first premolar Measurement table Morphology of maxillary first premolar Developmental variations and clinical considerations

M

axillary premolars are four in number, a first and a second premolar located on either side of the arch. These teeth are located between canine and molars. Premolars are the successors of deciduous molars and there are no premolars in deciduous dentition. They are called premolars because of their location before molars and along with molars they are called posterior teeth. The functions include assisting in mastication and maintenance of vertical height of face. Being a posterior teeth, they also shares some features with other posterior teeth. The common characteristics of posterior teeth are Greater faciolingual measurements when compared to mesio-distal measurements. Broader contact areas, nearly at the same level. Shorter crown, cervico-occlusally when compared to other anterior teeth. Marginal ridges are in horizontal plane while in anteriors are vertical plane.

From mesial and distal aspect the crest of convexity is not as much cervically as in case of anteriors. General characteristics of maxillary premolars (arch traits) Both maxillary premolars are more alike in morphology with two (one buccal and one lingual) well developed cusps. First premolar is larger than second premolar. In maxillary premolars, both buccal and palatal cusps are more or less equally developed and are functional cusps.

PERMANENT MAXILLARY FIRST PREMOLAR Maxillary first premolars belongs to the group of bicuspids and are situated distal to the maxillary canines on either side. The tooth resembles the canine from the buccal aspect with a few differences. The morphologic characteristics of maxillary first premolar can be described from five aspects, namely buccal, palatal/lingual, mesial, distal, and occlusal. Further, features on each aspect (except occlusal) is described under two subheadings, i.e. crown and root.

BUCCAL ASPECT Crown Shape of Buccal Aspect Shape of the crown from the buccal aspect is roughly trapezoidal with the shorter arm of the uneven side representing the cervical portion. This appearance is created by the cervical convergence. The mesio-distal dimension at the cervix is 2 mm less than its width at the points of its greatest mesio-distal measurement. Chronology of permanent maxillary first premolar

Outlines of Buccal Aspect Mesial outline shows a slight concavity from the cervical line to the contact area (crest of curvature) which lies immediately occlusal to the halfway point between the cervical line and the buccal cusp tip. Distal outline is relatively straight or less convex than the mesial outline from the cervical line to the contact area. The contact area (crest of curvature) on distal side lies nearly at the same level or slightly more occlusal to the contact area on mesial side, which is broader. Occlusal outline is represented by the buccal cusp and cusp slopes. The buccal cusp is long and has a pointed tip and shows mesial and distal cusp slopes. The mesial cusp slope is straighter and longer when compared to the distal slope which is shorter and more curved. The tip of the buccal cusp lies distal to a line bisecting the buccal surface of the crown. Cervical outline is semicircular with the convexity towards the root.

The Buccal Surface Buccal surface is convex with a prominent middle lobe. This is seen as a ridge on the buccal surface and is called the buccal ridge which runs vertically from the cervical region to the buccal cusp tip. On either side of the buccal ridge, i.e. mesial and distal, there may be shallow developmental depressions which demarcate the mesio-buccal and disto-buccal lobe from

the middle buccal lobe.

Root Although the maxillary first premolar has two roots, the buccal root superimposes the palatal root, and therefore only the buccal root is visible from this aspect. The buccal root is tapered apically with blunt apex.

PALATAL OR LINGUAL ASPECT Crown The palatal aspect of the crown is the reverse of the outline of the buccal aspect. The crown tapers towards the lingual and the tapering is more from the distal aspect, so that more of the distal surface is seen from this aspect. The palatal surface is spheroidal or smoothly convex with convex mesial and distal outline which are in continuation with the mesial and distal cusps slopes of the palatal cusp. The palatal cusp is 1 to 1.5 mm shorter than the buccal cusp which makes a portion of the buccal cusp visible from this aspect. The cusp slopes of lingual cusp meet at somewhat rounded angle. The cervical line is convex and regular with the curvature towards the root.

Root In most of the cases, only the palatal root is visible as it superimposes the buccal root. A portion of the buccal root may also be visible from this aspect.

MESIAL ASPECT Crown Shape of Mesial Aspect Shape of the crown from the mesial aspect is roughly trapezoidal with the

occlusal outline representing the shorter arm of the uneven side. Buccolingual dimension appears to be greater than the mesio-distal.

Outlines of Mesial Aspect Buccal outline is convex from the cervical line to the cusp tip. The crest of curvature is at junction of cervical and middle third from where the curvature becomes less. Palatal or lingual outline shows a smooth curvature from the cervical line to the cusp tip with crest of curvature at the middle of middle third. Occlusal outline is represented by the cusps and marginal ridge. Both the buccal and lingual cusps are visible from this aspect and the tips of the cusps are well within the confines of the root trunk. Buccal cusp is more prominent, longer and the tip is in line with the center of the buccal root. Lingual cusp is 1 mm shorter and less sharp than the buccal cusp and the tip is in line with the lingual border of the lingual root. The mesial marginal ridge is prominent, which is located at the level of junction of occlusal and middle third. The mesial marginal ridge is traversed by a distinct developmental groove which crosses the marginal ridge immediately lingual to the mesial contact area. This groove is called mesial marginal developmental groove and extends from the central groove to a short distance on the mesial aspect. The cervical line may be regular or irregular and is directed towards the occlusal aspect.

Mesial Surface Mesial surface appears to be convex at all points except for the marked depression called the mesial developmental depression or canine fossa, immediately cervical to the contact area which may extend up to the level of the root bifurcation.

Root The root begins at the cervix as a single trunk and shows bifurcation giving rise to a buccal root and a lingual root. The level of bifurcation varies from the middle half to apical third of root length. Because of the apical location of the furcation area, furcation involvement of this tooth is least likely to occur

in periodontal diseases. If involved prognosis is poor. A developmental groove and a depression are present on the root surface below the furcation area.

DISTAL ASPECT Gross morphology is similar to that of mesial aspect with a few differences. Surface shows convexity from almost all points except for a small flattened area cervical to the contact area. No developmental groove crossing the distal marginal ridge on this aspect. No developmental depression on this aspect. Curvature of cervical outline is very less and is almost straight from buccal to palatal. Bifurcation of root is at an apical level compared to mesial aspect.

OCCLUSAL ASPECT Shape and Outlines Shape is hexagonal; the six sides are mesio-buccal, disto-buccal, mesial, distal, mesio-lingual and disto-lingual. The sides are unequal and the length of each side depends on the location of four crests. Crest of the buccal outline (buccal crest) is located distal to the midline while the crest of the lingual outline (lingual crest) is mesial to the midline in a bucco-lingual direction. Crests of both mesial and distal outlines, i.e. mesial and distal crests are located in the buccal half, but the location of distal crest is more buccally when compared to the mesial crest. Distance between the buccal crest and mesial crest is more than the distance between the buccal crest and distal crest making the mesio-buccal outline longer than the disto-buccal outline. This is the result of distal location of buccal crest and buccal location of distal crest when compared to the relatively lingual location of mesial crest.

Because of the buccal placement of distal crest when compared to the lingually located mesial crest, the distal outline becomes longer than mesial outline. The mesial placement of lingual crest makes the disto-lingual outline longer than mesio-lingual outline. Greater bucco-lingual dimension of crown than the mesio-distal dimension can be appreciated from this aspect. Mesio-buccal cusp ridge and mesial marginal ridge meet at an angle of 90 degree and disto-buccal cusp ridge and distal marginal ridge meet at an acute angle. Occlusal aspect of maxillary first premolar shows slight convergence to the palatal aspect. The degree of lingual convergence is more from distal aspect. Mesial outline is relatively straight and the mesio-lingual line angle is more distinct while the disto-lingual line angle is rounded so that distal and disto-lingual outline together forms a semicircular outline.

Occlusal Surface Occlusal surface shows various anatomic landmarks such as cusps, ridges, fossae and grooves.

Cusps Premolar has two equally developed cusps, one buccal cusp and one palatal cusp. Buccal cusp is 1 mm longer and more pointed than palatal cusp.

Ridges Triangular ridges of buccal and palatal cusps are seen extending from the tip of cusp to the centre of occlusal aspect. Buccal triangular ridge is more prominent than palatal triangular ridge. Transverse ridge: The triangular ridge of the buccal cusp meets the triangular ridge of the palatal cusp to form a transverse ridge. Mesial marginal ridge: Forms the mesial boundary of the occlusal aspect and is located more occlusally than the distal marginal ridge. Distal marginal ridge: It is located at distal margin of occlusal aspect and is more cervically placed. Cusp ridges of buccal and lingual cusp forms the buccal and lingual boundary of occlusal aspect.

Grooves Developmental and supplemental grooves are seen.

Developmental grooves Central developmental groove divides the occlusal surface into equal halves, extending in a mesio-distal direction from mesial triangular fossa to distal triangular fossa. Mesio-buccal and disto-buccal developmental grooves: There are two collateral developmental grooves extending from mesial and distal pit respectively in a buccal direction. Mesial marginal developmental groove: This is a distinguishing feature seen in maxillary first premolar. This groove starts from the mesial pit as an extension from the central groove, runs in a mesial direction across the mesial marginal ridge immediately lingual to the mesial contact area and ends on the mesial surface.

Supplementary grooves Supplementary grooves may be seen in addition to the developmental grooves, and are relatively few in number.

Fossae and Pits Mesial triangular fossa is present as a triangular depression just distal to mesial marginal ridge. Distal triangular fossa is located mesial to distal marginal ridge. Pits may be present in triangular fossae where the grooves converge.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS Single root: In maxillary first premolars, root may remain undivided and present as single root.

Leongs premolar/Dens evaginatus: At times an accessory tubercle may be seen on occlusal aspect between buccal and lingual cusps. This is referred to as Leongs premolar/Dens evaginatus. This structure may interfere with occlusion. At times wearing away of covering enamel and dentin lead to exposure of pulp, necessitating root canal treatment. Mesial developmental depression on mesial aspect of crown that extends even onto the root may increase the possibility of periodontal diseases.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Buccal Aspect Mesial slope of the buccal cusp is longer than distal slope Buccal surface shows prominent depression mesial to buccal ridge Distal tilt of root

Palatal Aspect Lingual cusp tipped to the mesial side.

Proximal Aspect Cervical line is more curved on mesial aspect than distal. Distal marginal ridge is more cervically located compared to mesial marginal ridge. Presence of mesial marginal developmental groove. Presence of mesial developmental depression extending from crown to the mesial surface of root.

Occlusal Aspect Mesiobuccal cusp slope is longer. Straight mesial outline.

Angle between mesiobuccal and mesial outline is nearly 90 degree. Distolingual outline of occlusal aspect is curved with lingual convergence. Longer and convex distal marginal ridge. Large and deeper distal triangular fossa. Permanent Maxillary First Premolars

27 Permanent Maxillary Second Premolars

Introduction Chronology of maxillary second premolar Measurement table Morphology of maxillary second premolar Differences between maxillary first and second premolars Developmental variations and clinical considerations

M

axillary second premolars closely resemble the first premolar in its general morphology and supplement them in functions such as mastication and maintenance of vertical height of face. In contrast to first premolars, second premolars are more rounded and has single root. The morphologic characteristics can be described from five aspects, namely buccal, palatal/lingual, mesial, distal, and occlusal. Further, features on each aspect (except occlusal) is described under two subheadings, i.e. crown and root.

BUCCAL ASPECT Crown Shape of Buccal Aspect

Shape of the crown is squarish and is less angular. Crown shows lesser degree of cervical convergence.

Outlines Mesial outline is slightly curved with mesial contact area located near the junction of middle and occlusal 1/3rd. Distal outline is more convex than mesial outline and contact area on distal side is slightly more cervically placed than that of mesial. Occlusal outline is represented by buccal cusp and cusp slopes. Cusp slopes meet at an obtuse angle making the buccal cusp tip less pointed. In contrast to the first premolar, mesial cusp slope is shorter than the distal cusp slope. Cervical line is only slightly curved towards the root. Buccal surface: Buccal surface is smooth and convex. The middle buccal lobe is well developed to form a buccal ridge that extends cervico-occlusally up to the cusp tip. The buccal ridge is less prominent in second premolar than in first premolar. Very shallow depression may be present on either side of ridge.

Root Maxillary second premolar has only one root. Root is conical in shape with tapered apex bending distally. Chronology of permanent maxillary second premolar

PALATAL OR LINGUAL ASPECT Crown Palatal aspect is slightly narrower than the buccal due to palatal convergence, but the degree of convergence is less compared to that of first premolar. The palatal cusp is sharp and its height is almost same as that of buccal cusp. Tip of the palatal cusp is slightly mesially located to the center, in a mesiodistal direction.

Root From palatal aspect root is smooth with a little convergence.

MESIAL ASPECT Crown Shape of Mesial Aspect Shape of the crown from the mesial aspect is somewhat similar to that of first premolar and is roughly trapezoidal with the occlusal outline representing the

shorter arm of the uneven side.

Outlines of Mesial Aspect Buccal outline is convex from the cervical line to the cusp tip. The crest of curvature is near to the junction of cervical and middle one third. Palatal or lingual outline shows a smooth curvature from the cervical line to the cusp tip with crest of curvature at the center of middle third. Occlusal outline is represented by the buccal and palatal cusps and mesial marginal ridge. Both the buccal and palatal cusps are visible from this aspect and the tips of the cusps are well within the confines of the root trunk. Buccal and palatal cusps are nearly of the same height. The intercuspal distance between the buccal and palatal cusps is more, making the occlusal table wide. The mesial marginal ridge is slightly concave and is located more occlusally when compared to the distal marginal ridge. No developmental groove is found crossing the mesial marginal ridge of second premolar. The cervical line is directed towards the crown with a shallow curvature.

Mesial Surface Mesial surface appears to be convex and the mesial developmental depression observed in first premolar is not found in second premolar.

Root Root is conical and shows shallow depression running longitudinally on mesial surface.

DISTAL ASPECT Morphology is similar to that of mesial aspect. Distal marginal ridge is more cervically placed when compared to mesial. Therefore more of occlusal aspect can be seen. (Feature common to all posterior teeth except mandibular first premolar).

OCCLUSAL ASPECT

Shape Occlusal aspect of maxillary second premolar is less angular and is ovoid in shape. Greater bucco-lingual dimension of crown than the mesio-distal dimension can be appreciated from this aspect. Because of less lingual convergence, the buccal and palatal halves of occlusal surface are almost equal in width. Tooth is bilaterally more or less symmetrical. When observed from occlusal aspect contact area on mesial side is at the junction of buccal and middle 1/3rd and on distal side is slightly lingual to the position of mesial contact area. Buccal ridge appears to be less prominent. Lingual crest may be slightly mesially located.

Occlusal Surface Occlusal surface shows various anatomic landmarks such as cusps, ridges, grooves, fossae and pits. Differences between maxillary first and second premolars (type traits) Maxillary first premolar Maxillary second premolar Larger, longer and sharper buccal cusps

Smaller, shorter and less sharp buccal cusp

Mesial slope of buccal cusp is longer than distal slope

Distal slope of buccal cusp is longer than mesial slope

Prominent buccal ridge

Less prominent buccal ridge

More cervical convergence crown on buccal aspect

of

Less cervical convergence of crown on buccal aspect

Crown is narrower on lingual aspect

Lingual taper of crown is less

Buccal cusp is longer than palatal cusp

Both buccal and palatal cusps are nearly of same height

Mesial developmental groove is present crossing over the marginal ridge to the mesial surface

No such groove is seen

Mesial developmental depression on the mesial surface of crown and root

No mesial developmental depression on the mesial surface of crown and root

Cusps are relatively closer and occlusal table is narrower

Cusps are spread apart and occlusal table is wider

10. Occlusal aspect is asymmetrical and is hexagonal in shape

Occlusal aspect is symmetrical and is oval in shape

11. Central groove is longer

Central groove is shorter

12. Supplementary grooves relatively a few in number

are

Supplementary grooves are many making occlusal aspect irregular or wrinkled in appearance

13. First premolar usually has two roots

Second premolar usually has only one root

Cusps Second premolar has two equally developed cusps: One buccal cusp and one palatal cusp, Buccal and palatal cusps are nearly of same height. The intercuspal distance between the buccal and palatal cusp tips is more making the occlusal table wide.

Ridges Triangular ridges of buccal cusp and palatal cusp are seen extending from the tip of cusp to the center of occlusal aspect. Transverse ridge: A transverse ridge is formed by union of the triangular ridge of the buccal cusp and the triangular ridge of the palatal cusp.

Mesial marginal ridge: Forms the mesial boundary of the occlusal aspect and is located more occlusally than the distal marginal ridge. Distal marginal ridge: It is located at distal margin of occlusal aspect and is more cervically placed. Cusp ridges of buccal and palatal cusps forms the buccal and palatal boundary of occlusal aspect.

Grooves Central developmental groove is relatively short and it extends in a mesiodistal direction from mesial triangular fossa to distal triangular fossa, dividing the occlusal surface into buccal and lingual halves. Supplementary grooves are many in this tooth making occlusal aspect irregular or wrinkled in appearance.

Fossae and Pits Mesial triangular fossa is present as a triangular depression just distal to mesial marginal ridge. Distal triangular fossa is located mesial to distal marginal ridge. Pits may be present in triangular fossae where the grooves converge.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS Leongs premolar/Dens evaginatus-at times an accessory tubercle may be seen on occlusal aspect between buccal and lingual cusps. This is referred to as Leongs premolar/Dens evaginatus. This structure may interfere with occlusion. At times wearing away of covering enamel and dentin lead to exposure of pulp, necessitating root canal treatment.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE

Buccal Aspect Mesial slope of the buccal cusp is shorter than distal slope. Distal tilt of root.

Palatal Aspect Lingual cusp tipped to the mesial side.

Proximal Aspect Cervical line is more curved on mesial aspect than distal. Distal marginal ridge is more cervically located compared to mesial marginal ridge. Deeper developmental depression on distal aspect of root.

Occlusal Aspect Lingual cusp tipped to the mesial side. Distobuccal cusp slope is longer. Longer and convex distal marginal ridge. Large and deeper distal triangular fossa. Permanent maxillary second premolar

28 Permanent Mandibular First Premolars

Introduction Chronology of mandibular first premolar Measurement table Morphology of mandibular first premolar Differences between maxillary and mandibular first premolars Developmental variations and clinical considerations

M

andibular premolars are four in number, two first premolars and two second premolars located one on each side of the dental arch. They are successors of mandibular deciduous molars. The functions include mastication and maintenance of vertical dimension of face. Also assists canines to shearing the teeth and support the side of mouth and cheeks.

GENERAL CHARACTERISTICS OF MANDIBULAR PREMOLARS (ARCH TRAITS) Mandibular premolars do not resemble each other, in contrast to maxillary premolars of similar morphology. Mandibular second premolar is larger than first premolar. In mandibular premolars, buccal cusps are more developed than lingual cusp.

Lingual cusp is nonfunctional cusp in first premolar. From buccal and lingual aspects, mandibular premolar crown appears to be tilted distally (more significant in first premolar). From proximal aspect both mandibular premolars are tilted lingually, with lingual outline extending beyond the boundary of root outline.

PERMANENT MANDIBULAR FIRST PREMOLAR Mandibular first premolar is located between canine and second premolar and therefore bears resemblance to both, in certain features. The morphologic characteristics of mandibular first premolar can be described from five aspects, namely buccal, lingual, mesial, distal, and occlusal. Further, features on each aspect (except occlusal) is described under two subheadings, i.e. crown and root.

BUCCAL ASPECT Crown Shape of Buccal Aspect From this aspect the tooth is bilaterally symmetrical and has a trapezoidal shape with narrow cervix. Mesial outline is straight or slightly convex from cervix to contact area. Contact area is located slightly occlusal to midpoint of the tooth. Distal outline is more convex and the contact area is nearly at the same level as mesial or slightly more occlusal in its location. Chronology of permanent mandibular first premolars

Occlusal outline is represented by buccal cusp and cusp slopes. Buccal cusp is long and sharp and the cusp tip is located slightly mesial to the center. Mesial cusp ridge is shorter than the distal cusp ridge. The cusp ridges meet at an obtuse angle. Cervical line is slightly curved towards the root. Buccal surface: Buccal surface is convex with a well-developed buccal ridge extending vertically from cervical region to the cusp tip. On either side of buccal ridge, shallow depressions may be present.

Root Mandibular first premolar has only one root. Root is conical and tapers to a nearly pointed apex.

LINGUAL ASPECT Crown From this aspect mandibular premolars show many unique characteristics. Crown and root taper considerably to the lingual side, making a part of mesial and distal aspect visible from this aspect. Occlusal aspect slopes lingually in a cervical direction; therefore most of

occlusal aspect can be seen from lingual aspect. Contact areas and marginal ridges are more prominent because of narrow cervical region. Lingual cusp is short and poorly developed but is pointed. This cusp is a nonoccluding cusp. Both mesial and distal marginal ridges can be seen. Mesial marginal ridge is sloping and more cervically placed, while distal marginal ridge is relatively straight and more occlusally placed (more cervical location of the mesial marginal ridge is seen only in this tooth while in all other posterior teeth mesial marginal ridge is more occlusally placed than the distal marginal ridge). Another characteristic feature observed in this tooth is the mesio-lingual developmental groove which extends to lingual surface along the mesiolingual line angle, demarcating the mesial marginal ridge from mesial slope of lingual cusp.

Root Root is conical in shape and is narrow on lingual aspect.

MESIAL ASPECT Crown Shape of Mesial Aspect Crown is rhomboidal in shape with noticeable tilt to the lingual side at the cervix.

Outlines The buccal outline is curved from cervical line to the buccal cusp tip. A distinct inclination to lingual side is observed with the crest of curvature located at the junction of middle and cervical 1/3rd. Lingual outline is less curved than buccal and crest of curvature is located

nearly at middle 1/3rd. Because of extreme lingual tilting, lingual outline extend beyond the boundary of root outline giving an impression that the lingual side of the tooth is overhanging. Occlusal outline is represented by buccal and lingual cusps and mesial marginal ridge. The inclination of the occlusal aspect can be well appreciated from mesial aspect and the lingual height of the crown is only 2/3rds of buccal height. Buccal cusp is centered over the root and most of occlusal portion is occupied by buccal triangular ridge which also shows a cervical inclination. Lingual cusp is short but sharp and is nonfunctional. Lingual cusp tip is in line with lingual outline of root. The mesial marginal ridge is at a lower level compared to distal marginal ridge and it shows an inclination in a cervical direction. Because of this more of occlusal surface can be seen from this aspect. The direction of mesial marginal ridge is almost parallel to that of buccal triangular ridge but located at a lower level. Cervical line is slightly curved towards the crown. Mesial surface: Mesial surface is smooth. A prominent mesio-lingual developmental groove can be visible which demarcate the mesial marginal ridge from mesial slope of lingual cusp and extending to lingual surface along the mesio-lingual line angle.

Root Root is nearly straight at cervical 1/3rd and taper at apical 1/3rd to a blunt apex. A deep developmental groove may be present on root.

DISTAL ASPECT Distal aspect of mandibular first premolar shows various differences from mesial aspect. Distal marginal ridge is longer and is more occlusal than that of mesial marginal ridge. Distal marginal ridge is horizontal; nearly perpendicular to long axis of tooth; in contrast to mesial marginal ridge which shows a lingual inclination. No evidence of developmental groove on the distal aspect of crown.

Root is more convex on distal aspect.

OCCLUSAL ASPECT Shape The occlusal aspect is roughly diamond shaped. Tooth is not bilaterally symmetrical. The distal portion appears to be bulkier than mesial. Considerable degree of lingual convergence of the tooth can be appreciated from this aspect which is more from mesial aspect. Occlusal aspect is broadest at the buccal half, in the region of contact; which is located immediately lingual to buccal line angles. Because of lingual inclination more of buccal surface is seen which shows a distinct buccal ridge. The crest of lingual outline is located distal to center of tooth.

Occlusal Surface Occlusal surface shows anatomic landmarks such as cusps, ridges, fossa, grooves, and pits, etc.

Cusps Occlusal surface of mandibular first premolar shows two cusps: One buccal cusp and a lingual cusp. Buccal cusp and its triangular ridge make up the bulk of the occlusal surface of the tooth. Buccal cusp tip is near the center of crown and the cusp slopes are nearly in a straight line. Differences between maxillary first and mandibular first premolars (arch traits) Maxillary first premolar Mandibular first premolar Crown do not show lingual inclination

Crown shows a significant lingual inclination so that buccal cusp tip is inline with midline

Mesial slope of buccal cusp is longer than distal slope

Distal slope of buccal cusp is longer than mesial slope

Both buccal and lingual cusps are almost equally developed

Buccal cusp is well developed but the lingual cusp is much smaller and the crown height lingually is only 2/3rds of the buccal aspect

Palatal cusp is occluding cusp

Lingual cusp is non-occluding cusp

Occlusal aspect is hexagonal in shape with relatively less lingual convergence

Diamond shaped with significant lingual convergence

Occlusal aspect do not incline cervically

Occlusal aspect inclines cervically

Central groove is located at the Central groove is more lingually located center of occlusal aspect so that the buccal portion is much larger dividing it into equal buccal than lingual portion and palatal halves Triangular fossae are distinct

Not distinct

Mesial marginal developmental groove is present

No mesial marginal developmental groove, instead a mesio-lingual developmental groove is seen

10. Mesial marginal ridge is relatively straight and is at a higher level than that of distal marginal ridge

Mesial marginal ridge is sloping and is at a lower level than that of distal marginal ridge

11. Mesial aspect of crown and root has a developmental depression

No such developmental depression is seen

12. Usually has two roots

Only one root

Lingual cusp is small, sharp and is nonfunctional. Tip is at considerably

lower level than buccal cusp. Lingual cusp occupies only a small portion of occlusal surface.

Ridges Triangular ridges of both buccal and lingual cusps can be seen. Buccal triangular ridge occupies the major portion of occlusal aspect which also shows a lingual inclination. The triangular ridge of buccal cusp forms a transverse ridge with the small triangular ridge of lingual cusp. Marginal ridges are well developed and prominent. The mesial marginal ridge is at a lower level compared to distal marginal ridge and it shows an inclination in a cervical direction. Distal marginal ridge is horizontal; nearly perpendicular to long axis of tooth and is more occlusally placed than that of mesial marginal ridge.

Fossa and Pits Occlusal surface of mandibular first premolar shows two minor fossae, which are located on either side of the transverse ridge, namely mesial and distal fossae. The mesial fossa is linear and shallow when compared to circular and deeper distal fossa. Pits may be seen in fossae where the grooves converge.

Grooves A shallow central groove may be found extending from mesial to distal fossa across the transverse ridge. The central groove is placed more lingually, therefore, it divides the occlusal surface into two unequal parts. Mesial and distal developmental grooves are found in the fossae, which run in a buccolingual direction. Mesial groove is in continuation with mesio-lingual developmental groove which crosses onto lingual side along the mesiolingual line angle, separating mesial marginal ridge and mesial slope of lingual cusp.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS Leongs premolar/Dens evaginatus-at times an accessory tubercle may be seen

on occlusal aspect between buccal and lingual cusps. This is referred to as Leongs premolar/Dens evaginatus. This structure may interfere with occlusion. At times wearing away of covering enamel and dentin lead to exposure of pulp, necessitating root canal treatment. Mesio-lingual developmental groove at times extends even onto the root may increase the possibility of periodontal diseases.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Buccal Aspect Distal slope of the buccal cusp is longer than mesial slope. Distal tilt of root.

Lingual Aspect Mesiolingual developmental groove.

Proximal Aspect Cervical line is more curved on mesial aspect than distal. Presence of mesio-lingual developmental groove. Mesial marginal ridge is more cervically located and slopes from buccal to lin-glial. Distal marginal ridge is more occlusally placed compared to mesial marginal ridge and is perpendicular to long axis of tooth. Deep developmental depression on distal surface of root.

Occlusal Aspect Distobuccal cusp slope is longer. Mesiolingual developmental groove. Permanent mandibular right first premolar

29 Permanent Mandibular Second Premolars

Introduction Chronology of mandibular second premolar Measurement table Morphology of mandibular second premolar Differences between mandibular first and second premolars Developmental variations and clinical considerations

M

andibular second premolars are larger than first premolars and are located between first premolar and first molar. Except for buccal aspect, second premolar does not resemble first premolar in morphology. Second premolars are mainly seen in two forms; two cusp types and three cusp types which differ from each other mainly in occlusal morphology. The morphologic characteristics of mandibular second premolar can be described from five aspects, namely buccal, lingual, mesial, distal, and occlusal. Further, features on each aspect (except occlusal) is described under two subheadings, i.e. crown and root.

BUCCAL ASPECT Crown

Shape of Buccal Aspect Crown appears squarish from buccal aspect. This appearance is created due to short and less sharp buccal cusp and wider cervical 1/3rd which results from less cervical convergence.

Outlines of Buccal Aspect Mesial outline is curved and contact area is broader which is located occlusal to the junction of middle and occlusal 1/3rd. Distal outline is also slightly curved and the contact area is located relatively in a cervical position than that of the mesial contact area. Occlusal outline is represented by buccal cusp and cusp slopes. Buccal cusp is short, less pointed and tip is located slightly mesial to the center. Distal cusp ridge is longer than mesial cusp ridge. Cusp ridges meet at an obtuse angle making the cusp less sharp. Cervical line is curved towards the crown

Buccal Surface Buccal surface is convex with a buccal ridge which is inconspicuous, extending from cervical region to the cusp tip. On either side of buccal ridge shallow depression may be present.

