Orban's Oral Histology & Embryology 13th Edition PDF

 Orban’s Oral Histology and Embryology Prelims.indd i 6/27/2011 2:27:24 PM  “This page intentionally left blank" Prelims.indd ii 6/27/2011 2:27:24 PM ...

 Orban’s Oral Histology and Embryology Prelims.indd i 6/27/2011 2:27:24 PM  “This page intentionally left blank" Prelims.indd ii 6/27/2011 2:27:24 PM  Orban’s Oral Histology and Embryology Thirteenth Edition Edited by G S Kumar bds, mds (Oral Pathol) Principal KSR Institute of Dental Science and Research Tiruchengode, Tamil Nadu INDIA ELSEVIER A division of Reed Elsevier India Private Limited Prelims.indd iii 6/27/2011 2:27:24 PM  Orban’s Oral Histology and Embryology, 13/e Kumar ELSEVIER A division of Reed Elsevier India Private Limited ©2011 Elsevier; ©2007 Elsevier, Twelfth Edition (First Adaptation); ©1991 Mosby Inc., Eleventh Edition All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means-electronic or mechanical, including photocopy, recording, or any information storage and retrieval system-without permission in writing from the publisher. This edition contains content adapted from Orban’s Oral Histology and Embryology, 11/e by S.N. Bhaskar, DDS, and is published by an arrangement with Elsevier Inc. Original ISBN: 978-08-016-0239-9 Adaptation ISBN: 978-81-312-2819-7 Medical knowledge is constantly changing. As new information becomes available, changes in treatment, procedures, equipment and the use of drugs become necessary. The authors, editors, contributors and the publisher have, as far as it is possible, taken care to ensure that the information given in this text is accurate and up-to-date. However, readers are strongly advised to confirm that the information, especially with regard to drug dose/usage, complies with current legislation and standards of practice. Published by Elsevier, a division of Reed Elsevier India Private Limited. Registered Office: 622, Indraprakash Building, 21 Barakhamba Road, New Delhi–110 001. Corporate Office: 14th Floor, Building No. 10B, DLF Cyber City, Phase-II, Gurgaon–122 002, Haryana, India. Publishing Manager: Ritu Sharma Managing Editor (Development): Anand K Jha Commissioning Editor: Nimisha Goswami Copy Editor: Saroj K Sahu Production Manager: Sunil Kumar Production Executive: Arvind Booni Cover Designer: Raman Kumar Typeset by Olympus Infotech Pvt. Ltd., Chennai, India (www.olympus.co.in). Printed and bound at xxx Prelims.indd iv 6/27/2011 2:27:24 PM  To My Teachers Who Have Guided Me My Students Who Have Inspired Me My Family Who Have Encouraged Me My Associates Who Have Supported Me Prelims.indd v 6/27/2011 2:27:24 PM  “This page intentionally left blank" Prelims.indd ii 6/27/2011 2:27:24 PM  Preface to the Thirteenth Edition We, the editorial team, constantly strive to improve this book by incorporating not only additional information that we may have gathered, but also our readers’ valuable suggestions. Our contributors are dedicated to this cause and hence, within just three years, we have come up with the next edition of this book. A salient feature of this edition is the inclusion of Summary and Review Questions at the end of every chapter. ‘Appendix’ section has been removed and all chapters have been re-numbered to give their due identity. The redrawn diagrams and change in the style and format of presentation are bound to be more appealing than before. However, the most important change is the addition of a new chapter ‘Lymphoid Tissue and Lymphatics in Orofacial Region’. We have included this chapter because we believe that this topic is not given enough importance in General Histology lectures. I hope to receive feedback from all our readers to aid further improvement of this book. G S Kumar Prelims.indd vii 6/27/2011 2:27:25 PM  “This page intentionally left blank" Prelims.indd ii 6/27/2011 2:27:24 PM  Preface to the Twelfth Edition Orban’s Oral Histology and Embryology has been the all time favourite among the students of dentistry for many a decade. Its popularity is not only due to its elegant presentation but also due to its simplicity. To edit a book of this stature is challenging indeed but I and my contributors have put in our sincere efforts to do justice to both this book and its readers. Suitable additions and modifications have been incorporated owing to the developments that have changed the face of dental practice. Care though, has been taken to retain the old charm and flavour of the previous editions which have laid emphasis on scientific foundations in the field of dentistry. In this edition, a chapter “An Overview of Oral Tissues” has been included to give the student a bird’s eye view of Oral Structures. The chapter on Maxilla and Mandible has been enlarged to emphasize the basic structure of the bone and the factors which govern its dynamics. This edition also highlights the importance of molecular biological aspects that regulate the structure and function of oral tissues, which are yet to find their application in future therapies. Hope this edition caters to the needs and aspiration of students and practitioners alike and gets rewarded with your unstinted patronage. Your suggestions to improve the value of the book are most welcome. G S Kumar Prelims.indd ix 6/27/2011 2:27:25 PM  “This page intentionally left blank" Prelims.indd ii 6/27/2011 2:27:24 PM  List of Contributors Amsavardani S Tayaar Radhika M Bavle Professor and Incharge Professor Department of Oral Pathology Oral and Maxillofacial Pathology SDM College of Dental Sciences and Hospital Krishnadevaraya College of Dental Sciences and Hospital Dharwad Bangalore Chapters 3 and 17 Chapters 7, 8, 11 and 12 Arun V Kulkarni A Ravi Prakash Formerly Professor of Anatomy Professor and Head SDM College of Dental Sciences Department of Oral Pathology Dharwad G Pulla Reddy Dental College Kurnool Chapters 2, 15 and 16 Chapter 5 Dinkar Desai Professor and Head Sharada P Department of Oral Pathology and Microbiology Professor of Oral Pathology AJ Institute of Dental Sciences AECS Maruthi Dental College and Hospital Mangalore Bangalore Chapter 4 Chapters 7, 8 and 9 Karen Boaz Shreenivas Kallianpur Professor and Head Professor of Oral and Maxillofacial Pathology Department of Oral Pathology and Microbiology People’s College of Dental