Root Root is wider and longer; ending in a blunt apex which may be tilted distally. Chronology of permanent mandibular second premolar

LINGUAL ASPECT Crown From this aspect second premolar exhibits considerable morphological variations from that of first premolar. Difference can also be observed between two cusp type and three cusp type second premolars. The lingual aspect is narrower than buccal aspect but, the degree of convergence is not as prominent as in mandibular first premolar. In three cusp type the lingual side shows only minimal convergence. Lingual cusp is well developed and is only slightly shorter than buccal cusp making only part of occlusal aspect visible from this aspect. In two cusp types only one lingual cusp is seen, the cusp ridges of which merges with marginal ridges. In three cusp types two lingual cusps are seen separated by a groove that extends onto lingual surface. Mesio-lingual cusp is longer and broader than disto-lingual cusp therefore help in side identification. Lingual surface is smooth and convex.

Root Root is smooth and convex and taper apically to end in a blunt apex.

MESIAL ASPECT Crown Shape of Mesial Aspect From this aspect crown shows a lingual inclination, but to a lesser extent than that of first premolar. Crown and root are wider bucco-lingually than first premolar.

Outlines of Mesial Aspect The buccal outline is less convex than that of first premolar and crest of convexity of buccal outline is located at junction of middle in cervical 1/3rd. Lingual outline is convex and it extends beyond the root boundary in the region of crest of curvature which is located in the middle of middle 1/3rd. Occlusal outline is represented by buccal and lingual cusps and mesial marginal ridge. Buccal cusp is less sharp and is not so near to the midline of the tooth. Lingual cusp is well developed and larger in both two and three cusp types and is slightly shorter than that of buccal cusp (around 1.5 mm). The tip of lingual or mesio-lingual cusp is almost inline with lingual outline of the root. Because of well developed lingual lobe the occlusal surface does not show a lingual sloping. Mesial marginal ridge is more occlusally placed and horizontal making only a lesser portion of occlusal surface visible from this aspect. Cervical line is curved towards the crown. Mesial surface: Mesial surface is smooth and convex. In contrast to first premolar, there is no evidence of mesio-lingual developmental groove.

Root Root is conical and tapers apically to a blunt apex.

DISTAL ASPECT

Morphology is similar to the mesial aspect. More of occlusal aspect is seen from this aspect due to two reasons. Distal tilt of the crown on root base Concave and cervically located distal marginal ridge. In three cusp types the disto-lingual cusp is smaller; therefore part of mesio-lingual cusp is also visible from this aspect.

OCCLUSAL ASPECT Occlusal morphology varies considerably in two and three cusp forms.

Three Cusp Type Shape In three cusp types, occlusal aspect is squarish with minimal lingual convergence. In some teeth, lingual aspect is even wider than buccal portion.

Occlusal Surface Occlusal surface shows anatomic landmarks such as cusps, ridges, fossae and pits, grooves, etc. Differences between mandibular first and mandibular second premolars (type traits) Mandibular first premolar Mandibular second premolar Crown is longer

Crown is shorter

Buccal cusp is sharp

Buccal cusp is less pointed

Mesial contact area is more cervically placed than distal contact area

Distal contact area is more cervically placed than mesial contact area

From

From

buccal

aspect

cervical

buccal

aspect

cervical

convergence is more relatively narrow cervix

with convergence is less with relatively broader cervix

Buccal ridge is more prominent

Buccal ridge is less prominent

Crown shows considerable lingual Crown does not show much of convergence lingual Only one lingual cusp is seen

One or two lingual cusps are seen

No lingual groove is seen

In three cusp type a lingual groove is seen

Lingual cusp is very short, narrow and is non-occluding cusps

Lingual cusp is well developed and is only lightly shorter than buccal cusp

10. Crown shows much lingual inclination so that buccal cusp tip is in-line with root

The crown is lingually inclined to a lesser extent and buccal cusp tip is not so near to the midline

11. From the occlusal aspect crown is diamond shaped

Square shaped

12. Occlusal aspect is lingually inclined and most of the occlusal aspect can be seen from lingual aspect

No lingual inclination of occlusal aspect and only a little of occlusal aspect can be seen from lingual aspect

13. A transverse ridge is seen in the occlusal aspect between buccal and lingual cusp

No transverse ridge is seen

14. Mesial marginal ridge is at a lower level and is slopping lingually

Mesial marginal ridge is at a high level and is almost straight

15. Mesio-lingual developmental groove is found extending onto lingual surface

No mesio-lingual groove is present

developmental

Cusps In this type three cusps are seen: One buccal cusp and two lingual cusps. The buccal cusp is largest followed by mesio-lingual cusp and disto-lingual cusp is the smallest.

Ridges All the three cusps have well developed triangular ridges. Mesial and distal marginal ridges are found forming the mesial and distal boundaries of occlusal aspect. Distal marginal ridge is slightly concave and cervically located. Mesial marginal ridge is straight and is occlusally placed.

Fossae and Pits There are three fossae in three cusps type: A central fossa which is located nearly at the center of the occlusal surface and is slightly distal to the center in a mesio-distal direction and at the center in a bucco-lingual direction. The central fossa harbors a central pit. Triangular fossae: Inner to the marginal ridges, on either side of occlusal aspect mesial and distal triangular fossae are seen.

Grooves Mesial developmental groove: Starts from central pit runs in a mesial direction to end in the mesial triangular fossa. Distal developmental groove: Extends from central pit to distal triangular fossa. Lingual groove: Extends from central pit, travel in a lingual direction between the two lingual cusps and runs to a short distance onto the lingual surface. Supplementary grooves are seen radiating from developmental grooves. All the three developmental grooves converge at the central pit giving a Yshaped configuration.

Two Cusps Type In two cusp type second premolars, the occlusal aspect has a round shape with more lingual convergence. Mesio-lingual and disto-lingual line angles are rounded. Marginal ridges form the boundary of occlusal surface. Only two cusps are seen: One buccal cusp and one lingual cusp, both are well developed. Lingual cusp is located directly opposite to buccal cusp and triangular ridges of both cusps form a transverse ridge. Mesial and distal fossae are seen inner to marginal ridges which are roughly circular. Central fossa is absent in two cusp types. A central groove extends from mesial to distal fossa. This groove has a ‘U’ or crescent shape. Supplementary grooves are also present in the fossae.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS Leongs premolar/Dens evaginatus—at times an accessory tubercle may be seen on occlusal aspect between buccal and lingual cusps. This is referred to as Leongs premolar/Dens evaginatus. This structure may interfere with occlusion. At times wearing away of covering enamel and dentin lead to exposure of pulp, necessitating root canal treatment.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Buccal Aspect Distal slope of the buccal cusp is longer than mesial slope. Distal tilt of root.

Lingual Aspect Distolingual cusp is smaller than mesiolingual cusp.

Proximal Aspect

Cervical line is more curved on mesial aspect than distal. Distal marginal ridge is more cervically located. Mesial marginal ridge is more occlusally placed and almost straight. Deep developmental depression on distal surface of root.

Occlusal Aspect Distolingual cusp is smaller than mesiolingual cusp. Large, deeper distal triangular fossa. Permanent mandibular right second premolar

30 Permanent Maxillart First Molars

Introduction Chronology of maxillary first molar Measurement table Morphology of maxillary first molar Developmental variations and clinical considerations

T

here are three types of permanent molars: The first molar, second molar, and third molar. In the permanent dentition, there are 12 molars, three in each quadrant and are non-succedaneous teeth. The name molar comes from the Latin word for “grinding”. Molar teeth have a major role in mastication of food, giving support to the cheeks and also in maintaining vertical dimension of face and fullness of cheek.

GENERAL CHARACTERISTICS OF PERMANENT MAXILLARY MOLARS (ARCH TRAITS) Maxillary molars are larger than other maxillary teeth. Crown is bucco-lingually larger in contrast to the corresponding teeth on the mandibular arch which are larger mesio-distally. • Crown is centered over the root and has three primary cusps and a fourth relatively smaller cusp that

is disto-lingual cusp. All maxillary molars have an oblique ridge extending from the most prominent mesio-lingual cusp to the disto-buccal cusp. They have three roots, two buccal and one lingual. All the roots converge to a root base called root trunk.

PERMANENT MAXILLARY FIRST MOLAR There are two maxillary first molars, one on right and another on left side of the arch located between second premolar and second molar. Permanent maxillary molars are the largest and strongest of all maxillary teeth. Since these teeth are the first permanent teeth in the arch to erupt into the oral cavity, it is often the first one to be decayed. The first molars (maxillary and mandibular) are usually referred to as sixth year molars because they erupt at the age of 6 years. The morphologic characteristics of maxillary first molar can be described from five aspects, namely buccal, palatal/lingual, mesial, distal, and occlusal aspects. Further, features on each aspect (except occlusal) is described under two subheadings, i.e. crown and root.

BUCCAL ASPECT Crown Shape of Buccal Aspect Shape is trapezoidal with broader of the dissimilar outline being occlusal and narrower the cervical outline. Chronology of permanent maxillary first molar

Outlines of Buccal Aspect Mesial outline is nearly straight which becomes convex at mesial contact area located at the junction of occlusal and middle 13rd. The outline continues to join the mesial cusp slope of mesio-buccal cusp. Distal outline is convex with contact area in the middle of middle 1/3rd. Occlusal outline is represented by buccal cusps and cusp slopes. From the buccal aspect two cusps are seen; a mesio-buccal and a disto-buccal. Mesiobuccal cusp is wider and slightly longer than the disto-buccal cusp. The mesial and distal cusps slope of the mesio-buccal cusp meet at an obtuse angle making it less sharp while the disto-buccal cusp is sharper. Cervical outline is irregular and shows slight curvature to the root.

Buccal Surface On the buccal surface a buccal groove is seen separating the two buccal cusps. This groove extends to the middle 1/3rd and there may be a pit where the groove ends.

Root From the buccal aspect, a distinct root trunk (undivided part of the root) is visible. At a point about the junction of cervical and middle 1/3rd of the root (around 4 mm above the cervical line) the root trunk bifurcate giving rise to

two buccal roots a mesio-buccal and a disto-buccal. Both the roots are wellseparated, taper apically and often are curved distally.

LINGUAL/PALATAL ASPECT Crown Crown of maxillary first molar is often broader mesio-distally on the palatal side than on the buccal side, except in the cervical 1/3rd. The outline is reverse of that of buccal outline. Two well developed cusps are visible from this aspect, the larger mesiolingual cusp and smaller disto-lingual cusp. Mesio-lingual cusp is the longest cusp of this tooth and the cusp slopes meet at 90 degrees. The disto-lingual cusp is smallest and is more rounded. The lingual cusps are separated by a lingual developmental groove that extends from occlusal aspect to the lingual surface. Frequently a fifth cusp is found on the lingual surface of mesio-lingual cusp which is located 2 mm cervical to the tip of the mesio-lingual cusp. This cusp is separated from mesio-lingual cusp by a fifth cusp groove. The cusp is named as ‘cusp of Carabelli’ after the person who first described it. The presence or absence of this cusp is a racial characteristic and when present it may show variation in size and shape.

Root Only one root is present on the palatal side which is the longest of all three roots. The palatal root tapers to a blunt apex. From the palatal aspect along with the palatal root both mesio-buccal and disto-buccal roots are also visible.

MESIAL ASPECT Crown Shape of Mesial Aspect

Crown appears to be short and broad facio-lingually. A prominent curvature at cervical 1/3rd buccally and lingually is observed.

Outlines of Mesial Aspect Buccal outline is convex at cervical 1/3rd, followed by slight concavity and again convex as it progresses further to end at cusp tip. Crest of buccal outline is usually located immediately below the cervical line. Palatal outline is somewhat similar to buccal outline, but crest of the lingual outline is often found at the middle 1/3rd. Occlusal outline is represented by cusps and marginal ridge. Two cusps are seen; a mesio-buccal cusp and a larger, longer mesio-lingual cusp. The mesio-lingual cusp is inline with long axis of lingual root. Fifth cusp, the cusp of Carabelli is found on the lingual surface of mesio-lingual cusp. Confluent with the cusp ridges a distinct mesial marginal ridge is present which is irregular and curved cervically. Mesial marginal ridge is placed at an occlusal level than that of distal marginal ridge. Cervical outline is irregular and slightly curved towards the crown.

Mesial Surface Mesial surface is generally convex. A shallow concavity may be seen cervical to the contact area which may continue onto the root surface.

Root Two roots are visible from this aspect, mesio-buccal root and the lingual root. The level of bifurcation on the mesial aspect is closer to (less than 4 mm) the cervical line. The mesio-buccal root is broad in bucco-lingual direction and the apex is in-line with tip of mesio-buccal cusp. The palatal root is 1.5 mm longer than mesio-buccal root but narrower in a bucco-lingual direction. The roots are well-separated and the boundaries of the roots may extend beyond the crown. This feature helps to differentiate this tooth from that of 2nd molar.

DISTAL ASPECT

Crown Tooth shows a convergence distally making the buccal and palatal aspects visible from the distal aspect. Mainly two cusps, disto-buccal and disto-lingual cusps are visible from this aspect. Parts of other cusps including the ‘cusp of Carabelli’ can be seen. Of the two cusps, disto-buccal cusp is slightly larger than disto-lingual cusp. The distal marginal ridge is shorter, more concave and cervically placed than mesial marginal ridge making a part of occlusal aspect visible from distal aspect. Cervical line is less curved on distal aspect. Distal surface is generally convex except for a shallow concavity at cervical region which may continue onto the root surface up to the level of bifurcation.

Roots All the three roots are seen from this aspect. A portion of mesio-buccal root is seen because the disto-buccal root is shorter and narrow. The level of bifurcation on the distal side is more apical than on mesial side.

OCCLUSAL ASPECT Shape Occlusal outline is rhomboidal or parallelogram in shape. It has two acute angles and two obtuse angles. Acute angles are mesio-buccal and distolingual and obtuse angles are mesio-lingual and disto-buccal. Tooth is wider bucco-lingually (1 mm) than mesio-distally. Crown shows a buccal convergence and a distal convergence. The palatal half of the tooth is wider mesio-distally than buccal half. Similarly, the mesial half of the tooth is bucco-lingually wider than distal half.

Occlusal Surface Occlusal surface shows various anatomic landmarks such as cups, ridges, fossae, pits, grooves, etc.

Cusps Four major cusps are seen, i.e. mesio-lingual cusp which is longest and largest followed by mesio-buccal, disto-buccal and disto-lingual cusp. Of this four cusps mesio-lingual, mesio-buccal and disto-buccal forms the primary cusps of first molar. A fifth cusp the ‘cusp of Carabelli’ is also seen lingual to mesio-lingual cusp which is located 2 mm cervical to the tip of the mesiolingual cusp.

Ridges Triangular ridges of all the four major cusps are seen. Oblique ridge: The triangular ridge of the mesio-lingual cusp is divided into two parts by a groove named Stuart groove. The distal extension of the triangular ridge of the mesio-lingual cusp and the triangular ridge of distobuccal cusp meet and form a diagonal ridge called oblique ridge. A transverse ridge is formed by the triangular ridges of the mesio-buccal cusp and mesial portion of the triangular ridge of the mesio-lingual cusp. Mesial and distal marginal ridges form mesial and distal boundary of occlusal aspect. Cusp ridges: The buccal and lingual sides of occlusal surface are bounded by cusp ridges.

Fossae There are four fossae on the occlusal aspect of a maxillary first molar, two fossae are major and other two are minor.

Major fossae Central fossa: This is the largest fossa situated mesial to the oblique ridge, bounded by oblique, transverse and cusp ridges of buccal cusp. Distal fossa: This is also a major fossa, relatively smaller than central fossa, and is located distal to the oblique ridge. It is linear in shape.

Minor fossae

Mesial triangular fossa is a minor fossa, triangular in shape and is located adjacent (distal to) mesial marginal ridge. Distal triangular fossa is similar to mesial triangular fossa, but smaller and is located adjacent to distal marginal ridge.

Pits Pits are observed at the deepest part of all fossae as pin point depression where the grooves converge.

Grooves Both developmental and supplementary grooves are present.

Developmental grooves Central groove: Extends mesially from the central fossa, over the transverse ridge and ends in mesial triangular fossa. Transverse groove of the oblique ridge: This groove extends from the central fossa in a distal direction across the oblique ridge to the distal triangular fossa. Distal oblique groove: Extends from the distal triangular fossa, along the distal aspect of oblique ridge in a lingual direction between the mesio-lingual and distolingual cusps. Buccal groove: Extends from the central fossa, traverse in a buccal direction between the mesio-buccal and disto-buccal cusps and continues onto the buccal aspect of the tooth. Lingual groove: This is seen as a continuation of the distal oblique groove and extends onto the lingual surface of the tooth between mesio-lingual and disto-lingual cusps. Fifth cusp groove: This groove separates the fifth cusp from the mesiolingual cusp. Stuart groove: This is a small groove which extends from central groove to separate the two portions of triangular ridge of mesio-lingual cusp.

Supplementary grooves

In addition to developmental grooves, supplementary grooves are also present in triangular fossae extending to a buccal and a lingual direction.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS First molars are the teeth most often get decayed and the first tooth in permanent tooth to be lost, as they may be mistaken as primary teeth and neglected by parents. The deep pits and grooves present may act as the site of initiation of caries. The maxillary first molars may have an additional cusp on the buccal surface of the mesio-buccal cusp, which is termed as paramolar tubercle/parastyle. Taurodontism is a term used for developmental variation of molar teeth where the crown of the tooth is enlarged at the expense of root. This term is given as this tooth resembles that of a cud chewing animal. Bifurcation of the root will be shifted apically. This condition may exist as an isolated trait (autosomal dominant) or as part of several syndromes. Endodontic treatment of teeth affected by taurodontism needs special consideration. Concrescence is the fusion of cementum of adjacent teeth, a good reason to have radiographs before extraction of a tooth. Mulberry molar is dental defects specifically involving first molars, in congenital syphilis and caused by direct invasion of tooth germs by Treponema pallidum which can pass through the placenta. In mulberry molars the cusps are replaced by many globular masses of enamel, giving resemblance to mulberry fruit. Dens in dente/Dens invaginatus is a condition characterized by deep invagination in crown portion of tooth resulting in enamel being reflected into the tooth giving an appearance, tooth within a tooth. In affected teeth, caries may develop in the invagination and escape detection. Enameloma or enamel pearl is ectopic formation of enamel appear as small droplets of enamel on the root surface, mostly close to bifurcation.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE

Buccal Aspect Mesio-buccal cusp larger than disto-buccal cusp.

Palatal Aspect Mesio-lingual cusp is largest cusp. Cusp of Carabelli is present on lingual aspect of mesio-lingual cusp.

Proximal Aspect Cervical line is more curved on mesial aspect than distal. Distal marginal ridge is more cervically located. Mesial marginal ridge is placed at an occlusal level than that of distal marginal ridge. Deep developmental depression on distal surface of root.

Occlusal Aspect Distal and buccal convergence of occlusal aspect. Mesiolingual cusp is largest cusp and distolingual the smallest. Oblique ridge running from mesio-lingual to disto-buccal cusp. Permanent maxillary right first molar

31 Permanent Maxillary Second Molars

Introduction Chronology of maxillary second molar Measurement table Morphology of maxillary second molar Differences between maxillary first and second molars Developmental variations and clinical considerations

M

axillary second molars are situated distal to the first molars. Although they are relatively smaller, they assist first molars in function. These teeth may show considerable variation in morphology. The morphologic characteristics of maxillary second molar can be described from five aspects, namely buccal, palatal/lingual, mesial, distal, and occlusal. Further, features on each aspect (except occlusal) is described under two subheadings, i.e. crown and root.

BUCCAL ASPECT Crown Crown is shorter and less wider than first molars and is tipped distally on the root trunk. Mesial outline is slightly convex with contact area located at the junction

of occlusal and middle 1/3rd. Distal outline is shorter than the mesial outline. Distal contact area is located at the middle of middle 1/3rd. Two cusps can be seen on this aspect, mesio-buccal and disto-buccal. Mesio-buccal cusp is longer and wider than disto-buccal cusp. Smaller distobuccal cusp and distal tilting of the crown allows a part of the disto-lingual cusp visible from this aspect. Buccal groove present on buccal surface separates two buccal cusps, which is shorter than that of the buccal groove of first molar and only rarely end in a pit.

Root The maxillary second molar has two buccal roots and a palatal root and all three roots are visible from this aspect. The root trunk is distinct and the level of bifurcation is more apical when compared to that of the first molar making the root trunk longer. Both the buccal roots are nearly parallel, not spread apart and may show a distal tilt. The mesio-buccal root apex is in-line with buccal groove.

LINGUAL/PALATAL ASPECT Crown General outline of palatal aspect is reverse of that of buccal outline. Two cusps can be seen from this side, the mesio-lingual and disto-lingual cusps. The mesio-lingual cusp is longer and the disto-lingual cusp may be very small or even absent in some teeth. Part of the disto-buccal cusp may be visible. In contrast to maxillary first molars in this tooth a fifth cusp is not seen. Lingual groove separates both the lingual cusps. Chronology of permanent maxillary second molar

Root Only one palatal root is present which is almost of same length as that of buccal roots. Apex of the palatal root is in-line with disto-lingual cusp tip. Along with this palatal root, other two buccal roots are also visible from this aspect.

MESIAL ASPECT From the mesial aspect second molars resemble first molars. The differences observed are: Cusp of Carabelli is not present Roots are less separated Buccal and palatal roots are of equal length Buccal and palatal roots generally do not extend beyond the crown boundary.

DISTAL ASPECT From the distal aspect also the second molar shows similarity to first molar. The tooth is converging to the distal aspect; therefore buccal and lingual

surfaces are visible. Since the tooth shows a distal tilt and a cervical placement of the marginal ridge, the tooth appears shorter from this aspect and also much of occlusal aspect is seen. All the three roots are seen; palatal, mesio-buccal and disto-buccal. Apex of the palatal root is often in-line with that of disto-lingual cusps.

OCCLUSAL ASPECT Second molar shows similar morphological features as 1st molar with a few differences. Rhomboidal shape is more prominent with acute angles (mesio-buccal and disto-lingual) are less and obtuse angles (mesio-lingual and disto-buccal) are greater. It appears as though the lingual portion is pushed distally. Crown shows a lingual convergence and a distal convergence which is more pronounced than in the first molars. Tooth is bucco-lingually wider than mesio-distally with a difference of around 2 mm. There are four cusps, i.e. mesio-lingual, mesio-buccal, disto-buccal and disto-lingual. Greater difference in the cusp size is observed in this tooth. Mesio-buccal and mesio-lingual cusps are nearly of the same size and are noticeably larger than disto-buccal and disto-lingual cusps. Disto-lingual cusp is very small or even may be absent. No fifth cusp is observed. Occlusal surface shows more pits and grooves and the oblique ridge is less prominent than in first molar. Differences between maxillary first and second molars (Type traits) Maxillary first molar Maxillary second molar Larger in size

Smaller in size

Difference between buccolingual and mesio-distal diameter is less (around 1 mm).

Difference between bucco-lingual and mesio-distal diameter is more than in first molar (around 2 mm)

Buccal convergence of crown is observed

Lingual convergence observed

of

crown

is

Crown appears squarish or rhomboidal from occlusal aspect

Because the mesio-distal diameter is lesser, crown appears more oblong from occlusal aspect

Crown do not show distal tipping

Crown is tipped distally on root trunk

Disto-lingual cusp is relatively larger than that of second molars

Disto-lingual cusp is very small or absent

The mesio-buccal cusp is notably larger than disto-buccal cusp

Both the mesio buccal and disto-buccal cusps are notably larger

Cusp of Carabelli is present

Cusp of Carabelli is absent

Prominent oblique ridge

Less prominent oblique ridge

10. Relatively longer buccal groove, may end in a pit

Short buccal groove, may not end in pit

11. Root trunk is relatively shorter

Root trunk is longer

12. Roots of maxillary first molars are spread out

Roots do not spread out and all roots show adistal tilt

Similarities between Maxillary First and Second Molars Tooth is bucco-lingually broader Four major cusps as in the first molar Presence of oblique ridge and transverse ridge Fossae and groove pattern are similar Presence of 3 roots

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS Maxillary second molar may be prone to dental caries due to deep pits and fissures. Rarely cusp of Carabelli may be present on lingual aspect of mesiolingual cusp. Chances of concrescence with maxillary third molar is considerably more due to crowding of teeth in maxillary posterior region.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Buccal Aspect Mesio-buccal cusp larger than disto-buccal cusp.

Palatal Aspect Disto-lingual cusp is the smallest cusp.

Proximal Aspect Cervical line is more curved on mesial aspect than distal.

Occlusal Aspect Distal and lingual convergence of occlusal aspect. Mesio-lingual cusp is the largest and disto-lingual cusp is the smallest. Oblique ridge running from mesio-lingual to disto-buccal cusp. Permanent maxillary right second molar

32 Permanent Maxillary Third Molars

Introduction Chronology of maxillary third molar Measurement table Morphology of maxillary third molar

T

hird molars are the last tooth in the arch and erupt by the age of 17 to 21 years or later. These teeth show maximum variation in size and shape. The third molars are sometimes referred to as the “wisdom” tooth because they erupt last. The characteristics of maxillary third molars are: Smaller than 1st and 2nd molars. Crown shows significant convergence. Oblique ridge is less prominent. Disto-lingual cusp is much smaller or absent. Occlusal aspect may have many supplementary grooves giving wrinkled appearance. Root trunk is longer with point of bifurcation located more apically. Three roots, i.e. mesio-buccal, disto-buccal and lingual are seen. The roots are shorter than other maxillary molars and are less separated or often fused.

Chronology of permanent maxillary third molar

Permanent maxillary right third molar

33 Permanent Mandibular First Molars

Introduction Chronology of maxillary second molar Measurement table Morphology of maxillary second molar Differences between maxillary first and second molars Developmental variations and clinical considerations

P

ermanent mandibular molars are the largest group of teeth in the mandibular arch and are three in number on either side of the arch; 1st, 2nd and 3rd molars. Like maxillary molars, these teeth are also nonsuccessor teeth. Permanent mandibular molars help in mastication of food, to maintain proper vertical dimension of face, maintaining continuity of dental arches and also to provide fullness to the cheek. Unlike maxillary molars, all the mandibular molars are wider mesio-distally than bucco-lingually and have two roots.

GENERAL CHARACTERISTICS OF MANDIBULAR MOLARS (ARCH TRAITS) Shorter than other mandibular teeth but greater in other dimensions, Crown is broader mesio-distally than bucco-lingually.

Crown tapers distally and lingually. Crown tilts distally and lingually on the root base. Lingual cusps are relatively of same size, Two roots are present: Mesial and distal Root trunk is shorter.

PERMANENT MANDIBULAR FIRST MOLARS The mandibular first molars are the largest and strongest of all the mandibular teeth and have the widest crown of all teeth in the dentition. The mandibular first molar is the first permanent tooth to erupt into the oral cavity and is referred to as the “six-year-molar” as it erupts at 6 years. It normally erupts slightly before the maxillary first molar and is considered as the key of occlusion. Normally there are five functioning cusps on the occlusal surface of this tooth. The morphologic characteristics of mandibular first molar can be described from five aspects, namely buccal, lingual, mesial, distal, and occlusal. Further, features on each aspect (except occlusal) is described under two subheadings, i.e. crown and root.

BUCCAL ASPECT Crown Shape of Buccal Aspect Shape is roughly trapezoidal with the cervical and occlusal outlines representing the uneven sides. Crown is broader mesio-distally than cervicoocclusally. Chronology of permanent mandibular first molar

Outlines of Buccal Aspect Mesial outline is relatively straight or slightly concave from the cervical line to the contact area which is located at the junction of occlusal and middle third. Distal outline is straight or slightly convex from cervix to the contact area which is at the middle of middle third beneath the distal cusp. Occlusal outline is represented by the buccal cusps and the cusp slopes. On this aspect mainly three buccal cusps can be seen; the mesio-buccal cusp, disto-buccal cusp and a distal cusp. The mesio-buccal cusp is the longest and widest, followed by disto-buccal cusp which is smaller and shorter and a distal cusp which is the smallest and the pointed than the other buccal cusps. The smallest cusp on the buccal aspect is called the distal cusp because the major portion of the cusp is located on the distal part of the crown and only a small portion is seen on the buccal aspect. From the buccal aspect, a portion of mesio-lingual and disto-lingual cusps are also seen, as they are longer than the buccal cusps. Cervical line is nearly straight, regular and curves slightly to the root.

Buccal Surface Buccal surface of first molars is smooth and convex and shows two developmental grooves. The groove that separates the mesio-buccal and disto-buccal cusp is the mesio-buccal groove which extends up to the middle

third and ends in a pit. The disto-buccal groove separates disto-buccal and distal cusp, which ends at the cervical third without a distinct pit. The cervical portion of buccal aspect may show a prominent ridge running in a mesio-distal direction which is referred to as buccal cervical ridge.

Roots Mandibular first molar has two roots; mesial and distal. The level of bifurcation is 3 mm below the cervical line. Since the bifurcation is closer to the cervical line, the root trunk is short. The mesial root is the wider and the stronger of the two. The mesial and distal roots show a distal tilt. The tip of mesial root is almost in-line with the mesio-buccal cusps and of the distal root is often in-line or distal to the distal surface of crown.