Sciences and Research Centre Manipal College of Dental Sciences Bhopal Mangalore Chapter 6 Chapter 13 G Venkateswara Rao Pushparaja Shetty Dean and Principal Professor and Head Mamata Dental College Department of Oral Pathology Khammam AB Shetty Memorial Institute of Dental Sciences Chapter 3 Nitte University Mangalore Chapter 10 Prelims.indd xi 6/27/2011 2:27:25 PM  xii List of contributors Vinod Kumar R B G S Kumar Professor and Head Professor of Oral Pathology and Principal Amirta School of Dentistry KSR Institute of Dental Science and Research Kochi Tiruchengode Chapter 14 Chapters 1 and 18, Summary of Chapters 1, 3–16 and 18 Prelims.indd xii 6/27/2011 2:27:25 PM  Contents Preface to the Thirteenth Edition vii Preface to the Twelfth Edition ix List of Contributors xi 1. An Overview of Oral Tissues 1 2. Development of Face and Oral Cavity 5 3. Development and Growth of Teeth 24 4. Enamel 50 5. Dentin 93 6. Pulp 120 7. Cementum 151 8. Periodontal Ligament 172 9. Bone 205 10. Oral Mucous Membrane 238 11. Salivary Glands 291 12. Lymphoid Tissue and Lymphatics in Orofacial Region 317 13. Tooth Eruption 332 14. Shedding of Deciduous Teeth 348 15. Temporomandibular Joint 359 16. Maxillary Sinus 369 17. Histochemistry of Oral Tissues 380 18. Preparation of Specimens for Histologic Study 410 Index 417 Prelims.indd xiii 6/27/2011 2:27:25 PM  “This page intentionally left blank" Prelims.indd ii 6/27/2011 2:27:24 PM  Brief Contents Preface to the Thirteenth Edition vii Inner enamel epithelium 32 Preface to the Twelfth Edition ix Stratum intermedium 32 List of Contributors xi Stellate reticulum 33 Outer enamel epithelium 33 1. An Overview of Oral Tissues 1 Dental lamina 33 2. Development of Face and Oral Cavity 5 Dental papilla 33  Origin of Facial Tissues 5 Dental sac 35  Development of Facial Prominences 9 Advanced bell stage 35 Development of the frontonasal region:  Hertwig’s epithelial root sheath and olfactory placode, primary palate, and nose 9 root formation 35 Development of maxillary prominences  Histophysiology 37 and secondary palate 10  Initiation 37 Development of visceral arches and tongue 11  Proliferation 38  Final Differentiation of Facial Tissues 13  Histodifferentiation 38  Clinical Considerations 15  Morphodifferentiation 38  Facial clefts 15  Apposition 38  Hemifacial microsomia 18  Molecular Insights in Tooth Morphogenesis 38  Treacher Collins’ syndrome 18  Tooth initiation potential 39  Labial pits 19  Establishment of oral–aboral axis 40  Lingual anomalies 19  Control of tooth germ position 41  Developmental cysts 19  Functional redundancy and their complexities 41  Summary 21  Patterning of dentition 42  Regulation of ectodermal boundaries 43 3. Development and Growth of Teeth 24  Stomodeal thickening stage—Dental  Dental Lamina 25 lamina stage (E11.5–E12.5) 44  Fate of dental lamina 25  Bud stage (E12.5–E13.5) 44  Vestibular lamina 26  Bud stage–Cap stage (E13.5–E14.5) 44  Tooth Development 26  Enamel knot–Signaling center for tooth  Developmental Stages 27 morphogenesis 45  Bud stage 27  Clinical Considerations 46  Cap stage 28  Summary 46 Outer and inner enamel epithelium 28 Stellate reticulum 29 4. Enamel 50 Dental papilla 30  Histology 50 Dental sac (dental follicle) 30  Physical characteristics 50  Bell stage 31  Chemical properties 51 Prelims.indd xv 6/27/2011 2:27:25 PM  xvi Brief Contents  Structure 53  Age and Functional Changes 106 Rods 53 Vitality of dentin 106 Ultrastructure 53 Reparative dentin 107 Striations 54 Dead tracts 108 Direction of rods 54 Sclerotic or transparent dentin 108 Hunter–Schreger bands 57  Development 110 Incremental lines of Retzius 57 Dentinogenesis 110 Surface structures 58 Mineralization 112 Enamel cuticle 60  Clinical Considerations 115 Enamel lamellae 61  Summary 117 Enamel tufts 62 6. Pulp 120 Dentinoenamel junction 64  Anatomy 120 Odontoblast processes and enamel spindles 65 General features 120  Age changes 65 Coronal pulp 120  Clinical Considerations 66 Radicular pulp 121  Development 68 Apical foramen 121  Epithelial enamel organ 68 Accessory canals 122 Outer enamel epithelium 69  Structural Features 122 Stellate reticulum 69 Intercellular substance 122 Stratum intermedium 71 Fibroblasts 124 Inner enamel epithelium 71 Fibers 124 Cervical loop 71 Undifferentiated mesenchymal cells 125  Life cycle of the ameloblasts 72 Odontoblasts 125 Morphogenic stage 72 Defense cells 127 Organizing stage 74 Pulpal stem cells 130 Formative stage 74 Blood vessels 130 Maturative stage 75 Lymph vessels 135 Protective stage 75 Nerves 135 Desmolytic stage 75 Nerve endings 135  Amelogenesis 76  Molecular events following pulp injury Formation of the enamel matrix 76 and repair 138 Development of Tomes’ processes 77  Functions 139 Ameloblasts covering maturing enamel 81 Inductive 139 Mineralization and maturation of the Formative 139 enamel matrix 82 Nutritive 139  Clinical Considerations 86 Protective 139  Summary 87 Defensive or reparative 139  Differences in Primary and Permanent 5. Dentin 93  Physical and Chemical Properties 93 Pulp Tissues 139  Structure 94 Primary pulp 139 Dentinal tubules 94 Permanent pulp 140  Regressive Changes (Aging) 140 Peritubular dentin 95 Intertubular dentin 96 Cell changes 140 Predentin 96 Fibrosis 140 Odontoblast process 97 Vascular changes 141  Primary Dentin 98 Pulp stones (denticles) 141  Secondary Dentin 100 Diffuse calcifications 142  Development 142  Tertiary Dentin 100  Clinical Considerations 143  Incremental Lines 100  Summary 148  Interglobular Dentin 101  Granular Layer 103 7. Cementum 151  Innervation of Dentin 104  Physical Characteristics 151 Intratubular nerves 104  Chemical Composition 151 Theories of pain transmission through dentin 104  Cementogenesis 152  Permeability of Dentin 106  Cementoblasts 152 Prelims.indd xvi 6/27/2011 2:27:25 PM  Brief Contents xvii  Cementoid tissue 155  Elastic fibers 191  Structure 156  Reticular fibers 193  Acellular extrinsic fiber cementum 156  Secondary fibers 193  Cellular cementum 157  Indifferent fiber plexus 193 Cellular intrinsic fiber cementum (CIFC) 157  Ground substance 193 Cellular mixed fiber cementum (CMFC) 157  Interstitial tissue 193 Cellular mixed stratified cementum (CMSC) 157  Structures Present in Connective Tissue 193  Differences between cementocytes and Blood vessels 194 osteocytes 158 Lymphatic drainage 195  Differences between AEFC and cellular Nerves 195 intrinsic fiber cementum (CIFC) 159 Cementicles 196  Cementodentinal Junction 160  Functions 196  Cementoenamel Junction 162 Supportive 196  Functions 163 Sensory 197  Anchorage 163 Nutritive 198  Adaptation 163 Homeostatic 198  Repair 163 Eruptive 198  Hypercementosis 164 Physical 198  Clinical Considerations 166  Age Changes in Periodontal Ligament 199  Summary 168  Unique Features of Periodontal Ligament 199  Clinical Considerations 200 8. Periodontal Ligament 172  Summary 201  Development 173  Development of the principal fibers 174 9. Bone 205  Classification of Bones 205 Development of cells 174  Composition of Bone 206  Periodontal ligament collagen fiber  Bone Histology 209 attachment to the root surface 175  Bone Cells 211  Periodontal Ligament Homeostasis 175  Osteoblasts 211  Cell Biology of Normal Periodontium 176  Osteocytes 213  Cells 178  Osteoclasts 215  Synthetic cells 178  Bone Formation 216 Osteoblasts 178  Intramembranous ossification 216 Fibroblast 180  Differences between immature bone and Fibroblast-matrix adhesion and traction 180 Functions 181 mature bone 217  Intracartilaginous bone formation 217 Differences between periodontal ligament  Bone Resorption 220 fibroblasts and gingival fibroblasts 181  Bone Remodeling 222 Cementoblasts 182  Alveolar Bone 224  Resorptive cells 182  Development of Alveolar Process 224 Osteoclasts 182  Structure of the Alveolar Bone 225 Fibroblasts 182  Internal Reconstruction of Alveolar Bone 229 Intracellular degradation 183  Age Changes 231 Cementoclasts 183  Clinical Considerations 231  Progenitor cells 184  Therapeutic Considerations 233 Origin of the periodontal stem cells 184  Summary 234  Relationship between cells 185  Epithelial rests of Malassez 186 10. Oral Mucous Membrane 238  Defense cells 186  Classification of Oral Mucosa 239 Mast cells 186  Functions of Oral Mucosa 239 Macrophages 187 Defense 239 Eosinophils 188 Lubrication 239  Extracellular Substance 188 Sensory 239  Fibers 189 Protection 239  Collagen 189  Definitions and General Considerations 239  Sharpey’s fibers 190  Comparison of oral mucosa with skin and  Intermediate plexus 191 intestinal mucosa 239 Prelims.indd xvii 6/27/2011 2:27:25 PM  xviii Brief Contents  Basement membrane 242 Excretory ducts 300  Lamina propria 242  Connective tissue elements 300  Submucosa 243  Blood supply 302  Structure of the Oral Epithelium 244  Nerve supply and pattern of innervation 302 Cytokeratins 244  Classification and Structure of Human  Keratinized epithelium 245 Salivary Glands 303 Stratum basale 246  Major salivary glands 303 Stratum spinosum 248 Parotid gland 303 Stratum granulosum 249 Submandibular gland 303 Stratum corneum 252 Sublingual gland 304  Keratinocytes and nonkeratinocytes 253  Minor salivary glands 304 Keratinocytes 253 Labial and buccal glands 305 Nonkeratinocytes 253 Glossopalatine glands 305 Melanocytes 253 Palatine glands 305 Langerhans cell 254 Lingual glands 306 Merkel cells 254 Von Ebner’s glands 306  Nonkeratinized epithelium 254  Development and Growth 306  Subdivisions of Oral Mucosa 255  Control of Secretion 307  Keratinized areas 255  Composition of Saliva 308 Masticatory mucosa (gingiva and  Functions of Saliva 309 hard palate) 255 Protection of the oral cavity and Hard palate 255 oral environment 309 Gingiva 259 Digestion 310 Blood and nerve supply 263 Mastication and deglutition 310 Vermilion zone 264 Taste perception 310  Nonkeratinized areas 265 Speech 310 Lining mucosa 265 Tissue repair 310 Lip and cheek 265 Excretion 311 Vestibular fornix and alveolar mucosa 266  Clinical Considerations 311 Inferior surface of tongue and floor of  Summary 312 oral cavity 266 12. Lymphoid Tissue and Lymphatics in Soft palate 267 Orofacial Region 317  Specialized mucosa 267  Introduction to Lymphatic System 317 Dorsal lingual mucosa 267  Types of Lymphoid Tissues 317 Taste buds 270  Development of Lymph Nodes and Lymphatics  318 Gingival Sulcus and Dentogingival Junction 271  Functions of the Lymphatic System 318  Gingival sulcus 271  Lymph Nodes 318  Dentogingival junction 271  Anatomy 319 Development of dentogingival junction 272  Microscopic structure 319 Shift of dentogingival junction 274 Cortical (follicle) area 320 Sulcus and cuticles 277 Paracortex (paracortical area) 321 Epithelial attachment 277 Medullary area 322 Migration of epithelial attachment 278  Immunohistochemistry 323  Development of Oral Mucosa 281  Lymph sinuses 323  Age Changes in Oral Mucosa 281  Reticular network 323  Clinical Considerations 282  Lymphatic Vessels and Capillaries 324  Summary 284  Blood Vessels of Lymph Nodes 325 11. Salivary Glands 291  Clinical Significance of Lymph Nodes 325  Structure of Terminal Secretory Units 292  Lymph 325  Serous cells 293  Rate of lymph flow 326  Mucous cells 294  Tonsils 326  Myoepithelial cells 296  Lingual tonsils 326  Ducts 297  Palatine tonsils 326 Intercalated ducts 297  Pharyngeal tonsils 327 Striated ducts 298  Development of tonsils 327 Prelims.indd xviii 6/27/2011 2:27:25 PM  Brief Contents xix  Functions 327 17. Histochemistry of Oral Tissues 380  Clinical significance of tonsils 327  Overview of Histochemical Techniques 381  Lymphatic Drainage of Head and Neck 327  Structure and Chemical Composition of  Summary 329 Oral Tissues 382  Connective tissue 382 13. Tooth Eruption 332  Ground substance 383  Pattern of Tooth Movement 332 Proteoglycans 383 Pre-eruptive tooth movement 332  Cells and fibers 386 Eruptive tooth movement 334 Fibroblasts 386 Posteruptive tooth movement 335  Epithelial tissues and derivatives 386  Animal experimental studies in eruption 335  Enzymes 386  Histology of Tooth Movement 335  Histochemical Techniques 387 Pre-eruptive phase 335  Fixation procedures 387 Eruptive phase 335  Specific histochemical methods 388 Posteruptive phase 338  Mechanism of Tooth Movement Glycogen, glycoproteins, and proteoglycans 388 Proteins and lipids 389 (Theories of Tooth Eruption) 339 Enzymes 389 Bone remodeling theory 339 Phosphatases 389 Root formation theory 339 Immunohistochemistry 390 Vascular pressure theory 339  Histochemistry of Oral Hard Tissues 390 Periodontal ligament traction