LINGUAL ASPECT Crown Shape of Lingual Aspect From the lingual aspect the tooth shows a convergence lingually, making a part of mesial and distal surfaces visible. The degree of lingual convergence is more prominent distally. Tooth also tapers to the cervical region.

Outlines of Lingual Aspect Mesial outline is slightly convex. With the crest of contour located at the junction of occlusal and middle third. Distal outline is relatively straight with the crest of curvature located on the distal surface of the distal cusp. Occlusal outline is represented by the cusps and the cusp slopes. On this aspect mainly two cusps are seen; mesio-lingual and disto-lingual. Because of the lingual convergence, the distal portion of the distal cusps may be visible from this aspect. The mesio-lingual and the disto-lingual cusps are the longest and the sharpest of the five cusps. The mesio-lingual cusp is longer than the disto-lingual cusp and the width may be equal or slightly more than

that of the disto-lingual cusp. Cervical line is slightly irregular and relatively flat.

Lingual Surface Lingual surface is smooth and convex at the coronal 1/3rd and almost flat at the cervical region. Lingual developmental groove extends onto a short distance onto the lingual surface demarcating both the lingual cusps.

Root From this aspect both the mesial and distal roots are seen which show a lingual convergence. The root trunk appears to be longer because of the occlusal placement of cervical line. The level of bifurcation is 4 mm above the cervical line.

MESIAL ASPECT Crown Shape of Mesial Aspect Shape is rhomboidal with the crown tilted lingually on the root axis (arch trait). A greater bucco-lingual measurement of the crown and the root can be appreciated from this aspect.

Outlines of Mesial Aspect Buccal outline is noticeably convex at the cervical third where the crest of convexity is located, in the region of the buccal cervical ridge. As the buccal outline continues occlusally it becomes less convex and shows a lingual inclination. Lingual outline is relatively straight in the cervical third, becomes convex at the middle third where the crest of convexity is located. Occlusal outline is represented by the cusps and the marginal ridges. Two cusps can be seen from mesial aspect: Mesio-buccal and mesio-lingual.

Mesio-lingual cusp is longer and sharper and is in-line with the lingual surface of mesial root. A well developed mesial marginal ridge is seen which is slightly concave and is placed occlusally. Cervical line is irregular and slightly convex towards the occlusal aspect. The cervical line on the lingual surface is at a higher level than the buccal by about 1 mm. This difference in the level of cervical line can be appreciated from the mesial aspect.

Mesial Surface Mesial surface is smooth and relatively convex except for a slight concavity cervical to the contact area.

Root Only mesial root is visible from this aspect because the broad mesial root superimposes the narrower distal root. The outline of mesial root is relatively straight up to the junction of cervical and middle third and from there it tapers to a blunt apex. The apex is located directly below the mesio-buccal cusp.

DISTAL ASPECT Crown The general morphology of distal aspect is similar to that of mesial aspect. The crown is shorter distally than mesially. Due to the distal convergence of the crown, a part of buccal and lingual surface is also seen from this aspect. The distal convergence of the buccal surface is more pronounced than that of the lingual surface.

Differences are The distal marginal ridge is short, curved and is more cervically located than the mesial marginal ridge. Because of the distal tilt of the crown and cervical placement of the marginal ridge most of the occlusal surface and all cusps are seen from this aspect.

Curvature of cervical line is less than on mesial aspect.

Root The distal root and a part of the mesial root are visible from this aspect. The distal root is narrower than the mesial root and ends in a pointed apex.

OCCLUSAL ASPECT Shape The occlusal aspect is roughly quadrilateral in shape with the mesio-distal dimension more than bucco-lingual with a difference of 1 mm or more. The lingual and the distal convergence of the crown can be well appreciated. Because of the lingual tilt when tooth is viewed from occlusal aspect much of buccal surface also can be seen. The mesial outline is slightly convex and the contact area is centered in a bucco-lingual direction. The distal contact area is located buccal to the center point of distal marginal ridge.

The Occlusal Surface Occlusal surface shows various anatomic landmarks such as cusps, ridges, fossae, pits, and grooves.

Cusps Mandibular first molar has five cusps: Three buccal cusps and two lingual cusps. The buccal cusps are mesio-buccal, disto-buccal and distal. Of the three buccal cusps, the mesio-buccal cusp is the largest followed by distobuccal and the distal cusp. The distal cusp is the smallest and the sharpest and is located at the disto-lingual line angle. Lingually there are two cusps: The mesio-lingual and disto-lingual. The mesio-lingual and the disto-lingual cusps are the longest and tht sharpest of the five cusps. The mesio-lingual cusp is longer than the disto-lingual cusp and the width may be equal or slightly more than that of the disto-lingual cusp.

Ridges

Triangular ridges are seen extending from the tips of all five cusps towards the central part of occlusal surface. Triangular ridges of lingual cusps are longer than that of buccal cusps. Transverse ridge: The triangular ridge of the mesio-buccal cusp meets the triangular ridge of the mesio-lingual cusp to form a transverse ridge. Similarly, a transverse ridge is also formed by the triangular ridges of both the disto-buccal and disto-lingual cusps. Mesial marginal ridge: Forms the mesial boundary of the occlusal aspect and is located more occlusally than the distal marginal ridge. It is placed 1 mm below the level of the cusp tips. Distal marginal ridge: It is located at distal margin of occlusal aspect. It is shorter, concave and more cervically placed. Cusp ridges: Forms the buccal and the lingual boundaries of the occlusal aspect.

Fossae Three fossae can be seen; one major (central fossa) and two minor (mesial and distal triangular fossae). The central fossa is the largest fossa located at the center of the occlusal aspect. Central fossa is bounded by the distal slope of the mesio-buccal cusp, mesial and distal slope of the disto-buccal cusps, mesial slope of the distal cusp, triangular ridges of distal and disto-lingual cusps, mesial slope of distolingual cusp, distal slope of mesio-lingual cusp and the transverse ridge. Mesial triangular fossa is a triangular shaped depression located inner (distal) to the mesial marginal ridge. Distal triangular fossa is less distinct and is located inner (mesial) to distal marginal ridge.

Pits Pits are present as small pinpoint depression at the deepest part of all fossae, where the developmental grooves converge. The pits are named according to the fossa in which they are located: Central pit, mesial pit and distal pit.

Grooves Developmental and supplemental grooves are seen.

Developmental grooves Central groove: It is the major groove seen on the occlusal aspect and is centrally located dividing occlusal surface into buccal and lingual halves. It starts from the central pit and runs in a mesial direction between the mesiobuccal and mesio-lingual cusps to end in the mesial triangular fossa. The distal extension of the central groove runs between the disto-buccal and distolingual cusps to end in distal triangular fossa. The central groove follows a zigzag pattern. Mesio-buccal groove: This groove starts from the central fossa, slightly mesial to the origin of central groove and traverse in a buccal direction between the mesio-buccal and disto-buccal cusps and extend onto the buccal surface. Disto-buccal groove: It starts from the distal portion of the central groove and traverse in a buccal direction between the disto-buccal and distal cusps and extends onto the buccal surface. Lingual groove: It starts in the central pit, extends lingually between the two lingual cusps and onto the lingual surface.

Supplementary grooves In addition to the developmental groove there are supplementary grooves in triangular fossae extending to a buccal and lingual direction. Supplementary grooves are less distinct in the distal triangular fossa.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS First molars are the teeth most often get decayed and the first tooth in permanent tooth to be lost, as they may be mistaken as primary teeth and neglected by parents. The deep pits and grooves present may act as the site of initiation of caries.

Differences between maxillary first and mandibular first molars (arch traits) Maxillary first molar Mandibular first molar Crown is bucco-lingually broader than mesio-distally

Crown is mesio-distally broader than buccolingually

Have four major cusps: Two buccal and two lingual

Have five cusps: three buccal and two lingual

One accessory cusp, i.e. ‘cusp of Carabelli’ No such cusp is seen is also present and is seen lingual to mesiolingual Lingual cusps are of different size; large mesio-lingual and a smaller disto-lingual

Lingual cusps are nearly of equal size

Buccal surface is relatively flat

Buccal surface is convex and inclined lingually

Occlusal aspect is rhomboidal in shape

Occlusal aspect quadrilateral

Occlusal aspect shows buccal convergence

Occlusal aspect shows a lingual convergence

A prominent oblique ridge is seen on occlusal aspect extending from mesio-lingual to disto-buccal cusps

No such oblique ridge is seen on occlusal aspect

Occlusal aspect has four fossae: Two major and two minor

Occlusal aspect has only three fossae: One major and two minor

is

At times mandibular first molars may have only four cusps as in second molar with distal cusp missing. Sometimes an additional cusp on the buccal

surface of the mesio-buccal cusp may be present, at the middle third of the crown which is termed as paramolar tubercle/protostylid. An extra cusp, when located on distal marginal ridge between distal cusp and disto-lingual cusp, it is referred to as tuberculum sextum and when present between two lingual cusp, it is termed as tuberculum intermedium. Root division: Occasionally mesial root of mandibular molar may be divided into mesio-lingual and mesio-buccal roots making it three rooted. Taurodontism is a term used for developmental variation of molar teeth where the crown of the tooth is enlarged at the expense of root. This term is given as this tooth resembles that of a cud chewing animal. Bifurcation of the root will be shifted apically. This condition may exist as an isolated trait (autosomal dominant) or as part of several syndromes. Endodontic treatment of teeth affected by taurodontism needs special consideration. Concrescence is the fusion of cementum of adjacent teeth, a good reason to have radiographs before extraction of a tooth. Mulberry molar is dental defects specifically involving first molars, in congenital syphilis and caused by direct invasion of tooth germs by Treponema pallidum which can pass through the placenta. In mulberry molars the cusps are replaced by many globular masses of enamel, giving resemblance to mulberry fruit. Dens in dente/Dens invaginatus is a condition characterized by deep invagination in crown portion of tooth resulting in enamel being reflected into the tooth giving an appearance, tooth within a tooth. In affected teeth, caries may develop in the invagination and escape detection. Enameloma or enamel pearl is ectopic formation of enamel appears as small droplets of enamel on the root surface, mostly close to bifurcation.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Buccal Aspect Crown tilted distally

Mesio-buccal cusp largest and distal cusp smallest Distal tilt of root

Lingual Aspect Disto-lingual cusp is smaller than mesio-lin-gual cusp.

Proximal Aspect Cervical line is more curved on mesial aspect than distal. Distal marginal ridge is more cervically located. Mesial marginal ridge is more occlusally placed and almost straight. Deep developmental depression on distal surface of root.

Occlusal Aspect Disto-lingual cusp is smaller than mesio-lin-gual cusp. Large, deeper distal triangular fossa. Distal convergence of the crown. Smallest distal cusp. Permanent mandibular right first molar

34 Permanent Mandibular Second Molars

Introduction Chronology of mandibular second molar Measurement table Morphology of mandibular second molar Differences between mandibular first and second molars Developmental variations and clinical considerations

M

andibular second molars are two in number, one on either side of the arch, situated distal to the mandibular first molars. They supplement the first molar in function. Although the second molar resembles the first molar in its general morphology, a few differences can be observed. The morphologic characteristics of mandibular second molar can be described from five aspects, namely buccal, lingual, mesial, distal, and occlusal. Further, features on each aspect (except occlusal) is described under two subheadings, i.e. crown and root.

BUCCAL ASPECT Crown Shape of Buccal Aspect

Shape is roughly trapezoidal with the cervical and occlusal outlines representing the uneven sides. Tooth is wider mesio-distally than the crown length. The degree of cervical convergence is less therefore, the tooth appears to be wider at the cervix. Crown tilts distally so the distal side appears to be shorter.

Outlines of Buccal Aspect Mesial outline is straight with the contact area located at the junction of middle and occlusal 1/3rd. Distal outline is more convex and the contact area is at the middle of middle 1/3rd. Occlusal outline is represented by the buccal cusps and the cusp slopes. On this aspect mainly 2 buccal cusps can be seen: Mesio-buccal cusp and distobuccal cusp. Lingual cusps are also visible because they are longer than the buccal cusps. Cervical line is relatively straight or may curve sharply towards the root.

Buccal Surface Buccal surface is smooth and convex. The buccal groove extends between the mesio-buccal and disto-buccal cusps, which ends at the middle third of the surface in a pit. The cervical portion of buccal aspect may show a prominent ridge running in a mesio-distal direction which is referred to buccal cervical ridge (sometimes called buccal cingulum).

Root Two roots are present; mesial and distal. The level of bifurcation is more apical when compared to that of first molar. Both mesial and distal roots are usually closer together, nearly parallel and ending in a pointed tip. Chronology of permanent mandibular second molar

LINGUAL ASPECT Crown Tooth shows convergence lingually but to a lesser extent than that of first molar. Mesial and distal outlines are more convex. Occlusal outline is represented by the lingual cusps and the cusp slopes. Two lingual cusps are seen; mesio-lingual cusp and a disto-lingual cusp. The mesio-lingual cusp is slightly wider and longer of the two. Cervical line is regular. Lingual surface is smooth and convex. The lingual groove extends between the mesio-lingual and disto-lingual cusps, which is shorter than the buccal groove.

Root Two roots, mesial and distal roots are seen which end in a pointed apex.

MESIAL ASPECT

Crown Shape of Mesial Aspect From this aspect shape of second molar resembles that of first molar except for the differences in measurement.

Outlines of Mesial Aspect Buccal outline is noticeably convex at the cervical third (crest of convexity) in the region of the buccal cervical ridge. As the buccal outline continues occlusally it becomes less convex and shows a lingual inclination. Lingual outline is nearly straight or slightly convex with crest of convexity at the middle third. Cervical line is regular and straight with a slight curvature occlusally. Occlusal outline is represented by the cusps and the marginal ridge. Two cusps are seen: mesio-lingual and mesio-buccal cusps. Mesio-lingual cusp is longer. The mesio-buccal cusp tip is lingual to the buccal outline of mesial root. Mesial marginal ridge is concave and more occlusally placed.

Mesial Surface Mesial surface is smooth and convex.

Root Only mesial root is visible from this aspect because the mesial root is broad enough to hide the distal root.

DISTAL ASPECT General morphology of distal aspect resembles that of mesial aspect. Differences are: The distal convergence of the tooth makes a portion of buccal and lingual surfaces visible from this aspect. In addition to disto-buccal and disto-lingual cusps, a part of mesial cusps are

also seen. The distal marginal ridge is concave and more cervically located. Distal tilt of the crown and cervically located marginal ridge allows most of the occlusal aspect also to be visible from this aspect.

OCCLUSAL ASPECT Shape The occlusal aspect of mandibular second molar differs considerably from mandibular first molar. The tooth when viewed from occlusal aspect has a roughly rectangular shape which is wider in a mesio-distal direction than the bucco-lingual. The occlusal outline shows a distal and lingual convergence. The extent of the lingual convergence is lesser than the first molar. Mesial outline of the tooth is straight while distal outline is convex. Because of the lingual tilt when tooth is viewed from occlusal aspect much of buccal surface also can be seen. The mesio-buccal portion of the buccal surface shows a prominent bulge, representing the cervical ridge.

Occlusal Surface The occlusal surface shows various anatomic landmarks such as cusps, ridges, fossae, pits and grooves.

Cusps Four cusps are present; the mesio-lingual, disto-lingual, mesio-buccal and disto-buccal. The mesio-buccal and mesio-lingual cusps are larger than distobuccal and disto-lingual cusps. Unlike the mandibular first molars, distal cusp is absent in second molar.

Ridges Triangular ridges are seen extending from the tips of all the four cusps towards the central part of occlusal surface.

Transverse ridges-triangular ridges of mesio-buccal and mesio-lingual cusps meet to form a transverse ridge. Similarly, a transverse ridge is also formed by the triangular ridges of both the distal cusps. Mesial marginal ridge forms the mesial boundary of the occlusal aspect and is located more occlusally than the distal marginal ridge. Distal marginal ridge is located at distal margin of occlusal aspect. It is concave and more cervically placed. Cusp ridges forms the buccal and the lingual boundaries of the occlusal aspect.

Fossae The central fossa is the largest fossa located at the center of the occlusal aspect. Mesial triangular fossa is a triangular shaped depression located inner (distal) to the mesial marginal ridge. Distal triangular fossa is less distinct and is located inner (mesial) to distal marginal ridge.

Pits Pits may be present in any of the fossae where the grooves converge.

Grooves Occlusal surface shows both developmental and supplemental grooves.

Developmental grooves Central groove: Begins from central fossa and extends in a mesial and distal direction to end in the mesial triangular fossa and distal triangular fossa respectively. The central groove is relatively straight in second molar when compared to zigzag pattern in first molar. Buccal groove runs from the central fossa in a buccal direction separating two buccal cusps which also extends to the buccal surface. A lingual groove extends from central fossa between the two lingual cusps.

These developmental grooves arising from central fossa give a criss-cross pattern. Differences between mandibular first and second molars (type traits) Mandibular first molar Mandibular second molar Larger in all dimensions

Smaller in all dimensions

Has five cusps, three buccal cusps and two lingual cusps

Has only four cusps, two buccal cusps and two lingual cusps. Distal cusp is absent

Buccal surface shows two buccal grooves

Only one buccal groove

Less cervical constriction

More cervical constriction

Crown has a quadrilateral Rectangular shape from the occlusal aspect Roots are widely separated

Roots are close together

Grooves on occlusal aspect show a zigzag pattern

Grooves on occlusal aspect show a cross pattern

Supplementary grooves There may be many supplementary grooves radiating from the developmental grooves making the occlusal surface irregular.

DEVELOPMENTAL VARIATIONS AND CLINICAL CONSIDERATIONS The deep pits and supplementary grooves make the mandibular second molar, prone to caries. The mandibular first molars may have an additional cusp on the buccal

aspect similar to distal cusp of first molar. There is possibility of concrescence with third molar, i.e. the fusion of cementum. Enameloma or enamel pearl is ectopic formation of enamel appear as small droplets of enamel on the root surface, may be seen in this tooth, mostly close to bifurcation.

FEATURES TO BE CONSIDERED TO DIFFERENTIATE SIDE Buccal Aspect Crown tilted distally. Occlusal surface appears to be slopping cervically from mesial to distal. Mesio-buccal cusp wider than a disto-buccal cusp. Distal tilt of root.

Lingual Aspect Disto-lingual cusp is smaller than mesiolingual cusp.

Proximal Aspect Cervical line is more curved on mesial aspect than distal. Distal marginal ridge is more cervically located. Mesial marginal ridge is more occlusally placed. Deep developmental depression on distal surface of root.

Occlusal Aspect Distal convergence of occlusal surface. Mesial outline of occlusal aspect nearly straight while distal is more convex. Disto-lingual cusp is smaller than mesio-lingual cusp.

Permanent mandibular right second molar

35 Permanent Mandibular Third Molars

Introduction Chronology of mandibular third molar Measurement table Morphology of mandibular third molar

M

andibular third molars are extremely variable in morphology which may resemble a second molar (4 cusps) in most of the cases. A few specimens may also resemble a first molar (5 cusps). It supplements the mandibular second molar in function. Mandibular third molars are most likely to be impacted or congenitally missing.

General features are Rounded occlusal outline with a narrow occlusal table. Crown is bulbous and is tilted distally on root axis. Larger and longer mesio-lingual cusp, with short and rounded buccal cusps. Occlusal surface has irregular groove pattern. Roots are shorter, either fused or separated with more distal tilt. Chronology of permanent mandibular third molar

Permanent mandibular right third molar

36 Occlusion Dr Ajeesha Feroz Occlusion Deciduous dentition Mixed dentition Permanent dentition – –

Compensating curves Occlusal relationship between maxillary and mandibular posterior teeth

O

cclusion is defined as the contact of masticating and incising surfaces of opposing maxillary and mandibular teeth in function or in parafunction. The alignment and occlusion of the teeth are important in masticatory function.

OCCLUSION IN DECIDUOUS DENTITION At birth, teeth are not present in the mouth and over a period of time they erupt into the oral cavity. The maxillary and mandibular alveolar ridges, before the tooth eruption are called gum pads. The maxillary gum pads are wider than mandibular gum pads. The first deciduous tooth erupts into the oral cavity by six months of age and the dentition is completed by the age of 20–30 months. Natural spacing is seen in deciduous dentition and is more distinct in maxillary arch mesial to canine and in mandibular arch distal to canines. This space is called primate space, anthropoid space or simian space. Spacing in deciduous dentition is necessary for the proper alignment

of permanent dentition. Deciduous teeth are more or less upright in their arrangement in alveolar bone. The contact relations between the teeth vary in deciduous dentition with degree of bruxism present in the child. Normally, deep bite may be seen in deciduous anterior region which is reduced later by gradual attrition of incisors, forward movement of mandible and by the eruption of molars. The mesio-distal relationship between distal surfaces of deciduous second molars may be: Flush terminal plane (Fig. 36.1a): In this type of relation, the distal surface of maxillary second molar is in the same plane as that of distal surface of mandibular second deciduous molar. This type of relation results due to larger mesio-distal measurement of mandibular second molar when compared to maxillary second molar. This is considered as ideal relation which favors the development of proper occlusion of permanent molars. Mesial step terminal plane (Fig. 36.1b): In this type the distal surface of mandibular molar is anteriorly (mesially) located compared to distal surface of maxillary second molar. This causes a step directed mesially. Distal step terminal plane (Fig. 36.1c): When the maxillary second molar is in an anterior location than the mandibular second molar, the distal surface of mandibular second molar is located more distal to that of distal aspect of maxillary second molar resulting in a distal step.

Fig. 36.1: Molar relation in deciduous dentition

The mesial and distal step relations in a deciduous dentition suggest the possibility of disturbed occlusion in permanent dentition.

MIXED DENTITION

The mixed dentition begins with the emergence of the mandibular first molar at the age of 6 years and last up to 11–12 years, till all the deciduous teeth are replaced by permanent successors. Initially when the permanent first molars erupt distal to the deciduous molars they also show a flush terminal plane but the mesial movement of mandibular first molar results in class I molar relation. The space for the mesial shift of mandibular first molar is obtained by growth of mandible and by utilizing primate space and leeway space. At about the age of 8–9 years, by the eruption of larger anterior teeth crowding occur for a short period of time. The difference between the space available and space required to accommodate larger permanent incisors is called incisor liability. This liability is overcome by increase in width of dental arch in the intercanine region, by utilizing primate space and also by a labial inclination of permanent incisors which increase the dental arch circumference. Posterior successor teeth have a relatively lesser mesio-distal diameter than deciduous predecessors. Therefore the total mesio-distal measurements of deciduous canine and deciduous first and second molars are more than the total mesio-distal measurements of permanent canine, and two premolars. The difference between these two measurements is called leeway space of Nance. This space is around 0.9 mm on either side making up a total of 1.8 in maxillary arch and 1.7 on either side of mandible with a total of 3.4 mm in mandibular arch. This space is utilized by the permanent first molars to drift mesially to develop a class I molar relation.

PERMANENT DENTITION Alignment of Permanent Teeth in Dental Arch All the teeth in maxillary and mandibular arches are aligned with an angulation with respect to the alveolar bone. In the mandibular arch, both anterior and posterior teeth show a mesial inclination. Second and third molars are more inclined than the premolars. In the maxillary arch the anterior teeth are mesially inclined while the posteriors are distally inclined. A line drawn along the buccal cusp tips and the incisal edges of the mandibular teeth, is a curved line. By broadening this curved line to include the lingual cusp tips and extending it across the arch to the opposite side, a

curved plane can be established. This curved plane is called plane of occlusion. The curvature of occlusal plane is primarily due to the positioning of dental arch at varying degrees of inclination and this curved plane of occlusion permits maximum contact during function.

Compensating Curves The natural dentition shows compensating curves. Antero-posterior compensating curve runs in an antero-posterior direction, which can be appreciated from lateral (buccal) aspect. Curve of Spee (Fig. 36.2a): It is defined as anatomic curvature of occlusal alignment of teeth beginning at the tip of lower canine and following buccal cusp tips of the premolar and molar and continues to the anterior border of the ramus of mandible. This curve of dental arch was first described by von Spee. This imaginary curve is concave for mandibular arch and convex for maxillary arch. When the dental arches are placed into occlusion these concave and convex lines matches perfectly. Lateral compensating curve runs in bucco-lingual direction one side of the arch to other, which can be appreciated from frontal view. Wilson curve (Fig. 36.2b): When dental arch is observed from the anterior (front) region with mouth slightly open, a lateral (medio-lateral) compensatory curves can be appreciated in the maxillary molar region. Generally the posterior teeth in the maxillary arch have a slight buccal inclination while mandibular posterior teeth have a slight lingual inclination. If a line is drawn through the buccal and lingual cusps tips of both the right and left posterior teeth, a curved plane of occlusion is observed. The curvature is convex in the maxillary arch and concave in mandibular arch. When the arches are brought to occlusion both these curvatures match perfectly. This curvature in occlusal plane observed from frontal view is called curve of Wilson.

Interarch Tooth Relationship The relationship of teeth in one arch to those in the other arch is called interarch relationship. The maxillary and mandibular teeth occlude in a precise and exact manner. Since the arch width of mandible is slightly lesser than that of maxillary arch when the teeth occlude, maxillary teeth are more

facially placed than the occluding mandibular teeth.

Occlusal Relationship between Maxillary and Mandibular Posterior Teeth Centric relation refers to the relationship of the mandible to the skull as it rotates around the “hinge-axis” before any translatory movement of the condyles from their “uppermost and midmost position” in the mandibular fossa. In simple term centric relation can be defined as the position (or path of opening and closing without translation of the condyles) of the mandible in which the condyles are in their uppermost, midmost position in the mandibular fossae. It is irrespective of tooth position or vertical dimension.

Fig. 36.2a: Curve of Spee

Fig. 36.2b: Curve of Wilson

Centric occlusion refers to the relationship of the mandible to the maxilla when the teeth are in maximum occlusal contact, irrespective of the position or alignment of the condyle-disk assemblies. In other words, centric occlusion is the occlusion of opposing teeth when the mandible is at centric relation. This may or may not coincide with maximal intercuspation. The complete intercuspation of the opposing teeth independent of condylar position is referred to as maximum intercuspation. In complete occlusal closure (centric occlusion) the palatal cusps of the maxillary molars are seated in the central fossa of mandibular molars and buccal cusps of the mandibular teeth are seated in the central fossa of the maxillary molars. Therefore the buccal cusp of the mandibular posteriors and palatal cusps of maxillary posteriors are called supporting cusps/centric/functional/stamp cusps and are primarily responsible for maintaining the distance between maxilla and mandible. This distance supports the vertical facial height and is called vertical dimension of occlusion. The buccal cusp of the maxillary posteriors and lingual cusps of mandibular posteriors are called guiding or noncentriclnonfunctional cusps. The centric cusps are broad mid-rounded whereas noncentric cusps are sharp. When the teeth are in maximum intercuspation, only a small area of centric and noncentric cusps contact or remain in close relation and have functional significance. This area is located in the inner inclines of noncentric cusps near the central fossa and on the outer aspect of opposing centric cusps. The noncentric cusps give mandible stability so that when the teeth are in full occlusion, a tight definite occlusal relationship results.

Mesiodistal Occlusal Contact Relationship When the centric cusps contacts the opposing tooth, the occlusal contact can be observed in one of the two areas: 1. central fossa, 2. marginal ridge and embrasures. In the first type, when the cusp tip contacts with the central fossa only certain portions come in contacts at a given time, leaving other areas free of contact. In the second type, cusp tip contacts with the marginal ridge of the opposing tooth. Two variations in the occlusal contact patterns can result with respect to marginal ridge areas in some cases, the cusp contacts the embrasure area and the adjacent marginal ridges, resulting in two contacts on one area of cusp tip. In contrast, in some teeth cusp tip contacts only on marginal ridge, resulting in only one contact on cusp tip.

Normally each tooth occludes with two opposing teeth except for two teeth: Mandibular central incisors and maxillary third molars which occlude with only one tooth. Throughout the arch any given tooth is found to occlude with its counterpart in the opposing arch plus an adjacent tooth. This is described as one tooth to two teeth relationship. When the teeth are not in contact in mastication, swallowing, or speech, the lips are at rest and the jaws are apart. This is termed the postural position of the mandible, a term that is more appropriate than physiologic rest position. To maintain the mandible in this position, it is necessary to support it against the force of gravity. Thus, the masticatory muscles are in a mild state of contraction. The postural position is not constant; it varies with the position of the head and body and is affected by proprioceptive stimuli from the dentition, by prior jaw movements, and by emotional factors. Thus, there is no single, constant position of the mandible when the subject is at rest. The space between the mandibular and maxillary teeth when the mandible is in the postural position is called the free way space or the vertical dimension of rest. The average free way space ranges between 0 and 3 mm with an average of 1.7 mm.

Common Occlusal Relationships of the Posterior Teeth The molar relation of permanent posteriors can be class I, II or III as described by Angle. The key teeth for this classification are permanent first molars. Class I relation (Fig. 36.3a): Class I is the most typical molar relationship found in natural dentition and is considered as normal occlusion in permanent dentition. In this type of molar relation, the mesio-buccal cusp of the maxillary first molar is aligned directly over the mesio-buccal groove of the mandibular first molar. The mesio-buccal cusp of the mandibular first molar occludes in the embrasure area between the maxillary second premolar and first molar, whereas the mesio-lingual cusp of the maxillary first molar is situated in the central fossa area of the mandibular first molar. Class II relation (Fig. 36.3b): This type of molar relationship occurs when the mandibular arch is small or posteriorly positioned or the maxilla is large or anteriorly positioned. In this type of molar relationship, the mesio-buccal cusp of the maxillary first molar is aligned directly over the embrasure area between the mandibular second premolar and the first molar. The

buccal cusp of maxillary first molar is inline with mesio-buccal groove of the mandibular first molar. The mesio-buccal cusp of the mandibular first molar occludes with the central fossa of maxillary first molar whereas the distolingual cusp of the maxillary first molar occludes in the central fossae area of the mandibular first molar. Class III relation (Fig. 36.3c): This type of molar relationship corresponds to a predominant growth of the mandibular arch. Here, the mandibular molars are mesial to maxillary molar as compared to class I relation. Therefore mesio-buccal cusp of the maxillary first molar is found to be situated over the embrasure area between the mandibular first and second molars. The distobuccal cusp of the mandibular first molar is situated in the embrasure between the maxillary second premolar and first molar whereas the mesiolingual cusp of the maxillary first molar is situated in the mesial pit of the mandibular second molar.

Common Relationships of the Anterior Teeth Maxillary anterior teeth also show a labial placement compared to mandibular teeth. Both maxillary and mandibular teeth are aligned with slight labial inclination. In normal arch relation, when teeth are brought to occlusion, the anterior teeth of the maxillary arch overlaps the teeth of the mandibular arch. A horizontal overlap of approximately 2–4 mm, referred to as overjet is seen because the incisal edges of the maxillary anterior teeth are labial to the incisal edge of mandibular anterior teeth. The over jet is measured from the labial surface of the mandibular central incisor to the mid point of incisal edge of maxillary incisors. In normal relation, when viewed from labial aspect 3 to 4 mm of mandibular incisors are hidden because the incisal edge of maxillary incisors extend below the incisal edge of mandibular anteriors. This vertical overlap is called overbite and is measured as a distance between a horizontal line drawn between incisal edge of mandibular central to a point on labial aspect of maxillary incisors and a similar horizontal line drawn from maxillary incisor’s mesial edge and a point on mandibular incisor (Fig. 36.4).

Fig. 36.3: Molar relation in permanent dentition

Theories of Occlusion a. Bonwill Theory of Occlusion According to this theory of occlusion, the teeth move in relation to each other as guided by condylar and incisal guidance. The condylar guidance refers to the path that the trans-cranial rotation axis of the condyles travel during mandibular opening. The incisal guidance is a measure of amount of movement and angle at which the lower incisors and mandible must move from the overlapping position in centric occlusion to an edge to edge relationship with maxillary incisors.

Fig. 36.4: Overjet and overbite

Bonwill theory is also known as theory of equilateral triangle. According to this, the distance between the condyle is equal to the distance between the two condyles and the midpoint of mandibular incisors (the incisal point). A line drawn between the two condyles and from each condyle to the incisal point forms an equilateral triangle. The length of each arm of the equilateral triangle being 4 inches. Later, Monson, used Bonwill’s triangle and proposed a theory that sphere existed with radius of 4 inches, with centre that was an equal distance from occlusal surfaces of posterior teeth and from centres of condyles. The sphere formed is known as ‘Sphere of Monson’. ‘Curve of Monson’ can be defined as an ideal curve of occlusion in which each cusp and incisal edge touches the surface or confirms to a segment of an imaginary sphere 8 inches in diameter. Curve of Spee and Curve of Wilson form portions of the sphere of Monson.

b. Conical Theory of Occlusion This theory is proposed by RE Hall. According to this theory the lower teeth move over the surface of the upper teeth as over the surface of cone generating an angle of 45° with central axis of the cone tipped 45° to the occlusal plane.

c. Spherical Theory of Occlusion Proposes that the lower teeth move over the surface of upper teeth as over the surface of a sphere with a diameter of 8 inches. The surface of the sphere passes through the glenoid fossa along with articular eminences and the center of the sphere is located in the region of glabella.

Section 4

Oral Physiology 37. Eruption 38. Shedding 39. Saliva 40. Physiology of Taste and Speech 41. Mastication 42. Deglutition 43. Calcium Phosphorus Metabolism 44. Mineralization 45. Hormonal Influence on Orofacial Structures 46. Age Changanses of Oral Tissues

37 Eruption

Introduction Types of physiological tooth movements – – –

Pre-eruptive movements Eruptive movements Post-eruptive movements

Mechanism of tooth movement Clinical considerations

T

eeth undergo complex movements within the jaw bones during its development, as it moves from jaw bone to the functional position and also later to compensate for masticatory wear and to maintain their position in growing jaws. All these movements of teeth together are referred to as physiological tooth movements. The physiological movements of teeth are described under three headings. Pre-eruptive movements: The movements made by the developing tooth germ within the jaw bone. Eruptive tooth movements: The movements made by a developed tooth as it moves from the jaw bone to the functional position in the oral cavity. Post-eruptive movements: The movements made by a fully erupted tooth to compensate for occlusal wear and to maintain the occlusal plane as the jaw bone continues to grow.

Pre-eruptive Tooth Movements

Pre-eruptive tooth movements are preparatory to the eruption phase. These movements help in positioning the tooth germs within the jaw bone as the bone grows in length, width and height and also to position the tooth germs in a position favorable for eruption.

Pattern of Pre-eruptive Tooth Movement Deciduous teeth undergo pre-eruptive tooth movements to adjust their position in the developing jaw. As the jaw bone increases in length, anterior teeth drift forward and molars drift posteriorly to relieve the crowding of expanding tooth germs. Again as the jaw height increases, maxillary teeth move downwards and mandibular teeth move upwards to adjust their position. Tooth germs also move slightly outwards as the bone grows in thickness or width. Permanent successors also move in a complex fashion before it reaches its position from where they erupt. During initial stage of development, permanent anterior teeth are situated lingual and near to the occlusal level of its deciduous predecessor teeth in the same bony crypt. As the deciduous teeth erupt, they move apically to occupy their own bony crypt. Similarly, developing premolar tooth germs also move from a lingual position to the region between the divergent roots of deciduous molars. Permanent molar tooth germs also move considerably from the site of their initial development. Permanent maxillary molars develop with their occlusal aspect facing distally and mandibular molars with the occlusal aspect facing mesially. These teeth gradually become upright, only after sufficient space is created by distal growth of the jaw bones.

Histology of Pre-eruptive Tooth Movements Pre-eruptive tooth movements are combination of eccentric growth and total bodily movement. Eccentric growth results when the tooth germ increases in size and changes the shape. During this, only bone resorption occurs in the bony crypt to accommodate the growing tooth germ. Bodily movement is characterized by the movement of entire tooth germ which is brought about by selective resorption and deposition of the bony crypt wall. Bone resorption is observed on the side to which the tooth germ has to move followed by deposition on the opposite side. (For example, a

tooth germ moving in a mesial direction, show resorption of mesial aspect of crypt wall creating space. After the tooth germ moves mesially the bone is deposited on the distal aspect to fill in the space created by the movement of tooth germ.)

Eruptive Tooth Movements Eruption is defined as the axial or occlusal movement of the tooth from its developmental position within the jaw bone to its functional position in the occlusal plane. The term eruption is derived from a Latin word ‘erumpere’ which means ‘to breakout’. Although the breaking out of the tooth through the gingiva is the first clinical sign of eruption, eruptive tooth movements begin much earlier.

Pattern of Eruptive Tooth Movements Eruptive tooth movements are primarily in the axial or occlusal direction and ends when the tooth reaches the occlusal plane. During this phase, movements in other direction is also observed to position the teeth in the growing jaw bone.

Histology of Eruptive Tooth Movements During eruption along with movements of tooth, important events like root formation, orientation of periodontal ligament fibers and establishment of dento-gingival junction take place. Therefore histologically features related to these events are observed during eruptive tooth movements. Root formation: As a part of root formation, proliferation of Hertwig’s epithelial root sheath and dental papilla, formation of radicular dentin and cementum is observed. During initial stages of root formation, to accommodate the developing root, bone is resorbed at the base of bony crypt. During eruption, since the space is created for developing root by the occlusal movement of tooth, bone resorption stops and sometimes even bone formation is seen. Changes in periodontal ligament: As the root develops, organization of periodontal ligament takes place. During this phase periodontal ligament shows important changes that are required for eruptive tooth movement, These changes include: (a) presence of contractile elements in fibroblasts, (b)

formation of intercellular attachment between fibroblasts, (c) development of fibronexus (morphological relationship between intracellular microfilaments in the fibroblast and extracellular collagen fibers, mediated through a sticky glycoprotein called fibronectin), and (d) active remodeling of collagen fibers. Establishment of dento-gingival junction: After the erupting tooth comes out of the bone significant changes are observed in the overlying soft tissue such as: (a) degeneration of the connective tissue between reduced enamel epithelium and oral epithelium, (b) proliferation of both epithelia to form a solid plug of cells, and (c) degeneration of central cells of this epithelial plug, forming a canal through which tooth erupt without bleeding. Once the tooth erupts part of reduced enamel epithelium remain attached to the tooth and helps in establishment of dento-gingival junction. This part of epithelium is called junctional epithelium. Once the tooth break through mucosa it continues to move at the same rate till it reaches the occlusal plane to meet the antagonist tooth. Even after the tooth reaches the occlusal plane the major portion of tooth is covered by the soft tissue. Further exposure of the crown takes place by the apical shift of junctional epithelium. This is called passive eruption (for details refer page 124 and 125). In addition to the above mentioned changes, permanent successor teeth show an additional feature called gubemacular canal and guber-nacular cord. After the initial phase of development within the same bony crypt of deciduous teeth, permanent teeth move apically to occupy their own bony crypt. This bony crypt has a small opening or canal that contains connective tissue along with some remnants of dental lamina which connect the dental follicle to the lamina propria of overlying mucosa. The canal is called gubemacular canal and the connective tissue content of the canal is called gubemacular cord or gubernaculum dentis. In dry skull, this canal may be seen as small openings lingual to the deciduous teeth. This canal is widened by osteoclastic resorption as the tooth erupts and therefore may guide these teeth in eruption.

Post-eruptive Tooth Movements The teeth undergo physiologic movement even after it reaches the occlusal plane and this continues as long as the tooth remains in the oral cavity. Posteruptive tooth movements mainly occur due to three reasons.

To adjust their position in a growing jaw: This is seen up to the age of 18 years and stops once the growth of condyle is completed. As the condyle increases in length jaw get separated. To maintain the occlusal contact the upper and lower teeth move axially into a new occlusal plane. The same mechanism involved in eruptive movement is helping in this movement To compensate for occlusal wear—due to continuous tooth to tooth contact as in mastication, occlusal aspect of teeth undergoes continuous wear and tear. To compensate for this, the teeth move in axial direction which is also caused by the same mechanism as tooth eruption. To compensate for proximal wear due to wear and tear in the region of proximal contact, the teeth tend to drift mesially to maintain their contact. This is brought about by selective remodeling of bony socket wall and remodeling in periodontal ligament and contraction of transseptal fibers.

Histology of Post-eruptive Tooth Movement Histological changes observed during post-eruptive tooth movements include deposition of cellular cementum around the root apex; bone deposition at alveolar crest and base of socket and remodeling of collagen in periodontal ligament.

MECHANISM OF TOOTH MOVEMENT (THEORIES OF ERUPTION) The mechanism involved in eruption of teeth is not fully understood. It is considered that the eruptive tooth movement is brought about by multiple factors. Many factors have been considered to be playing significant role in tooth eruption. Accordingly, different theories have been put forward to explain the phenomenon of eruption. No single theory can be considered as complete explanation of eruption process.

1. Root Formation Theory According to this theory, the eruption of the tooth is brought about by the occlusal movement of the tooth that occurs to accommodate the growing root.

The increasing length of root during its formation can be accommodated either by an occlusal movement of the tooth or by resorption of bone at the base of the socket. It has been observed that the root growth produces force that is sufficient to move the tooth. But it is possible only if this apically directed force is translated to an occlusal direction for which a fixed base is essential. People who support this theory consider the existence of a strong base at the bottom of the socket in the form of a ‘cushion-hammock ligament’ that extends from one bony wall to other like a sling. But the histologic examination of developing tooth germs do not reveal any such structure. Instead only the pulp delineating membrane is observed that separates dental pulp from periapical tissue which do not have a bony attachment and is therefore unable to function as a fixed base. Although it would be possible to consider that the growing root pushes the tooth into the oral cavity, there are some clinical observations which question the importance of this theory. Some teeth move to a greater distance during eruption than the actual length of roots. Eruptive movement is observed even after the root completion. Experimental resection of developing root does not stop the eruption process. ‘Rootless teeth’ are found to be erupting. Although there are some demerits for this theory, root formation has an important role in helping in tooth eruption.

2. Bone Remodeling Theory This theory proposes that the eruption of tooth is brought about by selective deposition and resorption of bone that occur around the developing tooth. According to this theory, the resorption of bone in front and deposition of bone behind the erupting tooth, results in tooth movement. Results of some experimental studies have shown supporting evidences for this theory. In experimental studies where the tooth germ was removed and the dental follicle was left behind, the eruption pathway was created. In a similar experiment when a silica replica was placed after removal of developing

tooth, it erupted normally like a tooth. But when the dental follicle was removed, eruption pathway was not formed. It is observed that the tooth eruption is prevented in animals that have genetic deficiency of osteoclasts, the cells responsible for bone resorption. The supporters of this theory say that bone remodeling that occurs lead to formation of an eruption pathway through which the tooth erupts. The role of dental follicle could be indirect through the presence of blood vessels which provide a pathway for the osteoclasts that are derived from monocytes. Possible role of osteoblasts in bone remodeling is also been considered. The osteoblasts can secrete the collagenase and other proteolytic enzymes which can remove the osteoid layer to expose the mineralized bone; which in turn, can attract the osteoclasts to the site to cause bone resorption. Since the developing tooth is situated within the bony crypt, resorption and deposition of bone is very essential for tooth movement. But it is debatable whether the bone remodeling is the cause of tooth movements or the effect. Growth of alveolar bone: Growth of alveolar bone by apposition at the alveolar crest, was also thought to be helping in eruption process by the pulling action of periodontal ligament.

3. Vascular Pressure/Hydrostatic Pressure Theory According to this theory the local increase in tissue fluid pressure in the periapical region of the tooth causes the occlusal movement of tooth. The periapical tissue contains many blood vessels. The active fluid movement from these fenestrated capillaries into the local periapical tissue can cause swelling of the tissue to around 50%. Since the periapical tissue is in a closed space, swelling of tissue can cause local increase in the pressure and this pressure is exerted onto the tooth resulting in eruptive tooth movement. Since this pressure difference created is transient, it is doubtful whether this factor only is sufficient to cause a significant tooth movement as in eruption. Reduced eruption rate following severance of blood vessels to the periapical region has been observed. The role of vascular pressure in eruption cannot be confirmed only with this observation because lack of blood supply may affect other factors such as tissue growth, which could be even responsible for decrease in eruption rate.

4. Constriction of Pulp As the root formation continues, radicular dentin thickness increases resulting in decrease in size of the pulp cavity. It has been suggested that the pressure created within the constricting pulp is adequate to cause eruption of tooth. There is not much experimental evidence for this theory.

5. Pulp Growth The role of growing pulp in providing eruptive force has been suggested by Sicher. The supporters of this theory suggest that in the apical end of developing root there is active mitotic division of cells which bring about pulp growth and may provide at least a part of eruptive force. Rate of eruption is found to be decreased after injection of antimitotic drugs. This observation is insufficient to confirm the role of pulp growth alone, because the antimitotic drugs can adversely affect proliferation of other tissues that would be influencing eruption.

6. Periodontal Ligament Traction Theory This is the most accepted theory of eruption and it proposes that the contractile force created by the cells and fibers of the periodontal ligament is helping in pulling the tooth into occlusion. The fibroblasts in the developing periodontal ligament show intracellular contractile filaments, increased number of intercellular junctions and also a specialized structure called fibronexus providing connection between collagen fibers and intracellular filaments of the fibroblasts. Since the fibroblasts of periodontal ligament contain contractile filaments such as actin and myosin, they are called as myofibroblasts. These myofibroblasts contract and these contractile forces created by many cells are summated because of the intercellular attachments. These summated forces are transferred through fibronexus onto the collagen fibers, which are attached onto the tooth on one side. These forces applied onto an obliquely arranged collagen bundles are sufficient to pull the tooth upwards. The rapid remodeling of the collagen fibers and root elongation helps to maintain the oblique orientation of fibers when the teeth move occlusally. Force created by the fibroblasts and collagen fibers is able to pull the tooth only if the oblique orientation of periodontal ligament is maintained.

The activity of fibroblasts in pulling the tooth has been compared to a sailor (fibroblasts) pulling on a rope (collagen) attached to the sail (tooth). To move the sail (tooth) the sailor (fibroblasts) must remain stationary and pull on the rope (contraction) and coil it onto the deck (collagen remodeling). Many experimental evidences support the role of fibroblasts in eruption. Eruption process was stopped or slows down when periodontal ligament destruction was induced by injecting lathyritic agents or by denying vitamin C, which is an essential vitamin for development of collagen fibers of periodontal ligament. When fibroblasts were implanted on silicone rubber, they were found to be crawling on it and while doing so created wrinkles on the rubber indicating that traction forces have been created by moving fibroblasts. When fibroblasts of periodontal ligament were embedded in collagen gel, the fibroblasts were found to be aligning themselves parallel to collagen fibers and establishing a connection between each other and to collagen fibers thereby converting the collagen gel to a three-dimensional structure. Experiments in animals have showed the persistent movements of the tooth when only periodontal ligament was available to move the tooth and the possible effect of root growth and vascular pressure were eliminated. The force created by shrinkage of collagen fibers of periodontal ligament that occur during its development and maturation has also been considered as a source of eruptive force.

7. Dental Follicle Theory According to this theory dental follicle plays a very important role in eruption of teeth by: Playing important role in development of root Giving rise to periodontal ligament Providing a pathway for osteoclasts which is required for bone remodeling Acting as a source of osteoblasts Although many other mechanisms such as hormonal theory, foreign body theory, blood vessel thrust theory, pressure from muscular action, growth of

periodontal tissues, and resorption of the alveolar crest, etc. have been suggested by various investigators, these have not gained much importance. In conclusion, the eruption of the tooth is a multifactorial process initiated by pulling action of fibroblasts and periodontal ligament, facilitated by bone remodeling, root formation, vascular pressure, etc.

Clinical Considerations There are a number of clinical conditions, where eruption process is disturbed. 1. Premature eruption: Tooth erupts into oral cavity much earlier than normal time of eruption. Frequently involved tooth are deciduous mandibular central incisors. The term ‘natal’ teeth is used when the deciduous teeth are present at the time of birth. Deciduous teeth which erupt within first 30 days of life is termed as neonatal teeth. Premature loss of deciduous teeth causes premature eruption of permanent teeth which may be related to hormonal disturbances. 2. Delayed eruption: Tooth erupts into oral cavity much later than normal time of eruption. Both deciduous and permanent dentition may be affected. 3. Impacted teeth: Teeth which are prevented from eruption into oral cavity by some physical barrier in eruptive path or nonavailability of space. 4. Embedded teeth: It refers to those teeth that are unerrupted due to lack of eruptive forces 5. Ectopia: Remote location of a tooth away from its normal position. For example: Maxillary canine erupting in nasal cavity/maxillary sinus/at the inner canthus of eye, or mandibular 3rd molar erupting at angle of mandible/lower border of mandible/through the skin of cheek. 6. Transposition: Condition wherein 2 teeth exchange position. For example: Exchange of position between maxillary canine and premolar. 7. Rotation: Developmental partially/completely.

anomaly

wherein

a

tooth

turns

38 Shedding

Introduction Mechanism of shedding Histology of shedding Pattern of shedding Clinical considerations

S

hedding is defined as a physiological process by which the deciduous teeth are removed to allow the succeeding permanent teeth to take their functional position in the oral cavity. In simple words, shedding is the physiological process of eliminating the deciduous dentition. Shedding of the deciduous teeth is necessary as the teeth do not grow after they are formed and therefore they need to be replaced by more number of larger teeth, which can resist greater masticatory force and also, esthetically pleasing in a large jaw of adults.

Mechanism of Shedding Shedding of the deciduous teeth occur as a result of resorption of roots of the teeth and destruction of supporting periodontal ligament. The factors suggested to be playing role in shedding are: Pressure from erupting permanent successors: This pressure helps in the differentiation of odontoclasts that can resorb the dental hard tissues. In case of congenitally missing permanent successor, the deciduous tooth is retained for a longer time, supporting the role of pressure from the successor teeth in exfoliation.

Force of mastication: Although the deciduous tooth is retained for sometime when permanent successor is missing, ultimately it exfoliates suggesting the role of other factors on shedding. As individual grows, force of mastication increases and become greater than what deciduous periodontal ligament can withstand. This leads to trauma to the periodontal ligament, followed by destruction, initiating resorption and ultimately shedding. Combination of these two factors may be deciding the rate and patterns of resorption. When the root is resorbed, supporting tissue decreases making the tooth unable to bear the masticating forces. This makes the tooth mobile and accelerates the process of shedding.

Histology of Shedding Shedding is brought about by resorption of dental hard tissues and destruction of supporting periodontal ligament. The cells responsible for resorption of tooth are odontoclasts. Odontoclasts are highly specialized cells responsible for resorption of dental hard tissue including cementum, dentin and enamel. They are structurally and functionally similar to osteoclasts, the bone resorbing cells. Origin of odontoclast is from circulatory monocytes that are capable of giving rise to all different tissue macrophages. Light microscopically odontoclasts can be readily identified as large, multinucleated giant cells, occupying the irregular bays on the surface of resorbing dental hard tissue. They may also be found in the pulp. Electron microscopically these cells have a ruffled border adjacent to the resorbing hard tissue. This is formed by folding of cell membrane into a series of invaginations of 2–3 mm in depth. Mineral crystallites may be seen in the depth of these invaginations. Cytoplasm has large number of mitochondria and vacuoles which are seen close to the ruffled border. Histochemically the cells have increased levels of enzyme acid phosphatase. The mechanism by which odontoclasts actually resorb the hard tissues of teeth is not understood. Possibly during the initial stage, the crystallites are removed exposing the organic matrix. As a second step, organic matrix is removed by extracellular dissolution into smaller molecules and phagocytosis by odontoclasts.

Vacuoles in the odontoclasts, rich in acid phosphatase suggest that they are phagosomes causing break down of ingested materials. Histological sections show that periodontal ligament degeneration may be through apoptotic cell death or through a mechanism that interfere with formative function of fibroblasts.

Pattern of Resorption and Shedding The pressure exerted by the successor tooth leads to resorption. The developmental position and the physiological movement of the successors are very important factors in determining pattern of resorption and shedding. The developmental positions of permanent incisors are lingual to deciduous incisors, and later they occupy an apical position. Because of this, for deciduous anteriors first sign of resorption is on the lingual aspect of the root followed by resorption of the apical region. If the permanent tooth fails to take an apical position, it may erupt lingual to deciduous tooth which is still in function. The developmental position of premolars is between the roots of deciduous molars. So the first evidence of resorption is observed along the inner aspect of deciduous molar roots. Later the developing tooth occupies an apical position and slowly resorbs the root from its apex and continues till the root is completely resorbed and exfoliated. The process of resorption is not continuous but has periods of rest and repair. Overall the process of resorption predominates, over repair leading to exfoliation of tooth. If repair predominates it can lead to retention of deciduous tooth. The pattern of shedding is symmetrical for right and left side. Mandibular teeth exfoliate before their counterparts in maxillary arch. Maxillary and mandibular second deciduous molar exfoliate almost simultaneously. Girls exfoliate teeth earlier than boys. This discrepancy is most observable in case of mandibular canine. The sequence of shedding in mandibular follows anterior to posterior order of teeth in the jaw. In the maxilla, the sequence is disrupted by first molars exfoliating before canines.

Clinical Considerations 1. Retained deciduous teeth: Deciduous teeth remaining in the oral cavity

for longer time than the normal exfoliation period are referred to as retained deciduous teeth. The causes of retained deciduous teeth may be • • • •

Congenital absence of permanent successor Failure of eruption of permanent successor Ankylosis of deciduous tooth (fusion of tooth to alveolar bone). Eruption of permanent tooth in a lingual or labial position so that the deciduous tooth escape from the pressure exerted by them.

2. Remnants of roots of deciduous tooth Remnants of roots of deciduous tooth is commonly seen in maxillary premolar region. The roots of deciduous molars are so divergent that the distance between roots is more than the diameter of the developing permanent tooth. This allows a portion of root to escape from resorption. These unresorbed root pieces may get resorbed and replaced by bone. The remnants closer to the surface extrude through the mucosa and ultimately exfoliate, 3. Submerged tooth Submerged tooth is the one which remains below the level of normal occlusal plane of other teeth. This can occur if the tooth is ankylosed (fused) to the alveolar bone. Ankylosis may be due to trauma resulting in damage to periodontal ligament. Imbalance between resorption and repair that occur during the process of exfoliation may also lead to ankylosis of deciduous teeth. An ankylosed tooth is unable to undergo physiological tooth movement to compensate for increased height of alveolar bone. When adjacent teeth continue eruption, the ankylosed tooth remains submerged with occlusal plane of this tooth at a lower level. When the deciduous tooth is ankylosed, it fails to exfoliate and therefore prevent eruption of its successor. A submerged tooth should be extracted, after a radiographic confirmation of presence of permanent successor.

39 Saliva

Introduction Composition Function Synthesis Control of secretion

S

aliva is the fluid secreted by the salivary gland that keeps the oral cavity moist. Saliva is secreted by three pairs of major salivary glands, namely parotid, submandibular and sublingual glands and numerous minor salivary glands which are widely distributed in the oral mucosa. The total volume of saliva secreted varies from 600 to 700 ml per day or even may be up to 1 to 1.5 liters/day. pH of saliva varies from 6.2 to 7.6. During rapid secretion the saliva becomes more alkaline because of high bicarbonate content. Specific gravity of saliva is 1.002 to 1.012.

COMPOSITION OF SALIVA Saliva contains various constituents such as inorganic salts, both enzymatic and non-enzymatic proteins, dissolved gases, etc. Composition of saliva varies from unstimulated to stimulated saliva.

Saliva Contains Water-99%

Organic and inorganic components-1%

Organic Components Mucin or carbohydrate rich glycoproteins Antibacterial components – – – – –

Lactoferrin Kallikrein Lysozymes Peroxidase Thiocyanate

Digestive enzymes – – –

Ptyalin or amylase Maltase Lipase

Free amino acids, fatty acids, urea, uric acid, free glucose, peptides, blood clotting factors, blood group substances, epidermal growth factors, etc.

Inorganic Component Sodium, chloride, potassium, calcium, bicarbonates, phosphates, ammonia, magnesium, fluoride, iodine, etc.

Dissolved Gases Carbon dioxide, oxygen, nitrogen, etc. In addition, the whole saliva obtained from the mouth also contains desquamated epithelial cells and a few leukocytes from crevicular fluid and oral micro-organisms.

FUNCTIONS OF SALIVA Mechanical Function

Lubrication: Because of water and mucin content, saliva helps to keep oral cavity wet and helps in speech, mastication and deglutition. Salivary glycoproteins forms a lining of oral tissues and therefore prevent adhesion of microbes, microbial products and various other materials. Lavage: Saliva helps to flush out food debris and micro-organisms from the oral cavity. Saliva helps to dilute hot and other irritant materials therefore preventing trauma to mucosa.

Antimicrobial Actions of Saliva The high molecular weight glycoproteins in the saliva aggregate the microorganisms and therefore help in rapid clearance from oral cavity thereby preventing their adhesion to the oral tissue. Immunoglobulins: Saliva contains immunoglobulins mainly IgA, IgG and IgM also may be present. These immunoglobulins prevent the adhesion of micro-organisms to oral tissues. (Salivary IgA [sIgA] in contrast to serum IgA are always seen as diamers, i.e. two IgA molecules joined by a J chain. In addition, salivary IgA also has an additional protein called secretory component that helps in the transfer of IgA through the parenchymal cells and gives resistance to hydrolysis.)

Fig. 39.1: Functions of saliva

The antibacterial activity of sIgA can be • •

By binding to specific antigens responsible for adhesion. By agglutination or clumping of bacteria which are then easily

•

washed off. By affecting specific enzymes necessary for bacterial metabolism.

Peroxidase: This enzyme present in the saliva can catalyze the reaction between thiocyanate and hydrogen peroxide produced by micro-organisms, leading to formation of hypothiocyanate which oxidizes the bacterial enzymes and is bactericidal. Lysozymes: Saliva contains the enzyme lysozymes that can break down bacterial cell wall leading to lysis of bacteria. Lactoferrin is an iron binding protein present in the saliva that can combine with free iron; therefore depriving micro-organisms of iron which is essential for their multiplication.

Digestive Function Saliva contains amylase or ptyalin which can act on starch and split it into disaccharides. Lipase present in the saliva secreted by von Ebner’s gland is important in lipid digestion in newborn. Saliva also contains some maltase enzyme.

Buffering Action of Saliva The components that impart a buffering action to saliva are mainly bicarbonates and phosphates. When bicarbonate ions come in contact with acid ions, weak carbonic acid is formed which is rapidly dissociated into water and carbon dioxide. The glycoproteins having negatively charged residues and Sialin, a salivary polypeptide also reported to have buffer capacity. Buffering action of saliva is helpful in two ways It denies the optimum pH required for multiplication of micro-oganisms. Salivary buffers helps to neutralize the acids produced by micro-organisms therefore preventing demineralization of enamel and dental caries.