theory 340  Carbohydrates and protein 390 Posteruptive tooth movement 340  Lipids 392  Cellular and molecular events in eruption 341  Enzyme histochemistry of hard tissue 393  Clinical Considerations 344  Summary Alkaline phosphatase 393 345 Adenosine triphosphatase 394 14. Shedding of Deciduous Teeth 348 Acid phosphatase 394  Definition 348 Esterase 395  Pattern of Shedding 348 Aminopeptidase 396  Histology of Shedding 351 Cytochrome oxidase 396  Mechanism of Resorption and Shedding 355 Succinate dehydrogenase 397  Clinical Considerations 356 Citric acid cycle in osteoblasts and Remnants of deciduous teeth 356 osteoclasts 397 Retained deciduous teeth 356 Calcium-binding sites in enamel organ 397 Submerged deciduous teeth 357 Summary 397  Summary 358  Histochemistry of Oral Soft Tissues 397  Polysaccharides, proteins, and mucins 397 15. Temporomandibular Joint 359  Gross Anatomy Polysaccharides 397 359  Proteins and protein groups 398  Development of the Joint 361  Lipids 398  Histology 362  Mucins 398  Bony structures 362  Enzyme histochemistry 399  Articular fibrous covering 363 Alkaline phosphatase 399  Articular disk 364 Acid phosphatase 399  Synovial membrane 365  Clinical Considerations Esterase 399 365  Summary Aminopeptidase 399 367 β-Glucuronidase 399 16. Maxillary Sinus 369 Cytochrome oxidase 400  Definition 369 Succinate dehydrogenase and glucose  Developmental Aspects 369 6-phosphate dehydrogenase 401  Developmental Anomalies 369 Enzyme histochemical detection of  Structure and Variations 370 lymphatic capillaries 401  Microscopic Features 372 Angiogenic factor in inflamed gingiva 402  Functional Importance 375 Laminin-5 402  Clinical Considerations 376  Clinical Considerations 402  Summary 378  Summary 403 Prelims.indd xix 6/27/2011 2:27:25 PM  xx Brief Contents 18. Preparation of Specimens for Histologic Study 410  Preparation of Ground Sections of Teeth or Bone 414  Preparation of Sections of Paraffin-Embedded  Preparation of Frozen Sections 415 Specimens 411  Types of Microscopy 415 Infiltration of the specimen with paraffin 411  Summary 415  Preparation of Sections of Parlodion-Embedded Specimens 413 Index 417 Prelims.indd xx 6/27/2011 2:27:25 PM  Chapter |1| An Overview of Oral Tissues The oral cavity contains a variety of hard tissues and soft tissues. pulp are derivatives of dental papilla while cementum, periodontal The hard tissues are the bones of the jaws and the tooth. The ligament and alveolar bone, are all derivatives of dental follicle. soft tissues include the lining mucosa of the mouth and the The cells that form these tissues have their names ending in salivary glands. blast. Thus, ameloblast produces enamel, odontoblast dentin, The tooth consists of crown and root. That part of the tooth cementoblast, cementum and osteoblast bone. These synthesiz- visible in the mouth is called clinical crown; the extent of ing cells have all the features of a protein secreting cell—well which increases with age and disease. The root portion of the developed ribosomes and a rough endoplasmic reticulum (ER), tooth is not visible in the mouth in health. The tooth is sus- Golgi apparatus, mitochondria and a vesicular nucleus, which pended in the sockets of the alveolar bone by the periodontal is often polarized. The cells that resorb the tissues have their ligament. The anatomical crown is covered by enamel and the names ending in ‘clast’. Thus, osteoclast resorbs bone, cemen- root by the cementum. Periodontium is the term given to sup- toclast, cementum and odontoclast resorbs all the dental tissues. porting tissues of the tooth. They include the cementum, peri- The ‘clast’ cells have a similar morphology in being multinucle- odontal ligament and the alveolar bone. The innermost portion ated giant cells. Their ultra structural features include numerous of the crown and root is occupied by soft tissue, the pulp. The lysosomes and ingested vacuoles. dentin occupies the region between the pulp and enamel in the Dentin is the first hard tissue of the tooth to form. Enamel crown, and between pulp and cementum in the root. starts its formation after the first layer of dentin has formed. The enamel formation is from its junction with dentin outwards, first in the cuspal/incisal and later in the cervical regions. Dentin formation is similar, but from the dentinoenamel junction, the DEVELOPMENT OF TOOTH formation is pulpward. Cementum formation occurs after the root form, size, shape and number of roots is outlined by the epithelial The tooth is formed from the ectoderm and ectomesenchyme. root sheath and dentin is laid down in these regions. Formation The enamel is derived from the enamel organ which is differen- of enamel, dentin and cementum takes place as a daily event in tiated from the primitive oral epithelium lining the stomo- phases or in increments, and hence they show incremental deum (primitive oral cavity). Epithelial mesenchymal interactions lines. In dentin and cementum formation, a layer of uncalcified take place to determine the shape of the tooth and the differen- matrix forms first, followed by its mineralization. While in tiation of the formative cells of the tooth and the timing of enamel formation enamel matrix is calcified, but its matura- their secretion. The ectomesenchymal cells which are closer to tion or complete mineralization occurs as a secondary event. the inner margins of the enamel organ differentiate into dental Mineralization occurs as a result of supersaturation of calcium papilla and the ectomesenchymal cells closer to the outer mar- and phosphorus in the tissue fluid. The formative cells concen- gins of the enamel organ become dental follicle. Dentin and trate the minerals from calcium phosphate (apatite) and secrete 2 Orban’s Oral Histology and Embryology them into the organic matrix, in relation to specific substances like collagen, which act as attractants or nucleators for miner- PULP alization. The mechanism of mineralization is quite similar in all the hard tissues of tooth and in bone. The pulp, the only soft tissue of the tooth, is a loose connective tissue enclosed by the dentin. The pulp responds to any stimuli by pain. Pulp contains the odontoblast. Odontoblasts are ter- minally differentiated cells, and in the event of