Taste Sensation Taste sensation can be perceived only when the substance is dissolved and therefore the solvent action of saliva is very important in perception of taste.

Saliva also helps in cleaning the taste buds to ready them for the next taste perception. ‘Gustin’ present in saliva helps in development and maturation of taste buds.

Tissue Repair and Blood Coagulation Tissue repair function of saliva is considered because of the presence of epidermal growth factors which stimulate epithelial growth and therefore could be helping in wound healing. Saliva also contains blood coagulating factors such as IX, VII and platelet factor that reduces the clotting time.

Water Balance When the water content in the body is reduced, salivary secretion is decreased and mouth becomes dry. The nerve endings in the posterior aspect of tongue are stimulated and a dry mouth reflex is initiated which stimulate the salivary flow. If the body tissue is short of water, reflex does not occur leading to drying of mouth. Thus, encourages the individual to drink water and water balance is maintained.

Endocrine Function Saliva contains some biologically active materials. For example, parotin secreted by parotid gland. Parotin is reported to promote mesenchymal tissue growth, decreases serum calcium level, promoting the mineralization of dentin, etc. in animals. The status of parotin as a true hormone has not been identified.

Excretion Saliva act as a route through which certain substances are excreted such as mercury, lead, thiocyanate, ethyl alcohol, some drugs, etc. (Excretion of ethyl alcohol in saliva can be used as a method to determine whether the individual has consumed alcohol in case of medico legal cases.) Certain viruses like viruses of rabies, mumps and poliomyelitis are also excreted through saliva.

Tooth Integrity Saliva contains calcium and phosphate ions. Due to ionic exchange between saliva and tooth, enamel undergoes maturation. Due to this, enamel becomes

harder, less permeable and more resistant to caries. Ionic exchange can also lead to remineralization of initial caries lesion preventing further progression.

Nerve Growth Factor In experimental animals it has been observed that submandibular gland secretions contain rich nerve growth factor which greatly increases the growth of sympathetic ganglia and sensory nerves.

Temperature Regulation This function of saliva is significant in animals.

SYNTHESIS AND SECRETION OF SALIVA The organic components of the saliva are synthesized by the secretory cells of salivary gland utilizing the substrate provided by the nutrients that reach the cell from the blood vessels and stored in secretory granules. When the secretory unit is stimulated the stored products are expelled out. Water and electrolyte required for the sava reaches the cell from circulation and from tissue fluid. When there is nerve stimulation, chloride ions are actively transported into the cell. This increased electronegativity induces the influx of sodium ions. This increased sodium and chloride ions in the cell create an osmotic force resulting in transport of water into the cell causing cell swelling. The pressure in the cell results in minute rupture of secretory border of cell, expelling water and electrolytes. Kallikrein presents in saliva act upon plasma proteins to produce bradykinin. This produces the vasodilatation resulting in seepage of water into the tissue fluid, during active secretion of saliva. Decision for protein synthesis is taken in the nucleus ↓ Messenger RNA in ribosomes carry the message to the cytoplasm through ribosomes ↓ Ribosomes translates the message and initiate protein synthesis by adding

amino acids in required sequence. Thus, form a preprotein with a signal sequence attached to it ↓ With the help of signal sequence, protein synthesized enters the PER where the signal sequence is removed and protein assumes a helical structure ↓ Protein synthesized is transferred to Golgi complex ↓ Structural modification of protein in the Golgi complex by addition of carbohydrates ↓ Packing of secretory product into secretory granules (pro secretory granules) ↓ Further addition of molecules resulting in maturation of secretory granules ↓ Storage of secretory granules in the apical cytoplasm ↓ Secretion of stored material by a process of exocytosis

Control of Salivary Secretion Secretion of saliva is under nervous control. An increased secretion of saliva may result from thought, sight or smell of food, talking about food or even by the noise of food being prepared. Increased salivary secretion can be caused by: Taste: different tastes have varying capacities in stimulating salivary secretion. Smell Mechanical stimulation of oral mucosa Mechanical irritation of gingiva during dental treatment procedures Mastication: Stimulate salivary secretion by stimulating sensory impulses. Chemical irritation of oral mucosa by acids such as citric acid, salt, etc.

Irritation of esophagus, like in case of foreign bodies or pathology like carcinoma. Pregnancy: An increased salivary flow rate is reported. Irritation of the stomach wall leading to nausea.

Nervous Control of Salivary Secretion The salivary glands are innervated by both sympathetic and parasympathetic secreto-motor nerve fibers. The parasympathetic innervation is secretory and vasodilatory while sympathetic innervation is mainly vasoconstrictive which also promote secretion in some cases. Secretory activity of secretory cells is mediated by cholinergic agents in parasympathetic system and by adrenergic agents in sympathetic system. The secretory motor nerve endings are seen in relation to secretory cells, cells of striated and intercalated ducts, myoepithelial cells, smooth muscles of arterioles, etc. In relation to secretory cells, two types of association with nerve endings can be observed.

Intraepithelial or Hypolemmal Type In this type axons split off from the nerve bundle and penetrate the basal lamina to reach the space between two adjacent secretory cells. Here the nerve endings are in close proximity to the secretory cells and the distance in between is only 10–20 nm.

Subepithelial Type or Epilemmal Type In this type the nerve axons instead of penetrating the basal lamina, remain in the nerve bundle in the connective tissue. The distance between the secretory cell and the nerve axon is more and is around 100–200 nm. In both intraepithelial and subepithelial type of innervations, nerve axons show varicosities or thickening. These varicosities are called the neuroeffector sites. They contain chemical neurotransmitters such as norepinephrine and acetylcholine. When the nerves are stimulated, these neurotransmitters are released which stimulate secretory cells to synthesize and secrete saliva. The arterioles in the connective tissue component of the salivary gland also

get sympathetic and parasympathetic innervations. Stimulation of parasympathetic system causes vasodilatation while the sympathetic system causes vasoconstriction. Parasympathetic stimulation results in secretion of large volumes of watery saliva by the secretory cells. Sympathetic stimulation produces less quantity of thicker saliva. Sympathetic stimulation also has a greater influence on the composition of saliva and results in a higher concentration of organic substances due to increased exocytosis in the cell with decreased movement of water caused by vasoconstriction. There is no direct inhibition of salivary secretion by nerves.

Clinical Considerations •

Sialorrhea is the term used to describe a condition where there is an excessive flow of saliva. This may be associated with various conditions, such as acute inflammation of the mouth, mental retardation, mercury toxicity, teething, etc. Sialorrhoea is called hypersalivation or ptyalism.

•

Xerostomia is the term used for dry mouth due to a lack of saliva. Xerostomia can be caused due to a number of reasons: Psychological causes like anxiety and depression; dehydration due to diarrhoea, sweating, vomiting, diabetes mellitus and diabetes insipidus; use of antihistaminic drugs; diseases affecting salivary glands such as Sjogren’s syndrome, tumors or salivary gland aplasia. Xerostomia can cause difficulty in speech and eating. It also leads to halitosis (bad breath), dramatic increase in dental caries and infections.

40 Physiology of Taste and Speech Dr Usha Balan Physiology of taste – –

The sensation of taste Mechanism of taste stimulation

Physiology of speech – – –

Speech process Integration of speech Perception of speech

PHYSIOLOGY OF TASTE The Sense of Gustation or Taste Taste is a mixture of four elementary taste qualities salty, sweet, sour and bitter. Recently a fifth taste sensation called umami also has been identified. Taste depend on activation of chemoreceptors located in the taste buds (refer page 122 for details). Taste buds are widely distributed on the tongue, palate, pharynx and larynx, epiglottis, etc. The taste buds of the tongue are found on the dorsal and lateral aspects and are associated with specialized structures called papillae. The fungiform papillae contain up to five taste buds per papillae. The lateral walls of the circumvallate papillae contain large number of taste buds up to 100. Adults have a total of approximately 3000 to 10000

taste buds which decreases by as much as 60% in old age. Taste bud cells undergo rapid turnover with half life of 10 days. The sensitivity of tongue for different taste qualities vary with regions of tongue. Sweet sensations are detected best at the tip, salty and sour along the sides and bitter in the posterior region. When the taste substance is low in concentration each taste bud usually respond to one of the four primary taste stimuli, but at high concentrations taste buds can be excited by various primary taste stimuli.

Mechanism of Stimulation of Taste Buds The gustatory receptor cells are chemoreceptors that respond to substances dissolved in oral fluids. These substances act on exposed microvilli in the taste pore to evoke generator potentials in the receptor cells which generate action potential in the sensory neurons. The membrane of taste receptor cells are negatively charged on inside with respect to the outside. When a taste substance is applied, the taste chemical in the substance binds to the protein molecules in the microvilli. This in turn opens ion channels which allow sodium ions to enter and depolarize the cell. The type of receptor protein in microvilli determines the type of taste that will elicit the response. This change in potential in the taste cell leads to the receptor potential and the release of an excitatory neurotransmitter. This neurotransmitter evokes a generator potential in the primary afferent nerve fibers and cause a discharge that is transmitted to central nervous system. The taste buds are innervated by three cranial nerves. The chorda tympani branch of facial nerve supplies the taste buds of anterior 2/3rd of tongue and glossopharyngeal nerve supplies the taste buds of posterior 1/3rd of the tongue. A few taste buds on the posterior region such as pharynx (areas other than tongue) are innervated by vagus nerve. On each side, the taste fibers in these three nerves unite in the medulla oblongata to enter the nucleus of the tractus solitarius. Here they synapse on the second order neurons, the axons of which cross the midline and join the medial lemniscus ending with fibers of touch, pain and temperature, in the specific sensory relay nuclei of thalamus. The impulses are relayed from there to the taste projection area in the cerebral cortex. On first application of the taste stimulus, the rate of discharge of nerve fibers rises to the peak and comes down to a lower steady level. Thus, the taste is characterized by a strong immediate signal followed by weaker continuous signal as long as the taste bud is exposed to the taste stimulus.

PHYSIOLOGY OF SPEECH Speech is an ordered utterance of language. Speech is human achievement and is a signal output process through which an individual communicates with his or her surroundings. The development, maturation and maintenance of good speech depends greatly upon the integrity of the structural, neurological, physiological, psychological, social, and cultural processes.

Speech Process Speech is often described as “an overlaid process” secondary to vegetative functions. Phenomenon of speech process includes four mutually dependent divisions. Respiration Phonation Resonation Articulation These process co-ordinate to produce dynamic acoustic modulation of speech.

Respiration or Power Division First step in speech producing process is respiration where in the energy source for speaking is provided by the respiratory system. Exhaled air stream moves through the resonating cavities and is shaped into discrete sounds.

Phonation Second step in speech process is phonation. Breath stream emitted from the lungs strikes the vocal folds housed in the larynx. Phonation results from vibratory activity of vocal folds. Exhaled air stream is interrupted by vibratory pattern of vocal folds, and air puffs emerging from this process create sound. This sound is referred as source excitation which serves as an acoustic material from which speech sounds are later developed.

Processes concerned with shaping or modifying source sounds into identifiable speech sounds are resonance and articulation.

Resonation Resonation is the third step in speech process. Resonance system gives a distinguishing quality which is characteristic of each voice. Sounds are modified by selective alteration of size and shape of vocal tract. Depending on the configuration of vocal tract, certain frequencies are amplified whereas others are attenuated.

Articulation Articulation is the fourth step in speech producing sequence. Articulators and articulatory valves are responsible for this act. Vocal organs are the articulators and the articulatory valves are the place at which airstream is modified to produce speech sounds. When vocal organs assume a certain position they produce sound, simultaneously articulatory valves stop, constrict and narrow the air-stream, thus producing speech sounds. Articulation thus refers to placement and movement of lips, teeth, tongue, mandible, soft palate and associated structures during speech to produce speech sound.

Integration of Speech Processes of respiration, phonation, resonances and articulation are coordinated and integrated by the nervous system to produce the complex and dynamic behavior known as speech production.

Perception of Speech Speech is studied in terms of both production as well as perception of sound. Properties of each sound is influenced by speech sounds that preceed and succeed it. Perception of sound depends not only on factors like acoustic signals but also on adequacy of listeners auditory system, nature of perceptual environment and linguistic orientation of both speaker and listener.

41 Mastication

Introduction Objectives of mastication Forces of mastication Masticatory cycle/chewing cycle Changes in various structures during mastication Control of masticatory cycle

M

astication is the act of chewing food whereby the ingested food is cut or crushed into small pieces, mixed with saliva and formed into a bolus in preparation to swallowing.

Objectives of Mastication Mastication helps in deglutition by • •

Breaking the large food particles into smaller particles which otherwise may cause irritation to gastrointestinal tract. Forming a bolus that can easily be swallowed.

Mastication helps digestion by • • •

Stimulating salivary secretion Causing break down of food particles thereby increasing surface area for enzymatic action. Facilitating mixing of food with saliva and initiating digestion by salivary enzymes.

•

Exposing the digestible components present inside in some food materials.

Mastication also ensures healthy growth and development of oral tissues. The act of mastication is a complex process that uses masticatory muscles, teeth, periodontal supportive structures and also the lips, cheeks, tongue, palate and salivary glands. During mastication well coordinated functioning of masticatory muscles move the mandible to bring the teeth together. During this process of contact between teeth considerable force is exerted on the food particles resulting in reduction in size of food particles.

Forces of Mastication The maximum biting force that can be applied to the teeth varies from individual to individual. Males are able to exert more masticatory force than females. The masticatory force exerted on anterior teeth is 55 pounds (10–15 kg) and on molars is 200 pounds (around 50 kg) approximately. Biting force can be increased by exercise. Maximum biting force up to 150 kg has been recorded in traditional Eskimos who have lived on very tough diet requiring vigorous mastication.

Masticatory Cycle/Chewing Cycle Mastication is made up of rhythmic well-controlled separation and closure of maxillary and mandibular teeth. Each opening and closing movement of mandible represents a chewing stroke. During mastication similar chewing strokes are repeated over and over as the food is broken down. Along with straight opening and closing movements, the jaw also show protrusive, retrusive and lateral movements. Each chewing cycle lasts for approximately 0.8–1.0 second.

Chewing cycle comprises two phases An opening phase A closing phase The closing phase has been further subdivided into: Crushing phase which is at the first phase of closure during which food is

trapped between the teeth. Grinding phase which is at the later phase of closure which permit the shearing and grinding of food. Mastication is a complex process which causes rhythmic opening and closing movements of the jaws brought about by masticatory muscles and TMJ. The structures involved in masticatory cycle include masticatory muscles, temporomandibular joint, mandible, teeth and soft tissues such as tongue, cheeks and lips.

Changes Observed in Masticatory Muscle during Chewing Cycle During the opening phase and in the beginning of closing phase masticatory muscles undergo isotonic contraction (shorten to produce jaw movements against a constant load) and relaxation. In the latter part of closing phase and occlusal phase, tension builds up in the elevator muscles which undergoes isometric contraction (produce contractile tension with no change in length); when the teeth are in contact or when there is a hard object in between the teeth. During the opening phase the lateral pterygoid muscle is active and also the suprahyoid muscles (digastric, geniohyoid and mylohyoid). During the initial closing phase the depressor muscles are first activated, then gradually relaxed to allow the mouth to be closed by the passive tension in the elevator muscles. In the closing phase, first the temporalis on the working side become active followed by masseter and temporalis muscle of the nonworking side. Masseter and medial pterygoid are active during incisive movement and lateral pterygoid during protrusive movement.

Jaw Movement during Masticatory Cycle Both condylar head and the mandibular body move during masticatory cycle. The mandibular and condylar movements associated with mastication are the coordinated result of sequenced mandibular muscle contractions. When the movement of mandible is traced during masticatory stroke ‘a tear drop’ shaped tracing is observed. In the opening phase, the mandible drops downward from the intercuspal position to a point where the incisal

edge of the teeth are about 16 to 18 mm apart. The mandible then moves 5 to 6 mm laterally from the midline as the closing movement begins. The first phase of closure traps the food between the teeth and is called crushing phase. As the teeth approach each other the lateral displacement is lessened and the jaw occupies a position only 2 to 4 mm lateral to the starting position of the chewing stroke. As the mandible continues to close the bolus of the food is trapped between the teeth and the grinding phase starts. During grinding the mandible is guided by the occlusal surface of teeth which has come back to the intercuspal position. In early stages of mastication where the food has to be incised, the mandible moves forward to a greater distance. In later stages, the crushing of food is concentrated and very little anterior movement occurs. Similarly, the lateral movements are also greater when the food is initially introduced into the mouth and then becomes lesser as the food is broken down. The lateral movement also varies according to the consistency of food. The harder the food the more the lateral closure stroke becomes. Condylar heads rotate and translate to allow the jaw to open and close during masticatory movements. During opening phase the condyle on the working side move laterally whereas the opposing condyle on the balancing side moves medially downward and forward. These condylar movements make the mandible shift to the working side. In early closing phase the condyle of the working side resumes its position within the articular fossa whereas balancing side condyle returns only in late period of closing phase. The mandible swings back to intercuspal position.

Position of Teeth during Chewing Cycle In the opening phase, the incisal edges of teeth are about 16 to 18 mm apart. During the closing phase, the teeth approach each other with a distance of 2 to 3 mm in between. At this point teeth are so positioned that the buccal cusps of mandibular teeth are almost directly under the buccal cusps of maxillary teeth on the side, the mandible has been shifted. During grinding phase mandibular and the maxillary teeth come to intercuspal position and allows the cuspal inclines to move across each other permitting grinding of teeth similar to the action of pestle and mortar. During mastication most of the food particles are crushed by vertical movement of mandible and then sheared to make a bolus. Teeth do not come into occlusion in the initial period. The frequency of tooth increases after the

food particles are softened. In the final stage of mastication, just prior to swallowing, contacts occur during every stroke. Two types of contacts have been identified: Gliding, which occurs as the cuspal inclines pass by each other during opening and grinding phase of mastication, and single, which occurs in maximum intercuspal position. Occlusal contacts occur in centric occlusion in at least 90% of all chewing cycle, especially towards the end of the masticatory cycle. The number of teeth which may contact vary with food type and increases towards the end of chewing cycle. In some cases tooth contact occur only on one side.

Role of Soft Tissues (Lips, Cheeks, Tongue) in Mastication The lips, tongue and cheek play an essential role in mastication. As the food is introduced into the mouth, the lips guide and control intake. By sealing the oral cavity, the lips prevent the loss of fluid and food to outside. The lips are especially necessary when liquid is being introduced. When the food is introduced, the tongue often initiates the breaking up process by pressing the food against the hard palate. The tongue then pushes the food onto the occlusal surfaces of teeth where the food can be crushed. During the opening phase of next chewing cycle, the tongue repositions the partially crushed food onto the teeth for further break down. While the tongue is repositioning, the food from the lingual side the buccinator muscles of the cheek position food from the buccal side. The food is thus repositioned continuously until the particle size is small enough to be swallowed. Once the food is crushed and chewed it is moved back below the soft palate by squeezing action of tongue. Tongue is also important in collecting and sorting food that is suitable for swallowing while larger food particles are returned to occlusal table for further reduction. Tongue also has a hygienic function by removing the residues of food from, between the teeth and from the oral vestibule by sweeping action.

Control of Masticatory Cycle Mastication is a functional activity that is generally automatic and practically involuntary. Yet when desired, it can be readily brought under voluntary control. Mastication is controlled by nuclei in the brainstem and also areas in hypothalamus and cerebral cortex. The masticatory muscles are supplied by

the trigeminal nerve. The cyclic activity of the masticatory muscle is generated by the chewing center (neural pattern generator) in the brainstem and is influenced by peripheral afferents from face, mouth, etc. Sensory input generated by closing on a hard food initiate the generation of rhythmic activity; whereas closure on a softened bolus initiates a swallowing reflex and may consequently terminate rhythmic activity. Afferent fibers innervating the periodontal mechanoreceptors, mechanoreceptors at the corners of the mouth and jaw spindle afferent fibers exert peripheral control on masticatory cycle. The process of chewing is caused by chewing reflex that is repeated many times. When food is placed in the mouth the mandible drops down due to inhibition of muscles of mastication. This drop initiates a stretch reflex of muscles leading to rebound contraction. This leads to closure of jaw and compression of food between teeth and oral mucosa. This once again causes inhibition of muscles leading to drop of jaw which rebounds again. This process continues repeatedly till the food is sufficiently softened.

42 Deglutition

Introduction Phases of deglutition Infantile and adult swallow

D

eglutition or swallowing is a series of coordinated muscular contractions that move the ingested food and pooled saliva from the oral cavity through esophagus into the stomach. Swallowing consists of voluntary, involuntary and reflex muscular activity. Over a period of 24 hours swallowing occurs approximately 1000 times; which is highest while eating. Although swallowing is a continuous process, it can be divided into three basic phases: Oral phase during which a voluntary transfer of material from the mouth to pharynx takes place. Pharyngeal phase, which involves an involuntary or reflex mechanism that transfers material from the pharynx to the upper esophagus. Esophageal phase in which contents of the upper part of the esophagus are transferred into the stomach by involuntary peristaltic contraction of the esophageal muscles. In addition to these three phases, a preparatory phase can also be appreciated which merges with the terminal phase of mastication.

Preparatory Phase During this phase, the bolus is prepared and positioned on the tongue as a

preparation to swallowing. The tip of the tongue presses against the maxillary incisors or anterior part of palate and lateral aspect rises against posterior teeth and palate so that tongue develops a spoon-like depression. Posteriorly the pharyngeal part of the tongue arches up to the soft palate. At the same time, soft palate is depressed to create a glossopalatine seal, which prevent the bolus from escaping into the pharynx.

Oral Phase During oral phase, bolus is propelled from the oral cavity to the pharynx. The tongue muscles and the muscles of floor of the mouth play an important role in this phase. The oral phase starts after the bolus is positioned on the tongue. During this phase, the lips are closed and upper and lower teeth come in contact. This is followed by elevation of the anterior 2/3rds of the tongue, which presses against anterior part of the hard palate. Mean while the glossopalatine seal is opened by elevation of the soft palate with depression of posterior part of tongue. This allows the passage of food to pharynx. The entry of food into the nasopharynx is prevented by the elevation of the soft palate. The inward and forward constriction of posterior pharyngeal wall closes the palatopharyngeal isthmus. Oral phase of deglutition is under voluntary control and it lasts for 0.5 seconds. During this phase, airway is open and breathing continues normally.

Pharyngeal Phase In pharyngeal phase, the bolus is transported from the oropharynx into the esophagus by a peristaltic wave caused by contraction of the pharyngeal constrictor muscle. The pharyngeal phase begins, when the bolus makes contact with the posterior part of oral mucosa and mucosa of the pharynx. These contacts on sensitive areas act as stimuli for a series of reflexes that are responsible for the bolus being transferred into the esophagus and not into trachea or nasopharynx. In the beginning of pharyngeal phase tongue makes a rapid piston-like movement to propel the bolus through oropharynx to hypopharynx. The whole pharyngeal tube is elevated by stylopharyngeus and palatopharyngeus muscles. The entry of bolus to the esophagus is facilitated by the upward movement of larynx which stretches the opening of esophagus and elevation of larynx that lifts the glottis away from the food passage. Simultaneous

relaxation of the upper esophageal sphincter occurs followed by a wave of peristalsis caused by contraction of pharyngeal muscles propels the bolus into the esophagus. During this phase, there are possibilities of food entering back into oral cavity, upward into nasopharynx, forward into larynx and downward into esophagus. Due to co-ordinated movements of various structures, the entry of food to other passages is prevented. Bolus is prevented from moving back to oral cavity by the tongue which takes a position against the roof of the mouth and also by increased intraoral pressure created in the oral cavity by the movement of tongue. Entry of bolus to nasopharynx is prevented by upward movement of soft palate which becomes triangular in shape and contacts the adjacent pharyngeal wall. Several mechanisms operate to prevent aspiration of food to the larynx. a. b. c.

Larynx rises and is pulled up under the tongue. Epiglottis folds down from an upright to horizontal position over the laryngeal opening. The intrinsic muscle of the glottis approximate the vocal cords and the pyriform sinus create lateral food channels so that the bolus deviates around the laryngeal opening.

Temporary arrest of breathing occurs during the pharyngeal phase of swallowing and is referred to as deglutition apnea. The pharyngeal phase of deglutition takes around 0.7 seconds. The second phase of deglutition ends when the bolus is transferred from the pharynx into the upper part of esophagus and then the muscles of the tongue, palate, pharynx and larynx relax, the mandible is moved into rest position and respiration resumes.

Esophageal Phase During this phase, the bolus moves down the length of esophagus to the stomach. This is an involuntary stage. Esophagus helps to move food from pharynx to the stomach. The peristaltic movements (the alternative contraction and relaxation of muscle fibers of GIT) help in the movement of food in esophagus. When the bolus reaches esophagus these peristaltic waves are initiated which propel the food from pharynx to stomach.

The distal 2–5 mm is the lower esophageal sphincter. When bolus enters this part of the esophagus, the sphincter relaxes and the contents enter into the stomach. Later this sphincter contracts to prevent movement of food back to esophagus. This phase is somewhat longer, liquids take 3 seconds whereas solids take 9 seconds.

Immature Swallow/Infantile Swallow Swallowing in infancy prior to the establishment of occlusion has been termed as infantile swallow or visceral swallow. This type of swallowing is based on unconditioned reflex system in which facial and circumoral muscles initiate swallowing. Newborns and infants feed by a process called suckling. In infants, soft palate is large and more compact. During suckling, mouth acts as a piston within a cylinder. A negative pressure or suction is created in the mouth by lowering the jaw while the lips are sealed around the nipple to prevent entry of air to oral cavity. Respiration continues during the burst of suckling. As the infant grows, the soft palate becomes more mobile and orofacial structures develop. The epiglottis descends and assumes its mature functional role in swallowing. The duration of suckling becomes prolonged as the infant grows which is followed by deglutition.

Features of Infantile Swallow Alveolar ridges or teeth are apart and the tongue will be positioned between them. Mandible is stabilized by both tongue and facial muscles which are supplied by the 7th cranial nerve. Because of anatomic relationship of newborn pharynx and larynx, infants can swallow without interruption of breathing. With the eruption of teeth and emergence of canines at age of 12 years, there is transition to a teeth together swallowing which is termed as adult or somatic swallowing. Occasionally the transition from infantile swallow to adult swallow does not occur. This may be due to lack of tooth support because of poor tooth position or arch relationship. The infantile swallow also may be maintained when discomfort occurs during tooth contact because of caries or tooth sensitivity. Over retention of the infantile swallow can

result in labial displacement of anterior teeth by powerful tongue muscle, which may be presented clinically as anterior open bite.

Features of Adult Swallow Teeth are in contact and tongue does not come in between. Tongue will be positioned behind the teeth Mandible is stabilized by occluding teeth and masticatory muscles which are supplied by the 5th cranial nerve. Temporary arrest of respiration is observed during swallowing.

43 Calcium Phosphorus Metabolism

Calcium metabolism Phosphorus metabolism Hormonal control of serum calcium level Other hormones that have role in serum calcium level Functions of calcium and phosphorus Clinical considerations

C

alcium and phosphorus are considered as two essential elements required for normal growth and development. Adult human body contains approximately 1.1 kg calcium, of which 98 to 99% is in the bone and teeth. Normal serum calcium level varies from 9 to 11 mg%. 50% of serum calcium is in free or ionic form while 40% is bound to proteins and another 10% complexed with citrate phosphate or bicarbonate. The ionized calcium found free in the plasma performs its biological functions.

Requirement and Absorption Source Milk, diary products, egg, etc.

Daily Requirement

200 mg/day for infants 1000–1300 mg/day for children and adolescents 1000 mg/day for adults 1300 mg/day for pregnant and lactating females Absorption is mainly in jejunum and ileum and only about 1/3rd of the dietary intake of calcium is absorbed under normal circumstances.

Local Factors Increasing Absorption of Calcium Vitamin D Fat Citrates lower the pH of alimentary tract and produce calcium citrate which is relatively soluble. High protein diet produces soluble calcium compounds. Low pH of alimentary tract.

Local Factors Decreasing the Calcium Absorption Phytic acid present in cereals: Form insoluble calcium phytate with ingested calcium and make it nonabsorbable. Oxalic acid: Produces insoluble calcium oxalate. Hypochlorhydria or achlorhydria decreases calcium absorption because pH of intestine becomes high in the absence of hydrochloric acid.

Excretion Calcium is excreted both in urine and feces. In urine it is excreted as calcium chloride and calcium phosphate. Renal threshold is 7 mg/dl of serum calcium. Not only non absorbed calcium, even absorbed calcium is excreted through feces.

Phosphorus Normal serum phosphorus level

2–4 mg/dl in adults 3–5 mg/dl in children

Daily Requirement 240 mg for infants 800 mg for adults 1200 mg for pregnant and lactating females. Absorption of phosphorus takes place in small intestine in the form of soluble inorganic phosphate. Approximately 70% of dietary phosphate is absorbed in the form of orthophosphate. By the action of intestinal phosphatases food bound phosphorus is released during digestion. An excess of calcium, iron or aluminum may interfere with absorption. Excretion: Occurs primarily through urine in the form of phosphates of various cations. Fecal phosphorus is usually excreted as calcium phosphate.

Regulation of Serum Calcium Level/Hormonal Control of Serum Calcium Plasma calcium level is maintained with remarkable constancy despite with variation in calcium intake. Any gross decrease in plasma calcium leads to severe metabolic disturbances which may even lead to death. Calcium taken through the diet is absorbed from the intestine to the blood and distributed to various parts of the body. While passing through the kidney large amount is filtered in the glomerulus. 90% of calcium in this filtrate is reabsorbed in the renal tubules and only a small quantity of it is excreted. In the bone, calcium may be deposited or resorbed depending upon the level of calcium in the plasma. Therefore the serum calcium is maintained by regulating its absorption in intestine, reabsorption in the kidney and mobilization from the bone. All these processes are finely regulated by hormones. The level of blood calcium is maintained by action of two hormones: Parathyroid hormone and calcitonin and vitamin D (Fig. 43.1).

PARATHYROID HORMONE (PTH)

The parathyroid hormone is secreted by chief cells of parathyroid gland and its main function is to increase the serum calcium level. PTH performs this function by its direct effect on bone and kidney and also by an indirect effect on intestine through vitamin D. PTH secretion varies inversely with the level of ionized calcium in the plasma. Effect on kidney • • •

Increases the calcium reabsorption Enhances phosphate excretion Accelerates the conversion of 25-hydroxy-cholecalciferol to its active metabolic form 1, 25-dihydroxycholecalciferol which has a role in calcium absorption in the intestine.

Effect on bone • • •

Enhances osteoclastic activity in skeleton, thereby mobilizing the calcium from bone to the plasma. Increases net bone mass by its anabolic effect on bone. Decreases osteoblastic activity so new bone formation and utilization of serum calcium is decreased.

Effect on intestinal tract Enhances the absorption of calcium and phosphorus which is probably indirectly associated with the increased renal production of 1, 25dihydroxycholecalciferol (calcitriol).

VITAMIN D The most active metabolite of vitamin D is calcitriol which is formed in kidney. This calcitriol is considered as hormone like substance and is responsible for all the biological effect of vitamin D on calcium metabolism. Through the effect on intestine, kidney and bone, vitamin D increases the serum calcium level (Fig. 43.2).

Fig. 43.1: Hormonal control of serum calcium

Fig. 43.2: Schematic representation of metabolism and actions of vitamin D

Actions of Calcitriol Effect on intestinal tract Increases the intestinal calcium uptake: Calcitriol increases the transcription of calcium binding protein. This calcium binding protein acts as a carrier protein which facilitates the intestinal absorption of calcium and its transport. Other mechanisms by which calcitriol increases intestinal absorption of calcium may be by formation of calcium stimulated ATPase in the lining cells of intestine and by formation of alkaline phosphatase. Effect on bone Vitamin D enhances osteoclastic activity and increases mobilization of calcium from bone. Thus, increases serum calcium level. Effect on kidney Increases the reabsorption of calcium in kidney and decreases phosphate reabsorption.

CALCITONIN Calcitonin is a hormone produced by ‘C’ cells or parafollicular cells of thyroid gland. This hormone counteracts the actions of PTH and helps to decrease the serum calcium level. Although the main effect of calcitonin is on bone, it also acts on intestine and kidney to some extent. The level of this hypocalcemic hormone depends on plasma calcium level.

Actions of Calcitonin Effect on bone • • •

Acts directly on the osteoclasts causing an immediate inhibition of bone resorbing activity. Decreases development of new osteoclasts. Facilitates bone formation.

Effect on kidney Acts on kidney and increases excretion of calcium and decreases the

reabsorption. Effect on intestine Prevents absorption of calcium.

Other Hormones Playing Role in Serum Calcium Prolactin: Prolactin stimulate 1, 2-hydroxylase activity; thus increasing production of calcitriol, which in turn, increases the calcium absorption in lactating period. Growth hormone: Also have shown to have some influence on production of calcitriol. Sex hormones: Influence calcium and phosphate metabolism by increasing calcium absorption, decreasing calcium excretion and promoting mineralization of bone. Estrogen has a direct effect in reducing bone resorption. Thyroid hormone: Hyperthyroidism is accompanied by osteoporosis and increased excretion of calcium in urine. Fecal calcium is also increased indicating that this hormone decreases the absorption of calcium. This could be due to increased peristalsis, moving the gut contents too quickly preventing normal absorption.

Functions of Calcium Hemostasis: Calcium is necessary for activation of clotting factors in plasma. Calcium plays important role in formation of bone and teeth and in its maintenance. Calcium is essential for neurotransmitter release and helps in neuromuscular excitability. Calcium is bound to cell surface and has role in stabilization of cell membrane, normal membrane permeability and adhesion between cells. Calcium is essential for all secretory processes such as release of hormone by endocrine cells and release of secretory products of exocrine glands. Calcium is essential for activation of certain enzymes involved in inflammation and also acts as secondary messenger of hormonal action.

Functions of Phosphorus Phosphorus is important for formation of bone and teeth. Phosphates by their function in phosphorylation, is important in the metabolism of fat and carbohydrate. Phosphorus is used in building the more permanent organic phosphates including some catalyst essential for the structure and functions of cells. Phosphates are utilized for the formation of phosphoproteins, nerve phosphatides and nucleoproteins of cells. They provide energy rich bonds in such compounds as adenosine triphosphate and is important in muscle contraction. Phosphate form part of coenzymes as pyridoxal phosphate which is necessary for decarboxylation and transamination of certain amino acids such as tyrosine, tryptophan and arginine.

Clinical Considerations A low concentration of serum calcium which is less than 8 mg% produces hyperirritability of nerves and neuromuscular junction, leading to contraction of muscle spontaneously. This condition is called tetany which is characterized by carpopedal spasm and convulsions.

44 Mineralization

Introduction Booster theory Seeding theory Matrix vesicle theory

M

ineralization is the process of deposition of minerals in the organic matrix, which is capable of accepting the minerals. The process of mineralization is an important step in formation of hard tissue of the

body. The synthetic cells are responsible for deposition of calcifiable organic matrix with alkaline phosphatase enzyme activity. The mineral component of all hard tissues of the body are chiefly calcium hydroxyapatite crystals which is represented as Ca10 (PO4)6 (OH)2. The biologic apatite crystal has the shape of stubby rhombic prism which varies in size. The crystallites of mesenchymal hard tissues are approximately 100 × 200 × 50 × 50 A dimensions, whereas hydroxyapatite of enamel forms a considerably larger crystal which is 1,400 A long and 800 A wide. Although, tissue fluid contains calcium, phosphate and other minerals, spontaneous crystallization do not take place. This is probably because of presence of substances inhibiting crystal formation, requirement of energy for mineralization and formation of unstable insufficient amount of crystal, which is unable to cause mineralization. In this situation, mineralization can occur under following circumstances: If there is local increase in concentration of minerals which allows formation of sufficient ionic crystallites required for mineralization. Such process that

leads to mineralization is called homogenous nucleation. In presence of nucleating substances, mineralization process can be initiated even in the absence of increase in ionic concentration. This is called Heterogenous mineralization. These nucleating substances act as a template for crystal formation and therefore decrease the energy requirement for mineralization. The above mechanism will be effective if there are means to remove the inhibitors of mineralization. Once crystal formation is initiated, mineralization progresses rapidly, utilizing the calcium and phosphate ions from the tissue fluid even at a low concentration of ionic content. Based on above observations, theories have been put forward to explain the process of mineralization.

1. Booster Theory or Robinson’s Alkaline Phosphatase Theory This theory is put forward by Robinson in 1923 based on some of his experimental evidences. He proposed that, alkaline phosphatase enzymes present in the organic matrix of calcifying matrix can hydrolyze organic phosphates such as pyrophosphate or glucose 1,6-phosphate, etc. present in plasma and calcifying tissue fluid, and release inorganic orthophosphate resulting in local increase in phosphate ion concentration. This local increase in ionic component has a boosting effect which would increase the proportion of phosphate ions sufficient to cause spontaneous precipitation. The phosphate ions combine with the calcium ions available in tissue fluid to form hydroxyapatite crystals. According to him initially unstable amorphous calcium phosphate is formed which is then converted into stable calcium hydroxyapatite. He has evolved his theory based on his experiments on alkaline phosphatase. Robinson has observed in his studies that calcifying cartilage contains more alkaline phosphatase than noncalcifying cartilage. When slices of cartilage removed from bone of rachitic animals (affected by richets) were incubated with calcium and organic phosphates, hydroxyapatite crystals were formed. From this experiment Robinson came to a conclusion

that rachitic bone contains alkaline phosphatase which is capable of splitting organic phosphate to release inorganic phosphates. This phosphate combines with calcium to produce apatite crystals. Robinson’s theory of mineralization is not widely accepted and is criticized for various reasons. Robinson’s studies were on rachitic bone which is an abnormal tissue. Whether the result obtained in these studies can be applied to normal bone is doubtful. Alkaline phosphatase is observed in other tissues whith do not calcify. Inhibitors of certain other enzymes which do not inhibit alkaline phosphatase activity are found to be preventing mineralization. Experimental studies have shown that presence of inorganic phosphate and calcium is not sufficient to induce mineralization. Rather this also requires action of some other enzymes. The organic phosphate presents in tissue fluid of calcifying matrix is insufficient to produce sufficient inorganic phosphate ions to induce mineralization (Fig. 44.1). Although this theory is been criticized by various authors, the role of alkaline phosphatase in mineralization can’t be excluded. Alkaline phosphatase is a group of enzymes that can cleave phosphate ions from organic phosphates at an alkaline pH. This enzyme is found in cell membrane of hard tissue forming cells and in organic matrix of calcifying tissue. In addition to providing phosphate ions, alkaline phosphatase may also be involved in ion transport. Neumann has proposed that alkaline phosphatase may be playing important role in mineralization by hydrolyzing pyrophosphate which is a known crystal poison which prevents mineralization, therefore helping in crystal growth (Fig. 44.1).

Fig. 44.1: Alkaline phosphatase and mineralization

The possible role of alkaline phosphatase in mineralization can be: Hydrolyzing organic phosphates to provide inorganic phosphate ions required for mineralization. Ion transport May help in removing crystal poisons.

2. Collagen Seeding Theory/Nucleation Theory/Collagen Template Theory Some nucleating substances which have spatial arrangement as that of hydroxyapatite crystals, can act as a mould on template upon which crystals can be laid down. Nucleating substance can initiate mineralization even when the ionic, concentration is less and also reduce energy required for mineralization. Collagen is the most important seed playing a significant role in mineralization. It is suggested that certain amino acid residues in collagen with charged side chains, provide a specific, spatial arrangement that constitute a template matching for hydroxyapatite. Calcium and phosphate ions present in the extracellular fluid binds to these sites to form hydroxyapatite crystals which grow further by addition of ions. It has been observed that lysine and hydroxylysine groups are specific ion binding sites for phosphate ions, while carboxyl sites associated with aspartic acid and glutamic acid residues act as calcium binding sites to initiate nucleation of hydroxyapatite crystals. The role of collagen in mineralization was suggested based on some experimental evidences. When tendon collagen was added to a solution containing calcium and phosphate, crystal formation was found even when

the concentration of ions were lower than what is required for spontaneous mineralization and it was suggested that this had happened due to seeding capacity of collagen. Only the collagen with 64 nm periodic banding with three dimensional organization of collagen macromolecule has the capability of functioning as a seed. The gaps between the collagen molecules are filled with proteoglycans which bind to calcium. Calcium is released by enzymatic degradation of proteoglycans. After the removal of proteoglycans, phosphoproteins are attached to the collagen which is broken down by alkaline phosphatase to give rise to phosphate ion. These ions combine to form apatite crystals in the gap zone of collagen. Support for this template theory can be achieved from electron microscopic observation of parallel arrangement of hydroxyapatite crystals and collagen fibers. This theory is unable to explain mineralization in all tissues. For example, enamel is a highly mineralized tissue, but does not contain collagen. Mineralization of cartilage begins in ground substance and not in association with collagen. Therefore possibility of other mechanism should be considered. Similarly, another important question to be answered is why collagen does not initiate mineralization in all connective tissue. Possible explanation for this include: Collagen in connective tissue that does not calcify, may have spatial arrangement of charges that is different from the collagen in calcifiable tissue therefore unable to act as a suitable template. In collagen of soft tissues, the charged site could be protected by some ground substance components which prevent the attachment of the ions to initiate mineralization. These substances are called crystal poison. In calcifiable tissue these substances may be removed by certain mechanism leading to exposure of these charged sites, followed by binding of ions to initiate mineralization. Pyrophosphate is a known crystal poison which is hydrolyzed by alkaline phosphatase enzyme to expose the binding sites. Collagen exhibits intrafibrillar pores through which the calcium and phosphate ions should pass through to reach the nucleating sites located inside the fibrils. The gap between tropocollagen molecules in calcifiable tissues is 0.6 nm which is large enough to allow the passage of phosphate ions which are of 0.4 nm diameter. The gap in soft tissue collagen is only 0.3

nm through which the phosphate ions cannot pass, therefore cannot act as a template for hydroxyapatite crystals.

Other Nucleating Materials Lipids: Lipids have been identified as an important factor associated with mineralization process. Although the exact role of lipids in mineralization is not identified, experimental evidences suggest that phospholipids can act as a seed or a template for hydroxyapatite crystal formation. Phospholipids are also capable of stabilizing amorphous calcium phosphate which will later be transformed into hydroxyapatite crystals. Phospholipids are also found in matrix vesicle, which can participate in mineralization. Protein polysaccharides: It has been suggested by some investigators that protein polysaccharides act as a seed for mineralization. Experimental evidences show that proteoglycans and glycosaminoglycans have the capability of binding to calcium ions. Probably these protein polysaccharides regulate the rate of mineralization rather than initiating mineralization.

Matrix Vesicle Theory Matrix vesicles are membrane bound vesicles isolated from areas of calcification. These structures bud off from the synthetic cells and are released into the organic matrix. It has been observed that the matrix vesicles induces precipitation of hydroxyapatite crystals in vitro from solutions containing calcium and phosphate ions and also are capable of crystal formation even when the solubility of product of calcium and phosphate are as low as 2 millimoles2. The above factors suggest that the matrix vesicles have a capacity to initiate mineralization.

Two Types of Matrix Vesicles have been Identified Type I matrix vesicles are round or ovoid in shape resembling lysosomes. They contain enzymes such as acid phosphatase and aryl phosphatase. These enzymes can break down proteoglycans and glycosaminoglycans which are inhibitors of mineralization. Type II matrix vesicles are irregular membrane bound structures having enzymes such as ATPase, alkaline phosphatase, pyrophosphatase, proteoglycans and metalloproteinases but relatively less acid phosphatase.

These vesicles are also rich in phospholipids with great affinity for calcium and a large amount of annexin V.

Role of Matrix Vesicle in Mineralization Matrix vesicles provide a local environment for initial crystal formation. They have all the characteristics needed for the induction of calcification. Freshly isolated vesicles contain a relatively high Ca2+ content bound to phospholipids which act as a nucleating site within the vesicle. By being extremely rich in alkaline phosphatase activity, the vesicles have the capacity to hydrolyze a variety of organic phosphate substrates, to increase substantially the local availability of free phosphate ions which binds to calcium ions to initiate apatite crystallization. Such enzymatic activity may also remove putative inhibitors of mineralization, including pyrophosphate. In addition the vesicles also contain a large amount of annexin V, a membrane-associated protein which mediates the influx of Ca2+ into matrix vesicles, enabling intraluminal crystal growth. In addition, annexin V binds directly to type II and X collagen which may be important for anchoring the vesicles to the fibrous components of the matrix. Thus, the first crystal is formed in the matrix vesicle. Crystal growth continues in the vesicle by further addition of ions which is followed by rupture of vesicle membrane. The crystals are released into the organic matrix, where they grow by using ions in the tissue fluid and mineralization spreads to surrounding matrix. The mineralization progresses in the form of spherical or calcospheric masses which fuses with each other forming uniformly mineralized matrix. In summary, all the three mechanisms are involved in mineralization. Collagen acts as a seed and helps in intrafibrillar calcification. Similarly matrix vesicles help in extrafibrillar calcification. Alkaline phosphatase helps in providing more phosphate ions and also removing crystal poisons.

45 Hormonal Influence on Orofacial Structures

Introduction Effect of thyroid hormone Effect of parathyroid hormone Effect of pituitary hormones Effect of sex hormones Effect of adrenal hormones Effect of pancreatic hormone

T

he endocrine system consists of several glands which secrete hormones. Hormones are biologically active substances produced by these glands, directly released into the blood-stream in which it circulate continuously and exert their biological effect on different systems of our body. Hormones have a vital role in growth and development of orofacial structures and their functional activities and therefore altered levels of these can cause variety of manifestations in orofacial structures.

THYROID HORMONE Thyroid hormone is secreted by thyroid glands situated in the lower part of anterior region of neck. The hormone secreted by thyroid gland, i.e. thyroxin plays an essential role in regulation of metabolic activities of the body and

also in physical and intellectual development. Calcitonin produced by ‘c’ cells of thyroid gland is also important in maintaining serum calcium level. Abnormal functioning of the thyroid gland may cause hyperthyroidism or hypothyroidism, which can adversely affect growth by accelerating or retarding the growth. Hyperthyroidism causes increased metabolic activity. Affected persons are abnormally energetic. This condition, if occurs in early stages of life can lead to formation of large teeth, accelerated eruption of deciduous and permanent teeth and premature loss of deciduous teeth. Experimental studies have shown that excessive thyroid hormone can have a toxic effect on odontoblasts resulting in disturbed dentin formation. In advanced cases alveolar atrophy occurs. Hyperthyroidism in adults does not show any orofacial manifestations, but they may have increased sensitivity to epinephrine and due to hyperthyroidism they become poor dental patients. Hypothyroidism: This condition results from decreased functioning of thyroid gland. Congenital hypothyroidism or cretinism affects the mental and physical developments of a child depending upon severity of deficiency. Affected children have a characteristic facial appearance with depressed nasal bridge and flared nose. Face is wide and fails to develop in a longitudinal direction. The mandible is underdeveloped while maxilla is overdeveloped. Tongue is enlarged due to edema fluid and it protrudes out. Enlarged tongue exerts pressure on the teeth leading to malocclusion. In hypothyroidism, generalized retardation of skeletal growth takes place which also affect the jaw bones. Poor development of the jaw bones leads to anterior open bite and receded chin. In addition to skeletal development, teeth development is affected leading to decreased size of the tooth, delayed eruption, delayed exfoliation of deciduous teeth, etc. Hypothyroidism in children and adults leads to myxedema, a condition characterized by subcutaneous edema. Clinical and orofacial findings of myxedema are limited to soft tissues of face and mouth. Tongue is large and edematous, interfering with speech and mastication. Lips, nose, eyelids and suborbital tissue also show edema.

PARATHYROID HORMONE (PTH)

Parathyroid hormone is secreted by parathyroid gland situated on the posterior aspect of thyroid gland. This hormone has a significant role in maintaining the serum calcium level and therefore plays a vital role in orofacial development. Hypoparathyroidism: Dental changes can be observed in teeth which have formed during the time of PTH deficiency such as defective matrix deposition and mineralization of enamel and dentin. Delayed eruption also has been reported in individuals suffering from hypoparathyroidism which could be due to inhibitory effect on osteoclasts. A tooth cannot erupt without osteoclastic bone resorption, Exaggerated incremental lines and areas of interglobular dentin also have been reported in teeth of hypoparathyroid patients. Hyperparathyroidism: Parathyroid hormone can mobilize the calcium from bone causing bone resorption. This effect is applicable only on bone and not in fully formed teeth. Therefore in hyperparathyroidism, no visible changes occur in dental tissue. But the alveolar bone undergoes resorption. Loss of lamina dura is a very important observation in this condition. Alveolar bone becomes osteoporotic and soft leading to drifting of teeth and malocclusion. The jaw bones also may show areas of bone resorption which may be evident in the radiographs as large areas of radiolucencies. The areas of bone resorption will be filled with highly vascular connective tissue. These lesions are termed as brown tumor.

PITUITARY HORMONES Pituitary gland is the master endocrine gland, secretions of which control the functioning of many other glands and many body functions. Anterior pituitary produces at least six hormones: The somatotropic, the thyrotropic, the adrenocorticotropic and the lactogenic hormone. The posterior pituitary produces antidiuretic hormone. Decreased activity of this hormone results in excessive production of urine and general dehydration of the body. The main effect of pituitary hormones on teeth and orofacial structures are mainly through the effect of growth hormone and partly by thyroid stimulating hormone. Hypopituitarism can occur due to congenital defects or due to destructive

diseases. If it occurs before puberty it leads to a condition called dwarfism. In pituitary dwarf the eruption of teeth is delayed and the shedding time of deciduous teeth is also delayed, as is the growth of body in general. The size of the crown and root of the tooth is smaller than normal. The supporting structures of the teeth also show retardation of development. Because of incomplete eruption, clinical crown of the teeth may be smaller. Decreased growth hormone also causes retardation of development of maxilla and mandible. The dental arch is smaller than normal, therefore results in crowding of teeth and malocclusion. Pituitary dwarfs are reported to have a decrease in caries rate. Hypopituitarism in adults does not show any specific dental changes. Hyperpituitarism that occurs before the closure of epiphysis of long bones leads to a condition called gigantism and if it occurs later in life after epiphyseal closure, leads to acromegaly. Gigantism is characterized by symmetric overgrowth of the body. As a part of general overgrowth of bones, both mandible and maxilla are larger than normal. The teeth, both crown and root are larger and is in proportion to the size of the jaws. The eruption of both deciduous and permanent teeth is accelerated with premature shedding of deciduous teeth. In acromegaly, mandible continues to grow leading to abnormally long face and mandibular prognathism. Supra-eruption of teeth may occur leading to overgrowth of alveolar bone. Increase in length of mandibular arch may lead to malocclusion. The lips become thick and the tongue enlarged with indentations on the sides of the tongue. The enlarged tongue exerts pressure on the teeth leading to buccal or labial displacement of teeth and malocclusion.

SEX HORMONES The effect of sex hormones on oral tissues is not well understood. Experimental studies have confirmed that female sex hormones influence the growth of oral epithelium and its keratinization. They also cause dilatation of blood vessels in underlying connective tissue and increase their permeability. Tendency for gingivitis has been observed in females during puberty, menstruation and pregnancy. Hormonal alterations do not initiate the inflammation but exaggerate the existing inflammation by altering the

reaction of gingival tissue to inflammation.

ADRENAL HORMONES Various hormones have been secreted by adrenal cortex and medulla. The main secretions from adrenal medulla are epinephrine (adrenaline) and norepinephrine. Adrenaline is very essential for a quick physiological response to crisis situations. Adrenal cortex is concerned with liberation of steroids which involve in carbohydrate, mineral, fat and protein metabolisms and fluid electrolyte balance. Hydrocortisone also has a marked antiinflammatory effect. Chronic insufficiency of adrenal cortex leads to a condition called Addison’s disease which is characterized by pigmentation of oral mucous membrane involving buccal mucosa, tongue, gingiva and lip. Hyperfunctioning of adrenal cortex causes Cushing’s syndrome. The changes in orofacial region could be related to osteoporosis. Cortisone causes osteoporosis by suppressing the activity of osteoblasts resulting in defective matrix deposition.

PANCREATIC HORMONE—INSULIN Insulin is mainly concerned with carbohydrate metabolism and deficiency leads to diabetes mellitus. Diabetic patients have less resistance to infections, therefore these patients may show increased tendency to develop gingivitis and periodontitis. They also have delayed woundhealing and may complain of dryness of mouth.

46 Age Changes of Oral Tissues

Age changes in dental tissues Age changes in tooth supporting tissues Age changes in oral mucosa Age changes in salivary glands

T

he term age changes refer to all the changes that occur in the body from birth to death. However, it is usual to consider age changes as those which are evident in later life. Effects of aging in relation to the oral tissues can be discussed in the following headings: Changes in dental tissues Enamel Dentin Cementum Dental pulp Changes in supporting structures of teeth Periodontal ligament Alveolar bone Changes in oral mucosa Changes in salivary glands

ENAMEL Enamel is the hardest calcified tissue in the human body which forms the resistant covering of the teeth, rendering them suitable for mastication. Enamel being a nonliving tissue it is incapable of repair. But its surface can however be modified at a crystal level by ion exchange or grossly by attrition, abrasion, erosion or by dental caries. Age changes observed in enamel are Attrition: Attrition is the physiological wearing away of the teeth resulting from masticatory movements of teeth and friction from food particles. It is the most conspicuous change in the teeth with advancing age and can be appreciated on both occlusal and proximal surfaces. The amount of wear differs a great deal, due to variations in the type of occlusion present, habit, and muscular power, type of food and tooth loss. Attrition causes loss of vertical dimension of the crown, loss of enamel from the occluding surfaces of the teeth to produce polished attrition facets and flattening of proximal contour. Modification in surface layer: The enamel of newly erupted teeth are covered with pronounced rod ends and perikymata. With increasing age, these surface structures disappear. The rates at which they are lost depend on the location of the surface of the tooth and on the location of tooth in the mouth. Facial and lingual surfaces lose their structure more rapidly than proximal surfaces. Anterior teeth lose their structure more rapidly than posteriors. Increase of inorganic content: Due to exchange of ions with the oral environment during aging, superficial enamel surface of older teeth have increased inorganic content. The thickness of hypermineralized surface zone increases in older teeth and exhibits more resistance to decay. A steady increase in nitrogen and fluoride level in enamel with age has also been reported. Decrease permeability: Permeability of enamel decreases with age, possibly as a result of surface consolidation of crystals, formation of fluoroapatite and a reduction of matrix between individual crystals. Decrease in water content: The crystals in enamel acquire more ions and the

pores between them decreases. As the major portion of water in enamel lie in these pores, reduction in the pores in older enamel, results in decrease in their water content. Change in color of teeth: Color of the teeth becomes darker with age due to deepening of the color of the dentin. It is also possible that enamel itself either becomes darker with age or more translucent contributing to change in color of tooth.

DENTIN The dentin provides the bulk and general form of the tooth. Unlike enamel, dentin is deposited throughout life and is a vital tissue that can react and respond to various stimuli to which it is exposed. Age changes of dentin can be Changes in physical properties: Color of dentin becomes darker with age. Density and mineralization and hardness of dentin of both crown and root increases with age. Vitality of dentin: The vitality of dentin is decreased in advancing age probably due to decrease in the odontoblastic activity. Thickness of dentin: Although at a slower rate, dentin is laid down throughout life, resulting in gradual increase in thickness as age advances. Dentin tend to be deposited in greater amounts in certain areas of pulp such as in the floor of the pulp chamber. Secondary dentin deposition: This is the type of dentin formed after root completion and in the absence of obvious trauma to the tooth, such as attrition, abrasion, erosion, etc. Deposition of secondary dentin is a normal aging process that continues throughout life. Dead tracts: Dead tracts are empty dentinal tubules that are formed due to degeneration of odontoblast processes in the dentinal tubules. This usually occurs due to exposure of dentin following attrition, abrasion or erosion. These empty dentinal tubules are filled with air; thereby appear dark under transmitted light. Dead tracts may also develop in unerupted teeth and in teeth with a little or no visible damage, especially in the region of cusp or

incisal edges due to death of odontoblasts occurring as a result of overcrowding. Therefore dead tract can also be considered as an age change. Sclerotic or transparent dentin: Mild stimuli induce protective changes in the existing dentin. Continued deposition of intratubular dentin occurs in the tubules and this leads to gradual reduction in tubule diameter or even complete closure of tubules. The refractive indices of dentin in which the tubules are occluded are equalized and therefore such areas appear translucent or transparent in the transmitted light and dark in reflected light.  Sclerotic dentin is frequently found near the root apex in the teeth of elderly people as an age change. The sclerotic dentin is more brittle and less permeable. Reparative dentin: This is the type of response seen due to severe irritation caused by extensive abrasion, erosion, caries or operative procedures. Majority of the odontoblasts in this affected area degenerates, but a few may survive and continue to form dentin at a rapid rate to seal off the exposed tubules from the pulp. This dentin produced by survived odontoblasts is called reparative dentin. Dead odontoblasts are replaced by new odontoblasts differentiated from undifferentiated mesenchymal cells present in the pulp. The dentin produced by these new odontoblasts is called reactionary dentin.

CEMENTUM Cementum is the vital calcified tissue covering the periphery of root. Age changes of cementum can be Thickness of cementum: Cementum deposition is a continuous process that occurs throughout life. Cementum is deposited intermittently and its deposition in later life is mainly in response to stresses to which the tooth is subjected. It is said that, there is a triple increase in the thickness of cementum between 11 and 76 years of age. There is certainly a correlation between the thickness of cementum and age. Relatively thick layers of cementum are found on the roots of unerupted teeth in aged persons. Surface irregularity: Smooth surface becomes irregular due to calcification of periodontal ligament fiber bundles where they are attached to cementum.

Local injuries and mechanical stress cause resorptive changes which may be also responsible for surface irregularity. Reduced permeability: It becomes less permeable to dye molecules and ions. As the permeability reduces the nutritive molecules may not reach the deeper layers of cementum thus these deeper layers have less cementocytes in them. The fluoride content of cementum increases with age particularly in the acellular cementum of the cervical region, probably because this tends to be exposed to the oral environment. Structural changes: Resorption of root may occur with aging which will be repaired by cementum. Cementum also may show alternate periods of resorption and deposition creating reversal lines.