their injury and ENAMEL death, they are replaced from the pool of undifferentiated ecto- mesenchymal cells in the pulp. The pulp is continuous with the The enamel is the hardest tissue in the human body. It is the periodontal ligament through the apical foramen or through only ectodermal derivative of the tooth. Inorganic constituents the lateral canals in the root. Pulp also contains defense cells. account for 96% by weight and they are mainly calcium phos- The average volume of the pulp is about 0.02 cm3. phate in the form of hydroxyapatite crystals. These apatite crys- tals are arranged in the form of rods. All other hard tissues of the body, dentin, cementum and bone also have hydroxyapatite CEMENTUM as the principal inorganic constituent. Hydroxyapatite crystals differ in size and shape; those of the enamel are hexagonal and The cementum is comparable to bone in its proportion of inor- longest. Enamel is the only hard tissue, which does not have ganic to organic constituents and to similarities in its structure. collagen in its organic matrix. The enamel present in the fully The cementum is thinnest at its junction with the enamel and formed crown has no viable cells, as the cells forming it—the thickest at the apex. The cementum gives attachment to the ameloblast degenerates, once enamel formation is over. Therefore, periodontal ligament fibers. Cementum forms throughout life, all the enamel is formed before eruption. This is of clinical impor- so as to keep the tooth in functional position. Cementum also tance as enamel lost, after tooth has erupted, due to wear and forms as a repair tissue and in excessive amounts due to low tear or due to dental caries, cannot be formed again. Enamel, grade irritants. lacks not only formative cells but also vessels and nerves. This The cells that form the cementum; the cementoblast lines makes the tooth painless and no blood oozes out when enamel the cemental surface. Uncalcified cementum is usually seen, is drilled while making a cavity for filling. as the most superficial layer of cementum. The cells within the cementum, the cementocytes are enclosed in a lacuna and its process in the canaliculi, similar to that seen in bone, but in a far less complex network. Cementocytes presence is limited DENTIN to certain regions. The regions of cementum containing cells are called cellular cementum and the regions without it, are The dentin forms the bulk of the tooth. It consists of dentinal known as the acellular cementum. The acellular cementum is tubules, which contains the cytoplasmic process of the odonto- concerned with the function of anchorage to the teeth and the cel- blasts. The tubules are laid in the calcified matrix—the walls of lular cementum is concerned with adaptation, i.e. to keep the the tubules are more calcified than the region between the tooth in the functional position. Like dentin, cementum forms tubules. The apatite crystals in the matrix are plate like and throughout life, and is also avascular and noninnervated. shorter, when compared to enamel. The number of tubules near the pulp are broader and closer and they usually have a sinusoi- dal course, with branches, all along and at their terminus at the dentinoenamel or cementodentinal junction. The junction PERIODONTAL LIGAMENT between enamel and dentin is scalloped to give mechanical retention to the enamel. Dentin is avascular. Nerves are present The periodontal ligament is a fibrous connective tissue, which in the inner dentin only. Therefore, when dentin is exposed, by anchors the tooth to the alveolar bone. The collagen fibers of loss of enamel and stimulated, a pain-like sensation called sen- the periodontal ligament penetrate the alveolar bone and sitivity is experienced. The dentin forms throughout life with- cementum. They have a wavy course. The periodontal ligament out any stimulation or as a reaction to an irritant. The cells that has the formative cells of bone and cementum, i.e. osteoblast and form the dentin—the odontoblast lies in the pulp, near its bor- cementoblast in addition to fibroblast and resorptive cells—the der with dentin. Thus, dentin protects the pulp and the pulp osteoclast. Cementoclasts are very rarely seen as cemental resorp- nourishes the dentin. Though dentin and pulp are different tion is not seen in health. Fibroblast, also functions as a resorptive tissues they function as one unit. cell. Thus, with the presence of both formative and resorptive  An Overview of Oral Tissues 3 cells of bone, cementum and connective tissue, and along with the wavy nature of the fibers, the periodontal ligament is able ERUPTION AND SHEDDING OF TEETH to adjust itself to the constant change in the position of teeth, and also maintains its width. The periodontal fibers connect all The eruption of teeth is a highly programed event. The teeth the teeth in the arch to keep them together and also attach the developing within the bony crypt initially undergo bodily and gingiva to the tooth. The periodontal ligament nourishes the eccentric movements and finally by axial movement make its cementum. The presence of proprioceptive nerve endings pro- appearance in the oral cavity. At that time, the roots are about vides the tactile sensation to the tooth and excessive pressure on half to two thirds complete. Just before the tooth makes its the tooth is prevented by pain originating from the pain recep- appearance in the oral cavity the epithelium covering it, fuses tors in the periodontal ligament. with the oral epithelium. The tooth then cuts through the degenerated fused epithelium, so that eruption of teeth is a bloodless event. Root growth, fluid pressure at the apex of the erupting teeth and dental follicle cells ALVEOLAR BONE contractile force are all shown to be involved in the eruption mechanism. The bony crypt forms and resorbs suitably to adjust Alveolar bone is the alveolar process of the jaws that forms and to the growing tooth germ and later to its eruptive movements. supports the sockets for the teeth. They develop during the The deciduous teeth are replaced by permanent successor teeth eruption of the teeth and disappear after the tooth is extracted as an adaptation to the growth of jaws and due to the increased or lost. The basic structure of the alveolar bone is very similar masticatory force of the masticatory muscles, in the process of to the bone found elsewhere, except for the presence of imma- shedding. The permanent successor teeth during the eruptive ture bundle bone amidst the compact bone lining the sockets movement cause pressure on the roots of deciduous teeth and for the teeth. The buccal and lingual plates of compact bone induce resorption of the roots. The odontoclast, which has a enclose the cancellous bone. The arrangement and the density similar morphology to osteoclast and participates in this event, of the cancellous bone varies in the upper and lower jaws and has the capacity to resorb, all dental hard tissues. is related to the masticatory load, the tooth receives. The ability of bone, but not cementum, to form under tension and resorb under pressure makes orthodontic treatment possible. ORAL MUCOSA The mucosa lining the mouth is continuous anteriorly with the skin of the lip at the vermilion zone and with the pharyngeal TEMPOROMANDIBULAR JOINT mucosa posteriorly. Thus, the oral mucosa and GI tract mucosa are continuous. The integrity of the mucosa is interrupted by This only movable bilateral joint of the skull has a movable the teeth to which it is attached. The oral mucosa is attached to fibrous articular disk separating the joint cavity. The fibrous the underlying bone or muscle by a loose connective tissue, layer that lines the articular surface is continuous with the peri- called submucosa. The mucosa is firmly attached to the perios- osteum of the bones. The fibrous capsule, which covers the teum of hard palate and to the alveolar process (gingiva). The joint, is lined by the synovial membrane. The joint movement mucosa in these regions is a functional adaptation to mastica- is intimately related to the presence or absence of teeth and tion, hence, they are referred to as masticatory mucosa. Elsewhere, to their function. except in the dorsum of tongue, the mucosa is loosely attached as an adaptation to allow the mucosa to stretch. The mucosa in these regions is referred to as lining mucosa. The stratified squamous epithelium varies in thickness and is either kerati- MAXILLARY SINUS nized as in masticatory mucosa or non-keratinized as in lining mucosa. The submucosa is prominent in the lining and is nearly The maxillary posterior teeth are related to the maxillary sinus absent in the masticatory mucosa. The cells that have the abil- in that, they have a common nerve supply and that their roots ity to produce keratin, called keratinocytes, undergo matura- are often separated by a thin plate of bone. Injuries to the lining tional changes and finally desquamate. The non-keratinocytes, and extension of infection from the apex of roots are often do not undergo these changes, and they are concerned either encountered in clinical practice. Developing maxillary canine with immune function (Langerhans cells) or melanin production teeth are found close to the sinus. Pseudostratified ciliated (melanocytes). The mucosa that attaches to the tooth is unique, columnar epithelium lines the maxillary sinus. thin and permeable. The fluid that oozes through this lining 4 Orban’s Oral Histology and Embryology into the crevice around the tooth is called gingival fluid. It aids in defense against entry of bacteria, through this epithelium. STUDY OF ORAL TISSUES The mucous of the dorsum of tongue, is called specialized mucosa because it has the taste buds in the papillae. For light microscopic examination, the tissues have to be made thin and stained, so that the structures can be appreci- ated. The teeth (and bone) can be ground or can be decalcified before making them into thin slices. In the first method, all SALIVARY GLANDS hard tissues can be studied. In the second method, all the hard tissues except enamel, pulp and periodontal ligament can The major salivary glands (parotid, submandibular and sub- be studied. Soft tissues of the mouth require a similar prepara- lingual) and the minor salivary glands present in the submu- tion as soft tissues of other parts of the body for microscopic cosa, everywhere in the oral cavity except in gingivae and examination. anterior part of the hard palate; secrete serous, mucosa or mixed For traditional light microscopic examination, the tissues salivary secretion, into the oral cavity by a system of ducts. The have to be made into thin sections and differentially stained by acini, which are production centers of salivary secretion, are of utilizing the variations they exhibit in their biochemical and two types—the serous and the mucous acini. They vary in size immunological properties. There are various histochemical, and shape and also in the mode of secretion. The composition enzyme-histochemical, immunohistochemical, immunofluo- and physical properties of saliva differ between mucous and rescent techniques developed to enhance tissue characteristics. serous secretions. The ducts, act not merely as passageways for Apart from light microscopy, tissues can be examined using saliva, but also modify the salivary secretion with regard to electron microscope, fluorescent microscope, confocal laser quantity and electrolytes. The ducts, which vary in their struc- scanning microscope and autoradiography techniques for bet- ture from having a simple epithelial lining to a stratified squa- ter recognition of cellular details, functions and the series of mous epithelial lining, show functional modifications. events that take place within them.  