PULP Pulp is the soft tissue component of the tooth situated in the pulp cavity. Age changes in the dental pulp are Size and morphology: With age a progressive reduction in pulp size occurs due to secondary dentin deposition. The pulp horn becomes less prominent or may disappear. Similarly, the radicular dentin becomes narrow or even obliterated. Cellular and fibrous components: The pulp in older teeth becomes more fibrous with appreciable amount of mature collagen with proportionate reduction in the cellular components and ground substances. The collagen fibers of aged pulp is more resistant to enzymatic degradation. The number of cells in the pulp including fibroblasts and odontoblasts decreases with age. The odontoblast layer may show intercellular edema and vacuolation in sections of pulp, which could be even because of poor fixation. Changes in blood supply and innervations: Loss and degeneration of myelinated and unmyelinated axons occur which can be correlated with an age related reduction in sensitivity. As these progresses, the number of nerves gets greatly diminished. There is a decrease in the blood supply as the apical foramen is almost obliterated by secondary dentin and cementum which initiates most of the other changes in the pulp. Blood vessels decrease in

number may also show decrease in size of lumen, thickening of vessel walls with fibrosis and calcifications. Arteriosclerotic changes begin to develop from the age of 40 years. Reduction in sensitivity and healing potential: As age advances the sensitivity and healing or reparative capacity of pulp decreases. Decreased sensitivity can be directly related to nerve degeneration. Overall reduction in vascular supply and cellular component could be responsible for decreased reparative capacity of pulp. Pulpal calcifications: Calcification may occur in pulp tissue as a result of aging or external stimuli. These may be nodular, calcified masses referred to as pulp stones or diffuse calcifications. They are seen in functional as well as embedded, unerupted teeth. Although pulp calcifications are seen in young individuals, the incidence increases with age: 66% between the age group of 10 to 30 years, 80% between 30 to 50 years and 90% above 50 years.

Pulp Stones are Classified Based on its Relation to Adjacent Dentin into Three Groups Free pulp stones are those calcified structures lying free in the pulp without being attached to the dentin. Attached pulp stones: Those which are attached to the dentin. Embedded pulp stones: When pulp stone is completely surrounded by dentin it is called embedded pulp stone. They are believed to be formed as free pulp stones which later becomes attached or embedded due to progressive dentin formation.

Depending On Structure Pulp Stones can be Grouped into True denticles: True denticles are localized masses of calcified tissue having tubular structure containing odontoblast processes and thereby resembling dentin. They are very small and are seen only rarely. The true denticles are thought to be formed due to entrapped remnants of root sheath in pulp. These cells may induce the differentiation of odontoblasts which form calcified structures resembling dentin.

False denticles: False denticles are localized masses of calcified tissue having a laminated structure made of concentric layers of calcium deposited around a central nidus, which could be dead cells. They do not have a tubular structure or structural resemblance to dentin. They are larger than the true denticles and may fill the entire pulp chamber.

Diffuse Calcification Diffuse calcification is composed of small calcified particles with a few larger masses. The calcified structures are arranged as linear strands parallel to the long axis of pulp. They are found to be closely associated with blood vessels with an orientation parallel to the vessels and nerves. It is usually seen only in radicular pulp.

PERIODONTAL LIGAMENT Periodontal ligament is a soft tissue component that helps in the attachment of tooth to the bone. The age changes seen are: Width: The width of periodontal ligament is narrower in older individuals, due to continuum deposition of cementum and bone on either side of ligament. Vascularity: As age advances there is a decrease in vascularity of periodontal ligament. Cellular and fibrous components: The number of fibroblasts, collagen fibers and mucopolysaccharides content decreases with age, while the elastic fibers increases. Mitotic activity of cells of periodontium also decreases.

ALVEOLAR BONE It is that part of the maxilla and mandible that forms and supports the socket of the teeth. As age advances alveolar bone facing periodontal ligament becomes irregular. Bone also shows osteoporotic changes and decreased metabolic rate, vascularity, healing capacity, etc. Cancellous bone becomes dense with coarse trabecular pattern. Since the existence of alveolar bone

greatly depends on teeth, when the teeth are lost, it undergoes gradual atrophy.

ORAL MUCOSA It is defined as a moist lining of the oral cavity and shows various age changes such as: Clinically, the oral mucosa of an elderly person relatively has a smooth and dry surface than that of a youngster and may be described as atrophic or friable. Permeability of mucosa to water is reduced in older individuals. Histologically, the epithelium appears thinner and more or less regular epithelium-connective tissue interface resulting from the flattening or shortening of epithelial ridges. In the lamina propria, there is decreased cellularity with increased amount of collagen, which is reported to become more highly cross linked. The number of blood vessels decreases resulting in reduced blood flow to the oral tissues and decreased rates of metabolic activity. This leads to thinning of the mucosal layer and thus the oral mucosa is more susceptible to damage and infections as age advances. Gingiva may show a decrease in degree of keratinization. Sebaceous glands (Fordyce’s spots) of the lips and cheeks also increase with age. A striking and relatively common feature in elderly persons is nodular varicose veins on the undersurface of the tongue. The number of taste buds decreases as much as 60% in old age resulting in decrease or loss of taste perception. Threshold for salt and bitter tastes increases with age. The dorsum of the tongue may show a reduction in the number of filiform papillae. The reduced number of filiform papillae may make the fungiform papillae more prominent.

SALIVARY GLANDS

The salivary glands show various age changes which include: Structural changes: The salivary glands become less active with age due to relative decrease of acinar tissue with increase in fibrous and adipose tissue. Replacement of parenchyma with fatty tissue is more apparent in parotid gland. Salivary glands also show a progressive accumulation of lymphocytes. Quantity and quality of saliva: Since parotid is the major source of watery saliva, with advancing age the viscosity of saliva increase with a total reduction in the salivary secretion. Oncocytes: Altered epithelial cells found in the salivary glands that can be identified by their marked granularity and acidophilia under light microscope are thought to represent an age related change. The number of oncocytes increases with age.

Section 5

Allied Topics 47. Tissue Processing 48. Microscope 49. Muscles of Orofacial Region 50. Vascular and Nerve Supply of Orofacial Region

47 Tissue Processing Dr Rajeesh Mohammed PK and Dr Girish KL Introduction Soft tissue processing Hard tissue processing – –

M

Decalcification Ground sectioning

icroscopic examination is the method used to study the histological structure of the oral tissues. To study the histology or histopathology, the tissue should be appropriately prepared for microscopic examination. The tissue specimen received in the laboratory may be soft tissue, hard tissue or a combination of both which is taken from a living or dead organism. Tissues taken from the body for diagnosis of disease processes must be processed in the histopathology laboratory to make microscopic slides that can be viewed under the microscope by pathologists. For microscopic examination, the tissue specimen must be thin enough (4–6 (i) to permit the passage of transmitted light. The aim of tissue processing is to embed the tissue in a solid medium firm enough to support the tissue and give it sufficient rigidity to enable thin sections to be cut, which can be viewed under microscope. Depending on nature of specimen, preparation of tissue for microscopic study includes: Soft tissue processing and hard tissue processing. Hard tissue sections can be made either by grinding (ground sections) or by decalcification procedure (decalcified sections).

SOFT TISSUE PROCESSING The most commonly used method of preparing soft tissue for the light microscopic study is by embedding the tissue in paraffin and cutting and mounting the section on slides and staining. The procedure for soft tissue processing can be either manual or automatic. In manual method, all the procedures of soft tissue processing have to be done manually and require constant vigil. In automatic tissue processing, the tissue specimens are automatically transferred through all the processing solutions in the automatic tissue processor in which the time for the tissue to pass from one solution to the other can be preset.

Steps in Routine (Paraffin Embedded) Tissue Processing (Fig. 47.1) Obtaining the specimen Specimens for microscopic study are obtained through either biopsy or autopsy. Biopsy is the removal of tissue from a living organism for the purpose of microscopic examination and diagnosis. If tissue is taken for the same purpose from dead organisms it is called autopsy. After removal, the specimen should be kept in sufficient volume of fixative solution at the earliest and sent to the lab for tissue processing. Regardless of the type of tissue, the specimens received in the lab should be examined for: Relevant details about the patient and the lesion, adequate size of the tissue, labeling of the specimen bottle and fixative used. In the lab, steps should be taken to ensure complete fixation, before proceeding with further steps of tissue processing. Large tissues should be cut into smaller pieces. Care should be taken to send representative areas for processing. To avoid interchanging the specimen, a piece of paper with graphite pencil labeling is to be put in the tissue capsule in which the specimens are kept while processing. Fixation Fixation is the foundation in the sequence of events in tissue processing. It is a process involving series of chemical events which results in the

stabilization of proteins and makes the tissue resistant to damage during subsequent stages of processing and visualization.

The aims of fixation are To preserve the cells and tissue constituents in life like condition as closely as possible without loss or derangement. To prevent the process of autolysis and bacterial action or putrefaction of tissues. To coagulate the proteins, thus reducing the change in shape or volume during further processing of tissue and to make the tissue more readily permeable to the subsequent application of reagents. After fixation the specimen is washed in running water. 10% formalin is the most commonly used fixative. Formalin increases the cross linking and results in the stabilization of proteins.

Other fixatives used Glutaraldehyde Osmium tetroxide Chromic acid Methyl alcohol and ethyl alcohol Mercuric chloride Picric acid The amount of fixative should be 25 times more than the size of the specimen. Depending on the size and density of the specimen, the fixation time can vary from a few hours to days. Usually 24 hrs is sufficient for small specimens. Various factors which can influence the rate of fixation are specimen size (3–4 mm ideally), pH, agitation, heat, viscosity, vacuum, ultrasonic, etc. Dehydration Dehydration is done to remove the water content from tissues to allow the penetration of paraffin wax and is achieved by passing the tissue through

ascending grades of alcohol. Ascending grades of alcohol is used to prevent sudden shrinkage of tissue as a result of rapid leaching of water from the tissue. Example 50%, ... 70%, 90% and 100%

Solutions used Methyl and ethyl alcohol Isopropyl alcohol Acetone Clearing Paraffin and alcohol are not miscible. So impregnation of tissue by paraffin is not possible unless alcohol is replaced by a fluid that is miscible with both alcohol and paraffin. This process is called clearing. Xylene is one of the solutions that is miscible with both paraffin and alcohol. The term “clearing” comes from the fact that the clearing agents often have the same refractive index as proteins in the specimen. As a result, when the tissue is completely infiltrated with the clearing agent, it becomes translucent or clear. The presence of opaque areas after clearing indicates incomplete dehydration.

Ideal requirements of a clearing solution Speedy removal of alcohol Minimum tissue damage and toxicity Cost factor

Choice of a clearing agent depends on The type of tissues to be processed The type of processing to be undertaken The processor system to be used Processing conditions like temperature, vacuum and pressure Safety factors Cost and convenience

Reagents used Xylene (most commonly used) Chloroform Toluene Benzene CNP 30; inhibisol Food oil derivatives Cedar wood oil

Xylene Xylene is a colorless, clear, oily, liquid aromatic hydrocarbon (sweetsmelling), used as a solvent and clearing agent in the preparation of tissue sections for microscopic study. Also called xylol; di-methylbenzene C6H4 (CH3)2 and has a molecular weight of 106. Xylene is obtained from coal tar and sometimes from petroleum. Xylene is insoluble in water and is soluble in alcohol. Xylene is an organic solvent which is miscible with both alcohol and paraffin and is the most commonly used clearing agent in lab. It is widely used as a solvent and thinner for paints and varnishes, often in combination with other organic compounds and as a solvent in glues and printing inks, etc. Xylene is stable under ordinary conditions of use and storage, but is highly flammable under adverse conditions and can form explosive mixtures in air. Xylene is an irritant to the eyes and mucous membranes at low concentrations, and is narcotic at high concen trations. Although the carcinogenic effect of xylene is suggested, there is no direct evidence of carcinogenicity in humans. Impregnation Impregnation is the procedure where there is saturation of tissue cavities and cells by a supporting substance, which is generally, but not always, the medium in which they are finally embedded. Impregnation procedure replaces the xylene with paraffin and is achieved by immersion in molten paraffin wax (60°C).

Factors affecting impregnation Size and type of tissue Clearing agent employed Vacuum embedding Embedding or blocking Embedding is the process by which tissues are surrounded by a medium such as agar, gelatin, or wax, which when solidifies will provide sufficient external support during section. Impregnated tissue is transferred from wax bath to a mould filled with molten wax to get a block of wax with the tissue specimen at the center with the cutting surface facing the base of the block.

Procedure Embedding is done using Leuckhart’s L-shaped pieces, ice trays, paper boats or embedding cassette. The L-shaped block or paper boat is arranged to form a cube on a clean, flat surface. The cube is then filled with molten wax and the specimen is embedded into this with the help of a warm forceps. Make sure that there are no air bubbles trapped between the tissue and the molten wax. Care should be taken while embedding, so that the tissue to be embedded has proper orientation. The wax-filled mould containing the tissue is then allowed to cool. The wax blocks are labeled for easier identification. The hardened wax block is removed from the mould and trimmed using a sharp knife.

Fig. 47.1: Steps to be followed in tissue processing

Sectioning To view the specimen under microscope, the embedded tissues are to be cut into thin sections of 3–5 μ with a microtome. The microtome is a device used to cut the tissue into thin sections of specified thickness. The preparation of sections using a microtome also can be manual or automatic. The wax block is to be fixed onto a wooden or metal block to prevent wax block from crumbling during sectioning and the metal or wooden block is clamped onto the microtome for sectioning. The cut sections are transferred and floated on a warm water bath. The temperature of the water bath is to be maintained at 10° less than the melting temperature of wax. The inside of the water bath should be preferably of black color. This helps in easy visualization of the floated specimens against a dark background. The water bath helps to remove wrinkles and spread the specimen. Floated sections are picked up on an adhesive coated glass slide. Egg albumin with additives is the commonly used section adhesive. Glass slide should be kept on a slide warmer at 58° temperature for 20 mins to ensure complete adhesion. Staining Staining is the biochemical technique of adding a class-specific dye to a substrate (DNA, proteins, lipids, carbohydrates) to qualify or quantify the presence of a specific compound.

Hematoxylin and eosin (H and E) staining is the routinely used method in histopathology lab. Other commonly used staining procedures in histopathology tab are Gram staining Papanicolaou staining (Pap stain) Periodic acid-Schiff staining (PAS stain) H and E stain/hematoxylin and eosin stain Hematoxylin and eosin stain is the most popular staining method in histology and is the most widely used stain in medical diagnosis. The staining method involves application of the basic dye hematoxylin, which colors basophilic structures with blue-purple hue, and alcohol-based acidic eosin-Y, which colors eosinophilic structures bright pink. The basophilic structures are usually the ones containing nucleic acids, such as the ribosomes and the chromatin-rich cell nucleus, and the cytoplasmic regions rich in RNA. The eosinophilic structures are generally composed of intracellular or extracellular protein. Most of the cytoplasm is eosinophilic. Red blood cells are stained intensely red. Hematoxylin is a natural dye which is extracted from the logwood of the tree, Haematoxylon campechianum. Oxidation of this extract produces a colored substance hematein, which itself is a poor dye. This dye when used in conjunction with a mordant becomes a powerful dye. The color of dye is red which turns into blue when the tissue section is treated with weak alkali (blueing) following hematoxylin staining. Eosin is the second component of the H and E and is the counter stain. Eosin is a red dye formed by the action of bromine on fluorescein and is both water and ethanol soluble. Eosin-Y is the commonly used form of eosin. Eosin is used to stain cytoplasm, collagen and muscle fibers. Hematoxylin and eosin staining procedure can be carried out manually or using automated equipment. Methods of hematoxylin and eosin staining Remove wax with xylene. Rehydrate the tissues using descending grades of alcohol Wash sections in water Stain with hematoxylin

Differentiate in acid alcohol Wash in water Blueing by using tap water or Scott’s tap water substitute Rinse in water Stain with eosin Wash in running water Dehydrate using ascending grades of alcohol Removal of alcohol and clearing in xylene Mounting The stained section on the slide must be covered with a thin glass coverslip to protect the tissue from being scratched, to provide better optical quality for viewing under the microscope, and to preserve the tissue section for years to come. The mounting medium is used to adhere the coverslip to the slide. There are two types of mounting media: Water based mounting media and resinous mounting media. Distrene dibutyl phthalate xylene (DPX) and Canada balsam are the commonly used mounting media which are resinous mounting media.

Procedure Apply drops of mounting medium upon tissue section. Hold the coverslip at an angle of 45°. Allow the edge of the coverslip to contact the drop so that the drop spreads along the edge of the coverslip. Let go off the coverslip and allow the medium to spread slowly. Allow it to dry and the section is ready for viewing under microscope. Although paraffin embedded tissue processing is the one carried out routinely in a histopathology laboratory, another method termed as frozen section/cryosection procedure is performed, when rapid microscopic analysis of a specimen is required. In this case tissue to be examined is placed on a metal tissue disc which is then secured in a chuck and frozen rapidly to about –20 to –30°C. The entire process is done in a cryostat machine, which is a microtome inside a freezer, which is then used to cut thin sections. The

sections are taken on to a glass slide and stained with H and E stain. The preparation of the sample is much more rapid; however, the technical quality of the sections is much lower than formalin fixed paraffin embedded tissue processing. It is used most often in oncological surgery to ensure that the entire tumour and its surrounding borders are removed.

HARD TISSUE PROCESSING To study the structure of hard tissues of the body, two procedures can be adopted: Ground sections and decalcified sections.

Decalcified Sections Decalcification is the process by which calcium in the mineralized tissue is removed, so that the tissue becomes soft enough to make thin sections. The structure of all hard tissues of the body except enamel can be studied in decalcified sections. Enamel cannot be studied by this procedure because it is highly mineralized (96%) and is lost during decalcification. Decalcification is usually carried out between the fixation and processing steps. A variety of agents or techniques have been developed to decalcify tissues, each with advantages and disadvantages. Immersions in solutions containing mineral acids, organic acids, or EDTA are the commonly used methods. Electrolysis has also been tried. Mineral acids such as nitric acid and hydrochloric acids are used to decalcify dense cortical bone and teeth because they will remove large quantities of calcium at a rapid rate. Frequently used acid for decalcification is 5% nitric acid. Nitric acid may cause yellowing of the tissue, that may interfere with further staining procedure. To avoid this 0.1% urea is added to nitric acid. 10 to 15% formic acid is one of the best decalcifying agents. The use of EDTA is limited by the fact that it penetrates tissue poorly and works slowly. Electrolysis is slow and is not suited for routine daily use.

Procedure Hard tissue to be decalcified should be fixed in 10% formalin or formal saline. To reduce the time for decalcification, tissue can be cut into smaller pieces. Then, place the tissue in a container with decalcifying solution. The

solution should be changed daily for few days and then the specimen should be tested for completion of decalcification.

Methods to Check the Completion of Decalcification Checking the consistency of the tissue: Completely decalcified tissue will be soft without any hardness being felt. (Experienced hand can tell by the feel of the tissue.) Pressing the tissue with a needle: If it enters the tissue without resistance, the tissue is completely decalcified. This is not recommended because it may cause damage to tissue. Judicious bending or trimming of the tissue: This can be done to ensure completion of decalcification. Taking radiograph of the specimen: In the radiograph, if radiopaque specks are found, tissue is not completely decalcified. Chemical test: The basis of this test is to identify calcium in the decalcifying solution in which the specimen was kept. Sodium hydroxide or strong ammonia is added to 5 ml of decalcifying fluid, to neutralize the solution. Then 5 ml of saturated ammonium oxalate solution is added. After this, turbidity is checked. Absence of turbidity after 5 minutes indicate the fluid is free from calcium and thereby decalcification is complete. Turbidity is observed due to precipitation of calcium. If precipitation is observed after addition of sodium hydroxide, it indicates large amount of calcium is present in fluid. Precipitation seen only after addition of ammonium oxalate suggests decalcification is nearly complete. Checking the end point of decalcification is important because incomplete decalcification makes further cutting of specimens difficult. Prolonged decalcification is also not desirable because it may affect the staining procedure. Once the decalcification is complete the tissue should be washed in running water to remove all the acids. Hard tissue specimens after decalcification are treated like routine soft tissue specimens. The steps of processing can be continued like soft tissue processing, which include dehydration, embedding, sectioning and staining.

Ground Sections

Ground sections are of particular importance in the study of structure of dental hard tissues especially enamel. This method can also be used to study the structure of bone. In this method the hard tissue specimen is made into thin sections of desirable thickness by grinding, using abrasive stones.

Procedure The tooth to be examined should be cut into 2–3 sections using dental hand piece and diamond impregnated or carborundum disc. These sections should be ground using an Arkansas stone or by simply rubbing on a glass plate using abrasive slurry. Grinding should be continued till it is approximately 25–50 microns thickness. Fine abrasives should be used for final polishing. Most suitable abrasive is domestic scouring powders followed by soapy water. Once the desirable thickness is attained the section should be washed and dehydrated and mounted on a glass slide using synthetic resin or Canada balsam as mounting medium and is allowed to dry. Grinding of the tooth can also be done using a laboratory lathe. Initial grinding is done by holding the tooth in fingers and pressing it against the rotating course abrasive wheel of the lathe. When the tooth is thin, it is difficult to hold with fingers. Therefore a wooden block wrapped with adhesive plaster with sticky side directed outward can be used. Stick the tooth onto the plaster and press the wooden block to the rotating wheel of the lathe so that the tooth becomes thinner. Then change the coarse wheel to fine wheel and continue grinding till the section is sufficiently thin. To remove the adhesive plaster the sections can be soaked in water. The section removed from the plaster is then mounted on a glass slide using a mounting medium. Precision equipment like hard tissue microtomes are now available for the preparation of ground section.

48 Microscope

Types of microscopes Light microscopy

A

microscope is an important instrument used in histopathology laboratory to observe the tissues. The magnification it provides enables us to see the structures otherwise invisible to the naked eye. Robert Hooke developed an instrument that could truly be referred to as the forerunner of the modern day microscope.

Types of Microscopes Microscopes are broadly classified as: Simple microscopes and compound miroscopes. Simple microscopes It has a single lens system through which the upturned image of the object is seen. Compound microscopes These are again classified into two types: Light microscope Electron microscope Light microscopes They are of the following types Bright field microscope

Dark field microscope Phase contrast microscope Fluorescence microscope Ultraviolet microscope Interference microscope Electron microscopes They are of two types Transmission electron microscope (TEM) Scanning electron microscope (SEM)

Light Microscopy Microscope in which the final magnified image of the object, illuminated by visible light is seen through glass lenses is called optical or light microscopes or bright field microscope. The ordinary microscope is called a bright field microscope because it forms a dark image against a brighter background. The bright field microscope used in histopathology lab today is a compound microscope that uses multiple lens system to magnify the object, it has a light source, a condenser lens that focuses the light on the specimen and two sets of lenses—objective and ocular—that contribute to the magnification of the image. Through the refraction or bending of light rays by the system of microscope lenses, an image of the specimen is formed that is larger than the object itself, permitting the structures of the specimen to be seen.

Magnification The magnifying capability of a compound microscope is the product of the individual magnifying powers of the ocular lens and the objective lens.

Resolving Power Resolving power is the ability to distinguish two points as separate and distinct. Resolving power of the microscope depends upon the wavelength of

light and the numerical aperture of the lens (light gathering ability of the lens system).

Construction of Compound Light Microscope (Fig. 48.1) The compound microscope consists of a strong metal stand with a broad base or foot, from which rises a short, stout pillar supporting an upright, curved arm. Situated in the base is a strong light source either an adjustable, built in electric lamp or a mirror. Attached to the pillar, above the light source is a system of one or more horizontal iris diaphragms which regulate the passage of light and eliminate undesirable peripheral rays from the light source. Attached to the pillar above the iris diaphragm is one or two lenses vertically adjustable sub-stage condenser which concentrates the light rays on the object. Above the condenser is the horizontal working platform or the stage about 3 or 4 inches square or circular with an opening in the center to admit light from the condenser below. Attached to the curved upright arm is the vertical barrel or body tube. Modern binocular microscope contains a system of prism and reflectors that permit tilting of the barrel for ease in viewing. The barrel is mounted on a rack and pinion mechanism for vertical coarse and fine adjustment or focusing.

Fig. 48.1: Light microscope

At the lower end of the body tube is the objective lens system which consists of low power (10X, 45X) and oil immersion (100X) objectives. These are mounted on a “nose piece” on which they may be rotated into position under the body tube correctly aligned. The low power objectives are commonly used without immersion oil and are spoken of as “high-dry” objectives. The objective lens produces a real image within the instrument. At the top of the body tube is the ocular lens system or the eyepiece usually containing 2 or 3 lenses. These magnify the real image which then appears as a greatly enlarged virtual image seeming to be projected to a position just above the light source and below the iris diaphragm. Light rays from below the iris diapragm are refracted through the condenser and emerge from the top surface of the slide, at the plane of the object as a cone of light with the apex downwards.

49 Muscles of Orofacial Region

Muscles of mastication Muscles of soft palate Muscles of facial expression Muscles of pharynx Suprahyoid muscles Muscles of tongue Muscles of mastication (Figs 49.1a to d)

Fig. 49.1a: Temporalis

Fig. 49.1b: Messetei

Fig. 49.1c: Medial pterygoid

Fig. 49.1d: Lateral pterygoid

Muscles of the soft palate

Muscles of the facial expression (Fig. 49.2)

Muscles of the pharynx

Fig. 49.2: Muscles of facial expression

Suprahyoid muscles

Muscles of tongue

50 Vascular and Nerve Supply of Orofacial Region

Vasculature and nerve innervations of: – – – – – –

Face Teeth and supporting structures Palate Tongue Gingiva Cheek and lips

Face The facial artery is the chief artery of the face which is the branch of external carotid artery. This artery gives off anterior branches and posterior branches. Anterior branches include: Inferior labial supplying lower lip, superior labial supplying upper lip and lateral nasal supplying ala and dorsum of the tongue. The anterior branches anastomose with similar branches of opposite side. Posterior branches are smaller and anastomose with transverse facial artery which is a branch of superficial temporal artery. Other arteries supplying face include transverse facial artery, infra-orbital and mental branches of maxillary artery and dorsal nasal branch of the ophthalmic artery. Facial vein is the main vein draining the face. It begins at the medial corner of the eye by the confluence of supra-orbital and supratrochlear veins. The facial vein passes across the face following the course of facial artery. Below

the mandible this vein joins to the retromandibular vein to form the common facial vein which drains into internal jugular vein. The lymph from major part of forehead, lateral halves of eyelids, lateral part of the cheeks and parotid region is drained into pre auricular lymph nodes. The central part of lower lip and the chin drain into submental lymph nodes. The remaining region of face which include midportion of forehead, external nose, upper lip, lateral part of lower lip, medial part of eyelids, greater part of the lower jaw drain into the submandibular lymph nodes. The trigeminal nerve is the sensory nerve of the face. The ophthalmic division supplies the forehead, upper eyelid, and the nose. The upper lip, ala of the nose, lower eyelid, upper part of the cheek are supplied by maxillary division of trigeminal nerve. Mandibular division of trigeminal nerve provides sensory supply to lower lip, chin, lower part of cheek, lower jaw except for angle, lower margin, etc. Skin over the angle and lower margin of the lower jaw and parotid region are supplied by cervical plexus. The motor nerve supply of face is through the five branches of facial nerve: Temporal, zygomatic, buccal, mandibular, and cervical.

Teeth and Supporting Structures Mandibular teeth and supporting structures are supplied by inferior alveolar artery which is a branch of maxillary artery. Inferior alveolar artery passes through the mandibular foramen to enter into the mandibular canal and terminate as mental and incisive arteries. Maxillary teeth receive arterial supply from three different sources; posterior superior alveolar artery supplies the molars and premolars while anterior superior alveolar artery supplies anterior teeth. The veins related to the mandibular teeth may be collected into one or more inferior alveolar veins which may drain anteriorly into facial vein or posteriorly to pterygoid plexus of veins. In the maxilla also veins drain either into facial vein or pterygoid plexus of veins. The lymph vessels from teeth usually run directly into the submandibular nodes on the same side. Lymph from lower incisor teeth may drain into submental nodes. Sometimes molars may drain directly into jugulodigastric group of nodes. Inferior alveolar nerve innervates the mandibular premolars and molars while anterior teeth are innervated by incisive nerve. In the maxillary arch, anterior superior alveolar nerve supplies the anterior teeth and middle

superior alveolar nerve supplies the premolars and mesio-buccal root of first molar. The posterior superior alveolar nerve supplies all the molars except for mesio-buccal root of first molar.

Palate The palate receives its arterial supplies from greater and lesser palatine branches of maxillary artery. The veins of hard palate drain into pterygoid plexus while those of soft palate drain into pharyngeal plexus. The venous drainage of cheek is to pterygoid venous plexus via buccal veins. The veins of the lips drain into facial vein via superior and inferior labial veins. Lymphatic channels from the major part of palate drain into jugulodigastric group of nodes. Lymph vessels from posterior part of palate terminate in retropharyngeal lymph nodes. The nerve supply to most of the palate is from the maxillary division of trigeminal nerve. Anterior part of the palate is supplied by the nasopalatine nerve which emerges through the incisive foramen. The remaining part of the hard palate is supplied by greater palatine nerve while lesser palatine nerve supplies the soft palate. All muscles of soft palate except for tensor palati are supplied by pharyngeal plexus. The tensor palati is supplied by mandibular nerve.