Chapter |2| Development of Face and Oral Cavity CHAPTER CONTENTS Clinical Considerations 15 Origin of Facial Tissues 5 Facial clefts 15 Development of Facial Prominences 9 Hemifacial microsomia 18 Development of the frontonasal region: olfactory Treacher Collins’ syndrome 18 placode, primary palate, and nose 9 Labial pits 19 Development of maxillary prominences and Lingual anomalies 19 secondary palate 10 Developmental cysts 19 Development of visceral arches and tongue 11 Summary 21 Final Differentiation of Facial Tissues 13 This chapter deals primarily with the development of the (Fig. 2.2). In most vertebrates, including humans, the major human face and oral cavity. Consideration is also given to infor- portion of the egg cell mass forms the extraembryonic mem- mation about underlying mechanisms that is derived from branes and other supportive structures, such as the placenta. experimental studies conducted on developing subhuman The inner cell mass (Fig. 2.2D) separates into two layers, the embryos. Much of the experimental work has been conducted epiblast and hypoblast (Fig. 2.2E). Cell marking studies in chick on amphibian and avian embryos. Evidence derived from these and mouse embryos have shown that only the epiblast forms and more limited studies on other vertebrates including mam- the embryo, with the hypoblast and other cells forming sup- mals indicates that the early facial development of all verte- porting tissues, such as the placenta. The anterior (rostral) end brate embryos is similar. Many events occur, including cell of the primitive streak forms the lower germ layer, the endoderm, migrations, interactions, differential growth, and differentia- in which are embedded the midline notochordal (and pre- tion, all of which lead to progressively maturing structures chordal) plates (Figs. 2.2F and 2.3A). Prospective mesodermal (Fig. 2.1). Progress has also been made with respect to abnor- cells migrate from the epiblast through the primitive streak to mal developmental alterations that give rise to some of the form the middle germ layer, the mesoderm. most common human malformations (see Fig. 2.16). Further Cells remaining in the epiblast form the ectoderm, completing information on the topics discussed can be obtained by consult- formation of the three germ layers. Thus, at this stage, three ing the references at the end of the chapter. distinct populations of embryonic cells have arisen largely through division and migration. They follow distinctly sepa- rate courses during later development. ORIGIN OF FACIAL TISSUES Migrations, such as those described above, create new asso- ciations between cells, which, in turn, allow unique possibili- After fertilization of the ovum, a series of cell divisions gives ties for subsequent development through interactions between rise to an egg cell mass known as the morula in mammals the cell populations. Such interactions have been studied Chapter-02.indd 5 6/16/2011 4:04:15 PM  6 Orban’s Oral Histology and Embryology Fig. 2.1 Emergence of facial structures during development of human embryos. Dorsal views of gestational day 19 and 22 embryos are depicted, while lateral aspects of older embryos are illustrated. At days 25 and 32, visceral arches are designated by Roman numerals. Embryos become recognizable as “human” by gestational day 50. Section planes for Fig. 2.2 are illustrated in the upper (days 19 and 22) diagrams. Yolk sac Neural folds Buccopharyngeal Mandibular arch membrane Neural plate Amnion (see Fig. 2.2) Primitive node and streak Somite Day 19 Body stalk Posterior neuropore Anterior neuropore Day 22 Optic vesicle Otocyst Cardiac swelling Amnion Mandibular Eye prominence Yolk sac Medial nasal Anterior limb prominence bud Tail Posterior limb bud Day 25 Day 32 Auricular hillocks Eyelid External auditory Maxillary meatus Lateral nasal prominence prominence Hand plate Day 44 Day 50 experimentally by isolating the different cell populations or induction) the inductive influences appear to be able to act tissues and recombining them in different ways in culture or in between cells separated by considerable distances and to consist transplants. From these studies it is known, for example, that of diffusible substances. It is known that inductive influences a median strip of mesoderm cells (the chordamesoderm) extend- need only be present for a short time, after which the respond- ing throughout the length of the embryo induces neural ing tissue is capable of independent development. For example, plate formation within the overlying ectoderm (Fig. 2.3). The an induced neural plate isolated in culture will roll up into prechordal plate is thought to have a similar role in the ante- a tube, which then differentiates into the brain, spinal cord, rior neural plate region. The nature of such inductive stimuli and other structures. is presently unknown. Sometimes cell-to-cell contact appears In addition to inducing neural plate formation, the to be necessary, whereas in other cases (as in neural plate chordamesoderm appears to be responsible for developing Chapter-02.indd 6 6/16/2011 4:04:15 PM  Development of Face and Oral Cavity 7 Fig. 2.2 Sketches summarizing development of embryos from fertilization through neural tube formation. Accumulation of fluid within egg cell mass (morula, C) leads to development of blastula (D). Inner cell mass (heavily strippled cells in D) will form two-layered embryonic disk in E. It now appears that only epiblast (ep) will form embryo (see text), with hypoblast (hy) and other cell populations forming support tissues (e.g., placenta) of embryo. In F, notochord (n) and its rostral (anterior) extension, prechordal plate (pp), as well as associated pharyngeal endoderm, form as a single layer. Prospective mesodermal cells migrate (arrows in F) through primitive streak (ps) and insert themselves between epiblast and endoderm. Epiblast cells remaining on surface become ectoderm. Cells of notochord (and pre- chordal plate?) and adjacent mesoderm (together termed chordamesoderm) induce overlying cells to form neural plate (neurectoderm). Only later does notochord separate from neural plate (G), while folding movements and differential growth (arrows in G and H) continue to shape embryo h, heart; b, buccal plate; op, olfactory placode; ef, eye field; nc, neural crest; so, somite; lp, lateral plate. (Modified from Johnston MC and Sulik KK: Embryology of the head and neck. In Serafin D and Georgiade NG, editors: Pediatric plastic surgery, vol. 1, St Louis, 1984, The CV Mosby Co). ep hy A B C D E h b Epilblast op pp ef nc Neurectoderm s n Ip Skin (surface) ectoderm ps ps F G ps H the organizational plan of the head. As noted previously, the to skeletal and connective tissues (i.e., cartilage, bone, dentin, notochord and prechordal plates arise initially within the endo- dermis, etc.). In the trunk, all skeletal and connective tissues derm (Fig. 2.3A), from which they eventually separate (Figs. are formed by mesoderm. Of the skeletal or