Tongue The arterial supply of the tongue is from lingual artery, a branch of the external carotid artery. The veins of dorsum and sides of the tongue form the lingual veins which follow the course of lingual arteries to drain into internal jugular veins. The deep lingual veins from ventral surface of the tongue join the facial, internal jugular or lingual veins. The lymphatic vessels from the tip of the tongue drain into the submental nodes. The remaining part of anterior two-thirds of the tongue drain unilaterally into submandibular lymph nodes. The posterior one-third drains bilaterally into jugulo-omohyoid node. The sensory innervations of tongue are from three different sources. The anterior one-third of the tongue is supplied by lingual nerve although the taste sensation is mediated by chorda tympani. The posterior one-third including

the circumvallate papillae are supplied by glossopharyngeal nerve which carries taste and general sensations. The posterior most part of the tongue is innervated by vagus nerve via internal laryngeal branch. Lingual nerve supplies the mucosa on the ventral aspect of the tongue. The motor supply to intrinsic and extrinsic muscles of the tongue is hypoglossal nerve except for palatoglossus which is supplied by cranial part of accessory nerve through the pharyngeal plexus.

Gingiva The labial gingiva around the mandibular anterior teeth are supplied by mental artery and perforating branches of incisive artery. The buccal artery and perforating branches from inferior alveolar artery supplies the posterior buccal gingiva. The lingual gingiva is supplied by the lingual artery and perforating branches from the inferior alveolar artery. The arterial supply to the buccal gingiva around maxillary posterior teeth is by gingival and perforating branches from posterior superior alveolar artery and by buccal artery. The labial gingiva of anterior teeth is supplied by labial branches of infraorbital artery and by perforating branches of the anterior superior alveolar artery. The palatal gingiva is primarily supplied by branches of greater palatine artery. The venous drainage of gingiva could be via buccal, lingual, greater palatine and nasopalatine veins which drain into internal jugular vein or to pterygoid plexus of veins. The lymphatic drainage from labial and buccal gingivae of both maxillary and mandibular teeth drain into submandibular lymph node though the gingiva in the labial region of mandibular incisors drain to the submental node. The palatal and lingual gingiva drain into jugulodigastric nodes directly or indirectly through submandibular node. In the mandibular arch the entire lingual gingiva is innervated by lingual nerve. The labial and buccal gingiva in relation to the anterior teeth and premolars are supplied by mental nerve while the gingiva of molar region receives nerve supply from long buccal nerve. The labial gingiva in relation to maxillary anterior teeth is innervated by anterior superior alveolar nerve and infra-orbital nerve. The buccal gingiva of premolars gets the nerve supply from middle superior alveolar nerve and infraorbital nerve. The posterior superior alveolar nerve supplies the posterior buccal gingiva in relation to molars. The major portion of palatal gingiva is innervated by

greater palatine nerve except for the anterior gingiva which is supplied by nasopalatine nerve.

Cheek and Lips The cheek is supplied by buccal branch of maxillary artery, and the floor of the mouth by lingual arteries. The superior and inferior labial branches of facial arteries provide arterial supply to the lips. The buccal vein of the cheeks drains into pterygoid venous plexus. The venous blood from the lip drains into the facial veins via superior and inferior facial veins. The lymphatics of cheek mainly drain into submandibular and preauricular nodes. Lymphatics from the lips except for central part of the lower lip drain into submandibular lymph nodes. The central part of the lower lip drains into submental lymph nodes. The submandibular nodes also drain the anterior part of floor of the mouth. The mucosa of upper lip is supplied by infraorbital branch of maxillary division of trigeminal nerve. The mental branch of mandibular division of trigeminal nerve innervates the mucosa of the lower lip. The cheek mucosa is supplied by buccal branch and floor of the mouth by lingual nerve.

Appendix Test Yourself (Expected Questions) SECTION 1: ORAL EMBWOLOGY Chapter 1: General Embryology Short Answers for 4–5 marks Derivatives of germ layer (Page 4) Neural crest cells (Page 4) Branchial arches and pouches (Page 5) Short Notes for 2–3 marks Morula (Page 3) Blastocyst (Page 3)

Chapter 2: Development of Orofacial Structures Short Answers for 4–5 marks Formation of palate (Pages 9–10) Formation of tongue (Pages 11–13) Formation of mandible (Pages 12–13) Formation of salivary glands (Page 14)

Meckel’s cartilage and its role in development of mandible (Page 12) Development of face (Page 8)

SECTION 2: ORAL HISTOLOGY Chapter 3: Development of Tooth Essay Questions of 10 or more marks Enumerate the stages of development of teeth. Discuss in detail cap stage of tooth development. (Both morphological and physiological stages should be enumerated. (Pages 22–24) Enumerate the stages of development of teeth. Discuss in detail bell stage of tooth development (Pages 20 and 25–28) Enumerate the stages of development of teeth. Discuss in detail various physiological stages of tooth development and its clinical importance. (Pages 19–21) Discuss in detail development of roots (Pages 28–30) Short Answers for 4–5 marks Dental lamina (Pages 17–19) Tooth germ in bud stage (Pages 21–22) Enamel organ in cap stage (Pages 22–24) Enamel organ in early bell stage (Pages 25–26) Tooth germ in late bell stage (Pages 27–28) Hertwig’s epithelial root sheath and its role in root formation (Pages 28–30) Functions of enamel organ (Pages 30) Stellate reticulum (Pages 23) Transitory structures/temporary structures of enamel organ seen in cap stage of tooth development (Page 24) Short Notes for 2–3 marks

Vestibular lamina (Page 17) Primary epithelial band (Page 17) Cell rests of Serres’ (Page 19) Successional lamina (Pages 26–27) Components of tooth germ and derivatives of each components (Pages 19– 20) Enamel knot and enamel cord (Page 24) Cervical loop (Page 26) Cell rests of Malasses (Pages 30 and 85–86) Stratum intermedium (Pages 25–26)

Chapter 4: Enamel and Amelogenesis Essay Questions of 10 or more marks Enumerate and discuss in detail stages of life cycle of ameloblasts (Pages 33– 36) Enumerate stages of life cycle of ameloblasts and discuss in detail Amelogenesis (Pages 33 and 36–38) Describe the light microscopic and electron microscopic characteristics of enamel rods (Pages 38–40) Discuss in detail various structures observed in ground section of enamel under light microscope. (Answer should include light microscopic structure of enamel rod, structural lines, Gnarled enamel, Hunter Schreger bands, enamel lamellae, tufts and spindles, DEJ, etc.) (Pages 38–47) Discuss in detail the hypocalcified structures of enamel. (Answer should include enamel lamellae, tufts, spindles and various structural lines.) (Pages 41–42, 44–46) Short Answers for 4–5 marks Unique features of enamel (Page 32) Chemical composition and physical properties of enamel (Pages 31 and 32)

Secretory stage of ameloblasts (Page 35) Ameloblast modulation (Page 35–36) Enamel proteins (Page 36) Light microscopic characteristics of enamel rods (Pages 38 and 39) Electron microscopic/submicroscopic characteristics of enamel rods (Pages 39 and 40) Structural lines of enamel (Pages 41–42) Striae of Retzius (Pages 41–42) Hunter Schreger bands (Pages 43–44) Dentinoenamel junction (Page 44) Enamel lamellae and tufts (Pages 45–46) Surface structures of enamel (Pages 46–47) Age changes of enamel (Pages 320–321) Short Notes for 2–3 marks Tomes’process (Page 35) Reciprocal induction (Page 34) Reversal of polarity (Page 34) Reduced enamel epithelium (Page 36) Immediate partial mineralization of enamel (Page 38) Direction of enamel rods (Pages 40–41) Cross striation (Page 41) Neonatal lines of enamel (Page 42) Enamel spindles (Pages 44–45) Enamel tufts (Page 45) Perikymata (Page 47) Nasmyth’s membrane/enamel cuticle (Page 47)

Chapter 5: Dentin and Dentinogenesis Essay Questions of 10 or more marks Discuss in detail various structures observed in ground section of dentin under light microscope. (Answer should include description of dentinal tubules, peri- and intertubular dentin, interglobular dentin, Tomes’ granular layer, structural lines and functional changes) (Pages 54–62) Discuss in detail dentinogenesis (Pages 51–53) Enumerate and discuss in detail various types of dentin. (Answer should include description of primary (mantle and circumpulpal), secondary, tertiary dentin, interglobular dentin, sclerotic dentin, predentin, osteodentin, etc.) (Pages 53–54, 56–58, 60–62) Short Answers for 4–5 marks Chemical composition and physical properties of dentin (Pages 49–50) Differences between enamel and dentin (Page 50) Primary dentin (Pages 53–54) Dentinal tubules (Pages 54–55) Peritubular and intertubular dentin (Pages 56–57) Hypocalcified structures of dentin (answer should include interglobular dentin, structural lines of dentin and Tomes’ granular layer) (Pages 58–59) Interglobular dentin (Page 58) Structural lines of dentin (Page 59) Age and functional changes of dentin (Pages 60–62) Dead tracts (Pages 60–61) Reparative dentin (Pages 61–62) Sclerotic dentin (Page 62) Theories of dentin sensitivity (Pages 62–63) Short Notes for 2–3 marks Mantle dentin (Page 53)

Circumpulpal dentin (Pages 53–54) Secondary dentin (Page 54) Tomes’ granular layer (Pages 58–59) Incremental lines of dentin (Page 59) Contour line of Owen (Page 59) Neonatal lines of dentin (Page 59) Predentin (Page 60) Hydrodynamic theory of dentin sensitivity (Page 63)

Chapter 6: Pulp Essay Questions of 10 or more marks Discuss in detail histological/microscopic structure of pulp. Add a note on functions of pulp (Pages 67–71) Discuss in detail structural components of pulp (Pages 68–70) Short Answers for 4–5 marks Pulp stones/pulp calcifications (Pages 72–73) Age/regressive changes of pulp (Pages 71–73) Histological zones of pulp (Pages 67–68) Functions of pulp (Pages 71) Odontoblasts (Pages 68–69) Short Notes for 2–3 marks Morphological characteristics of pulp (Pages 65–66) Accessory canals (Pages 66–67) Zone of Weil (Page 68) Plexus of Rashkow (Page 68) Undifferentiated mesenchymal cells of pulp (Page 69)

Chapter 7: Cementum

Essay Question of 10 or more marks Classify cementum and discuss in detail structure of cementum (Pages 76– 81) Short Answers for 4–5 marks Physical properties and chemical composition of cementum (Pages 74–75) Cementogenesis (Pages 75–76) Structure of acellular cementum (Pages 77–78) Structure of cellular cementum (Pages 78–79) Cementoenamel junctions (Page 80) Differences between acellular and cellular cementum (Page 80) Hypercementosis (Page 82) Functions of cementum (Pages 81–82) Short Notes for 2–3 marks Classification of cementum (Page 76–77) Cementocytes (Page 78) Acellular afibrillar cementum (Page 79) Intermediate cementum (Page 79) Cementodentinal junction (Page 81) Mixed stratified cementum (Page 79) Age changes of cementum (Page 322)

Chapter 8: Periodontal Ligament Essay Questions of 10 or more marks Discuss in detail microscopic structure of PDL (Pages 84–89) Discuss in detail cellular components of PDL (Pages 84–86) Discuss in detail extracellular components of PDL (Pages 86–89) Discuss in detail principal fibers of PDL add a note on functions of PDL

(Pages 86–88) Short Answers for 4–5 marks Epithelial cell rests of Malassez (Pages 85–86) Principal fibers of PDL (Pages 86–87) Gingival group of fibers (Page 87) Functions of PDL (Pages 89–90) Age changes of PDL (Page 323) Short Notes for 2–3 marks Progenitor cells of PDL (Page 85) Intermediate plexus (Pages 88–89) Oblique fibers of PDL (Page 87) Sharpey’s fibers (Page 88) Cementicles (Page 89) What is periodontium and what are the various components (Page 83)

Chapter 9: Alveolar Bone Short Answers for 4–5 marks Briefly describe parts of alveolar bone (Pages 91–93) Histology of bone (Pages 94–97) Cells of bone (Pages 95–97) Osteoclasts (Pages 96–98) Bone remodeling (Pages 98–99) Short Notes for 2–3 marks Alveolar bone proper/lamina dura/cribriform plate (Page 92) Bundle bone (Page 92) Define alveolar bone and enumerate the parts (Page 91) Resting lines and reversal lines (Page 98)

Osteon/Haversian system (Page 95) Chemical composition of bone (Page 93) Supporting alveolar bone (Pages 92–93)

Chapter 10: Oral Mucosa Essay Questions of 10 or more marks Define and classify oral mucosa. Describe in detail structure of keratinized mucosa (Pages 100 and 103–106 and brief mention of basal complex and desmosomes) Define and classify oral mucosa. Describe in detail structure of nonkeratinized mucosa (Pages 100 and 107–109 and brief mention of basal complex and desmosomes) Describe in detail structure of buccal mucosa (Pages 107–109 and brief mention of basal complex and desmosomes) Describe in detail microscopic and macroscopic structures of gingiva (Pages 115–117 and 103–104 and brief mention of basal complex and desmosomes) Discuss in detail microscopic and macroscopic structures of tongue (Pages 120–123) Discuss in detail microscopic and macroscopic structures of palate (Pages 118–120 and brief description of keratinized epithelium) Discuss the differences between keratinized and nonkeratinized mucosa (Page 110) Short Answers for 4–5 marks Light microscopic structure of keratinized epithelium (Pages 103–104) Light microscopic structure of nonkeratinized epithelium (Page 107) Nonkeratinocytes of oral epithelium (Pages 112–113) Papillae of tongue (Page 121–122) Basal complex (Page 112) Desmosomes (Page 109)

Vermilion border of lip (Pages 114–115) Taste buds (Pages 122–123) Dentogingival junction (Pages 124–125) Passive eruption (Pages 124–125) Lamina propria (Page 102) Junctional epithelium (Page 117) Short Notes for 2–3 marks Classification of oral mucosa (Pages 100–101) Function of oral mucosa (Page 100) Submucosa (Page 102) Mucoperiosteum (Pages 102 and 118) Odland bodies (Pages 106 and 108) Keratohyaline granules (Page 106) Melanocytes (Pages 112–113) Langerhans cells (Page 113) Merkel cells (Page 113) Circumvallate papilla (Page 122)

Chapter 11: Salivary Glands Essay Questions of 10 or more marks Classify salivary glands. Discuss in detail structure of parotid/serous gland (answer should include detailed description of serous acini, and brief description of myoepithelial cells, ductal system and connective tissue component. Pages 127, 129–130, 135, 137–138) Classify salivary glands. Discuss in detail structure of sublingual/mucous gland (answer should include detailed description of mucous acini, and brief description of myoepithelial cells, ductal system and connective tissue component. Pages 127, 133–134, 135, 137–138)

Classify salivary glands. Discuss in detail structure of submandibular/mixed gland (answer should include brief description of mucous, serous and mixed acini, myoepithelial cells, ductal system and connective tissue component. Pages 127, 129–134, 135, 137–138) Classify salivary glands. Discuss in detail structure of serous acini (answer should include detailed description of light and electron microscopic structure of serous cells with a mention of arrangement of cells into acinus and myoepithelial cells) Classify salivary glands. Discuss in detail structure of mucous acini. Add a note on differences between serous and mucous acini (answer shoul include detailed description of light and electron microscopic structure of mucous cells and myoepithelial cells with a mention of arrangement of cells into acinus and difference with serous acini) Classify salivary glands. Discuss in detail structure of parenchymal components of serous/mucous/mixed glands. (Answer should include description of mucous/serous/mixed acini, myoepithelial cells and ductal system) Short Answers for 4–5 marks Ductal system of salivary glands (Pages 137–138) Myoepithelial cells (Pages 135–136) Short Notes for 2–3 marks Differences between serous and mucous acini (Page 140) Functions of salivary ductal systems (Pages 138–139) Mixed acinus (Page 135) Striated ducts (Page 137) Excretory ducts (Page 138) Lingual glands (Page 128) von Ebner’s glands (Page 128) Synthesis and secretion of saliva (Pages 130–132)

Chapter 12: Temporomandibular Joint

Short Answers for 4–5 marks Histology of articular fibrous covering (Page 142) Articular capsule (Pages 142–143) Synovial membrane (Page 143) Articular disc (Pages 144–145) Functions of articular disc (Pages 144–145) Ligaments of TMJ (Page 145) TMJ movements (Pages 145–146) Short Notes for 2–3 marks Histology of condyle (Page 141) Bennett movement (Page 146)

Chapter 13: Maxillary Sinus Short Answers for 4–5 marks Anatomy of maxillary sinus (Pages 148–149) Histology of maxillary sinus lining (Page 149) Functions of maxillary sinus (Pages 149–150) Short Notes for 2–3 marks Pseudostratified ciliated columnar epithelium (Page 149) Goblet cells (Page 149) Oro-antral fistula (Page 150)

SECTION 3: ORAL AND DENTAL ANATOMY Chapter 14: Introduction to Dental Anatomy Short Answers for 4–5 marks Tooth numbering systems (Pages 161–163)

Ridges and grooves (Pages 166–167) Fundamental curvatures of teeth (Pages 169–171) Chronology of human dentition (Page 159) Short Notes for 2–3 marks Dental formula for deciduous and permanent dentition (Pages 157–158) Sequence of eruption of deciduous and permanent dentition (Page 159) Mamelons (Page 168) Embrasures/spillway spaces (Page 170) Line angles and point angles of anterior and posterior teeth (Pages 164–165)

Chapter 15: Deciduous Maxillary Anterior Teeth Short Answers for 4–5 marks Morphological features of deciduous maxillary central incisor (Pages 172– 173) Morphological features of deciduous maxillary canine (Page 173)

Chapter 16: Deciduous Mandibular Anterior Teeth Short Answers for 4–5 marks Morphological features of deciduous mandibular central incisor (Page 176) Morphological features of deciduous mandibular canine (Pages 176–177)

Chapter 17: Deciduous Maxillary Molars Essay Questions of 10 or more marks Discuss in detail morphology of deciduous maxillary first molar (Pages 180– 183) Discuss in detail morphology of deciduous maxillary second molar (Pages 183–185) Short Answers for 4–5 marks Occlusal aspect of deciduous maxillary first molar (Pages 182–183)

Occlusal aspect of deciduous maxillary second molar (Pages 184–185)

Chapter 18: Deciduous Mandibular Molars Essay Questions of 10 or more marks Discuss in detail morphology of deciduous mandibular first molar (Pages 187–189) Discuss in detail morphology of deciduous mandibular second molar (Pages 189–191) Short Answers for 4–5 marks Occlusal aspect of deciduous mandibular first molar (Page 189) Occlusal aspect of deciduous mandibular second molar (Page 191) Differences between deciduous mandibular second molar and permanent mandibular first molar (Page 192)

Chapter 19: Comparison between Deciduous and Permanent Dentition Essay Questions of 10 or more marks Discuss in detail differences between deciduous and permanent dentition (Pages 194–196)

Chapter 20: Permanent Maxillary Central Incisors Essay Question of 10 or more marks Describe in detail morphology of permanent maxillary central incisors. Add a note on its chronology (Pages 197–200) Short Answer for 4–5 marks Class trait of permanent maxillary central incisors (Page 204)

Chapter 21: Permanent Maxillary Lateral Incisors Short Answers for 4–5 marks Briefly describe morphology of permanent maxillary lateral incisors (Pages

202–204) Class trait of permanent maxillary lateral incisors (Page 204) Morphologic differences between of permanent maxillary central and lateral incisors (Page 204)

Chapter 22: Permanent Mandibular Central Incisors Short Answers for 4–5 marks Morphological characteristics of permanent mandibular central incisors (Pages 207–209) Class trait of permanent mandibular central incisors (Page 214) Class trait of permanent mandibular incisors (Page 209) Morphologic differences between of permanent maxillary and mandibular incisors (Page 209)

Chapter 23: Permanent Mandibular Lateral Incisors Short Answers for 4–5 marks Morphological characteristics of permanent mandibular lateral incisors (Pages 212–214) Class trait of permanent mandibular lateral incisors (Page 214) Morphologic differences between of permanent mandibular central and lateral incisors (Page 214)

Chapter 24: Permanent Maxillary Canine Essay Question of 10 or more marks Describe in detail morphology of permanent maxillary canine. Add a note on chronology (Pages 216–219) Short Answers for 4–5 marks Class trait of permanent canines (Page 216) Class trait of permanent maxillary canine (Page 223) Short Note for 2–3 marks

Anatomic landmarks on lingual aspect of permanent maxillary canine (Pages 217–218)

Chapter 25: Permanent Mandibular Canine Essay Question of 10 or more marks Describe in detail morphology of permanent mandibular canine. Add a note on chronology (Pages 221–225) Short Answers for 4–5 marks Class trait of permanent mandibular canine (Page 223) Differences between maxillary and mandibular canines (Page 223)

Chapter 26: Permanent Maxillary First Premolar Essay Question of 10 or more marks Describe in detail morphology of permanent maxillary first premolar. Add a note on its chronology (Pages 226–231) Short Answers for 4–5 marks Class trait of permanent maxillary first premolar (Page 234) Occlusal morphology of permanent maxillary first premolar (Pages 229–230) Mesial aspect of permanent maxillary first premolar (Page 228) Short Note for 2–3 marks Canine fossa (Page 228)

Chapter 27: Permanent Maxillary Second Premolar Essay Question of 10 or more marks Describe in detail morphology of permanent maxillary second premolar. Add a note on its chronology (Pages 232–235) Short Answers for 4–5 marks Class trait of permanent maxillary second premolar (Page 234) Occlusal morphology of permanent maxillary first premolar (Pages 234–235)

Morphological differences between maxillary first and second premolars (Page 234)

Chapter 28: Permanent Mandibular First Premolars Essay Question of 10 or more marks Describe in detail morphology of permanent mandibular first premolar. Add a note on chronology (Pages 237–241) Short Answers for 4–5 marks Class trait of permanent mandibular first premolar (Page 240) Occlusal morphology of permanent mandibular first premolar (Pages 239– 241) Mesial aspect of permanent mandibular first premolar (Page 239) Differences between permanent maxillary and mandibular first premolar. (Page 240)

Chapter 29: Permanent Mandibular Second Premolars Essay Question of 10 or more marks Describe in detail morphology of permanent mandibular second premolar. Add a note on chronology (Pages 243–247) Short Answers for 4–5 marks Class trait of permanent mandibular second premolar (Page 245) Occlusal morphology of three cusp type/two cusp type permanent mandibular second premolar (Pages 245–246) Differences between permanent mandibular first and second premolars (Page 245)

Chapter 30: Permanent Maxillary First Molars Essay Question of 10 or more marks Describe in detail morphology of permanent maxillary first molar. Add a note on its chronology (Pages 248–252)

Short Answer for 4–5 marks Occlusal morphology of permanent maxillary first molar (Pages 251–252) Short Note for 2–3 marks Ridges in occlusal aspect of permanent maxillary first molar (Page 251)

Chapter 31: Permanent Maxillary Second Molars Essay Question of 10 or more marks Describe in detail morphology of permanent maxillary second molar. Add a note on chronology (Pages 254–257) Short Answer for 4–5 marks Differences between permanent maxillary first and second molars (Page 256)

Chapter 32: Permanent Maxillary Third Molars Short Note for 2–3 marks Morphological characteristics of permanent maxillary third molar (Page 258)

Chapter 33: Permanent Mandibular First Molars Essay Question of 10 or more marks Describe in detail morphology of permanent mandibular first molar. Add a note on chronology (Pages 260–264) Short Answer for 4–5 marks Occlusal morphology of permanent mandibular first molar (Pages 263–264) Short Note for 2–3 marks Anatomic landmarks in the occlusal aspect of permanent mandibular first molar (Pages 263–264)

Chapter 34: Permanent Mandibular Second Molars Essay Question of 10 or more marks Describe in detail morphology of permanent mandibular second molar. Add a note on chronology (Pages 267–271)

Short Answer for 4–5 marks Differences between permanent mandibular first and second molars (Page 270)

Chapter 35: Permanent Mandibular Third Molars Short Note for 2–3 marks Morphological characteristics of permanent mandibular third molar (Page 272)

Chapter 36: Occlusion Short Answers for 4–5 marks Molar relation in permanent dentition (Pages 277–278) Molar relation in deciduous dentition (Pages 247–275) Compensating curves (276) Theories of occlusion (Page 279) Centric relation and centric occlusion (Pages 276–277) Short Notes for 2–3 marks Leeway space of Nance (Page 275) Primate space (Page 274) Curve of Spee (276) Curve of Wilson (276) Overjet and overbite (Page 278) Bonwill theory of occlusion (Page 279) Centric and concentric cusps (Page 277)

SECTION 4: ORAL PHYSIOLOGY Chapter 37: Eruption Essay Question of 10 or more marks

Enumerate and discuss in detail the various theories of eruption (Pages 285– 288) Short Answers for 4–5 marks Define eruption and describe pattern of pre-eruptive movements (Pages 283– 284) Histology of eruptive tooth movement (284–285) Post eruptive tooth movements (Page 285) Enumerate theories of eruption and discuss the most accepted one (Pages 283–284. Answei should include names of all theories and detailed description of PDL traction theory) Root formation theory of eruption (Pages 285–286) Bone remodeling theory of eruption (Pages 286–287) Role of dental follicle in eruption (Page 288) Short Notes for 2–3 marks Gubernacular canal and cord (Page 285) Enumerate and define types of physiological movements of teeth (Page 283) Pre-eruptive movements of permanent molars (Page 284) Histological changes observed in PDL during eruption (Page 284) Cushion hammock ligament (Page 286) Importance of pre-eruptive tooth movements (Pages 283–284)

Chapter 38: Shedding Short Answers for 4–5 marks Mechanism of shedding (Page 289) Histology of shedding (Pages 289–290) Odontoclasts (Pages 289–290) Pattern of shedding (Page 290) Short Notes for 2–3 marks

Submerged tooth (Page 291) Causes of retained deciduous teeth (Page 291) Definition of eruption and shedding (Pages 284 and 289)

Chapter 39: Saliva Short Answers for 4–5 marks Composition and functions of saliva (Pages 292–294) Antimicrobial functions of saliva (Page 293) Factors controlling the secretion of saliva (Page 295) Synthesis and secretion of saliva (Pages 294–295) Short Notes for 2–3 marks Salivary IgA (Page 293) Buffering action of saliva (Page 293)

Chapter 40: Physiology of Taste and Speech Short Answers for 4–5 marks Physiology of taste/mechanism of taste perception (Pages 297–298) Physiology of speech (Pages 298–299)

Chapter 41: Mastication Short Answer for 4–5 marks Masticatory/chewing cycle (Pages 300–303) Short Note for 2–3 marks Definition and objectives of mastication (Page 300)

Chapter 42: Deglutition Short Answer for 4–5 marks Phases of deglutition (Pages 304–305)

Short Notes for 2–3 marks Enumerate and define various phases of deglutition (Page 304) Infantile swallow (Page 306)

Chapter 43: Calcium Phosphorus Metabolism Short Answers for 4–5 marks Factors affecting calcium absorption (Page 307) Role of hormones in serum calcium level (Pages 308–311) Short Notes for 2–3 marks Role of parathyroid hormone in serum calcium level (Page 308) Role of vitamin D in serum calcium level (Pages 308–309 ) Role of calcitonin in serum calcium level (Page 310)

Chapter 44: Mineralization Short Answers for 4–5 marks Booster theory/Robinson’s alkaline phosphatase theory (Pages 312–313) Seeding theory/nucleation theory/role of collagen in mineralization (Pages 314–315) Matrix vesicle theory/role of matrix vesicle in mineralization (Pages 315– 316) Short Note for 2–3 marks Alkaline phosphatase (Page 313)

Chapter 45: Hormonal Influence on Orofacial Structures Short Answers for 4–5 marks Effect of hormones on oral tissues (Pages 317–320) Effect of thyroid hormone on oral tissues (Page 317)

Effect of parathyroid hormone on oral tissues (Page 318) Effect of pituitary hormones on oral tissues (Page 318)

Chapter 46: Age Changes of Oral Tissues Short Answers for 4–5 marks Age changes of enamel (Pages 320–321) Age changes of dentin (Pages 321–322) Age changes of pulp (Pages 322–323) Age changes of cementum (Page 322) Age changes of oral mucosa (Page 324) Age changes of salivary glands (Page 324) Short Note for 2–3 marks Oncocytes (Page 324)

SECTION 5: ALLIED TOPICS Chapter 47: Tissue Processing Short Answers for 4–5 marks Enumerate the steps in routine tissue processing (name various steps in order) (Pages 327–331) Ground sectioning (Pages 327–328) Decalcification techniques/preparation of decalcified section (Pages 326– 327) Short Notes for 2–3 marks Frozen section (Page 331) Fixation and fixatives (Page 322) Dehydration (Page 322)

Clearing (Pages 322–323) Hematoxylin and eosin stain (Page 325–326)

Chapter 48: Microscope Short Answer for 4–5 marks Compound microscope (Pages 335–336)

Chapter 49: Muscles of Orofacial Region Short Answer for 4–5 marks Muscles of mastication (Page 337)

Chapter 50: Vascular and Nerve Supply of Orofacial Region Short Answers for 4–5 marks Nerve supply of orofacial region (Pages 342–344) Vasculature of orofacial region (Pages 342–344)