connective tissue 2.2G and 2.3B). The mesodermal portion differentiates into of the facial region, it appears that tooth enamel (an acellular well-organized blocks of cells, called somites, caudal to the develop- skeletal tissue) is the only one not: formed by crest cells. The ing ear and less-organized somitomeres rostral to the ear (Figs. enamel-forming cells are derived from ectoderm lining the oral 2.2 and 2.6). Later these structures form myoblasts and some cavity. of the skeletal and connective tissues of the head. Besides The migration routes that cephalic (head) neural crest cells inducing the neural plate from overlying ectoderm, the chorda- follow are illustrated in Figure 2.4. They move around the sides mesoderm organizes the positional relationships of various neu- of the head beneath the surface ectoderm, en masse, as a sheet ral plate components, such as the initial primordium of the eye. of cells. They form all the mesenchyme* in the upper facial A unique population of cells develops from the ectoderm region, whereas in the lower facial region they surround meso- along the lateral margins of the neural plate. These are the dermal cores already present in the visceral arches. The pharyn- neural crest cells. They undergo extensive migrations, usually geal region is then characterized by grooves (clefts and pouches) beginning at about the time of tube closure (Fig. 2.3), and give in the lateral pharyngeal wall endoderm and ectoderm that rise to a variety of different cells that form components of many approach each other and appear to effectively segment the tissues. The crest cells that migrate in the trunk region form mostly neural, endocrine, and pigment cells, whereas those *Mesenchyme is defined here as the loosely organized embryonic that migrate in the head and neck also contribute extensively tissue, in contrast to epithelia, which are compactly arranged. Chapter-02.indd 7 6/16/2011 4:04:15 PM  8 Orban’s Oral Histology and Embryology Fig. 2.3 Scheme of neural and gastrointestinal tube formation in higher vertebrate embryos (section planes illustrated in Fig. 2.1). (A) Cross- section through three-germ layer embryo. Similar structures are seen in both head and trunk regions. Neural crest cells (diamond pattern) are initially located between neural plate and surface ectoderm. Arrows indicate directions of folding processes. (B) Neural tube, which later forms major components of brain and spinal cord, and gastrointestinal tube will separate from embryo surface after fusions are completed. Arrows indicate directions of migration of crest cells, which are initiated at about fourth week in human embryo. (C) Scanning electron micrograph (SEM) of mouse embryo neural crest cells migrating over neural tube and under surface ectoderm near junction of brain and spinal cord following removal of piece of surface ectoderm as indicated in B. Such migrating cells are frequently bipolar (e.g. outlined cell at end of leader) and oriented in path of migration (arrow). Neural crest Ectoderm Mesoderm Endoderm A Neural plate Notochord Neural crest Surface ectoderm Neural tube Neural tube Notochord Gastrointestinal tube B mesoderm into a number of bars that become surrounded by Almost all the myoblasts that subsequently fuse with each crest mesenchyme (Figs. 2.4C, D and 2.7A). other to form the multinucleated striated muscle fibers are Toward the completion of migration, the trailing edge of the derived from mesoderm. The myoblasts that form the hypo- crest cell mass appears to attach itself to the neural tube at loca- glossal (tongue) muscles are derived from somites located tions where sensory ganglia of the fifth, seventh, ninth, and beside the developing hindbrain. Somites are condensed masses tenth cranial nerves will form (Fig. 2.4C and D). In the trunk of cells derived from mesoderm located adjacent to the neural sensory ganglia, supporting (e.g., Schwann) cells and all neurons tube. The myoblasts of the extrinsic ocular muscles originate are derived from neural crest cells. On the other hand, many of from the prechordal plate (Fig. 2.2F). They first migrate to the sensory neurons of the cranial sensory ganglia originate poorly condensed blocks of mesoderm (somitomeres) located from placodes in the surface ectoderm (Fig. 2.4C and F). rostral to (in front of ) the otocyst, from which they migrate to Eventually, capillary endothelial cells derived from meso- their final locations (Fig. 2.6). The supporting connective tis- derm cells invade the crest cell mesenchyme, and it is from this sue found in facial muscles is derived from neural crest cells. mesenchyme that the supporting cells of the developing blood Much of the development of the masticatory and other facial vessels are derived. Initially, these supporting cells include only musculature is closely related to the final stages of visceral arch pericytes, which are closely apposed to the outer surfaces of development and will be described later. endothelial cells. Later, additional crest cells differentiate into A number of other structures in the facial region, such the fibroblasts and smooth muscle cells that will form the ves- as the epithelial components or glands and the enamel organ sel wall. The developing blood vessels become interconnected of the tooth bud, are derived from epithelium that grows to form vascular networks. These networks undergo a series of (invaginates) into underlying mesenchyme. Again, the connec- modifications, examples of which are illustrated in Figure 2.5, tive tissue components in these structures (e.g., fibroblasts, before they eventually form the mature vascular system. The odontoblasts, and the cells of tooth-supporting tissues) are underlying mechanisms are not clearly understood. derived from neural crest cells. Chapter-02.indd 8 6/16/2011 4:04:16 PM  Development of Face and Oral Cavity 9 Fig. 2.4 A and B, Migratory and C and D, postmigratory distributions of crest cells (stipple) and origins of cranial sensory ganglia. Initial gan- glionic primordia (C and D) are formed by cords of neural crest cells that remain in contact with neural tube. Section planes in C and E, pass through primordium of trigeminal ganglion. Ectodermal “thickenings,” termed placodes, form adjacent to distal ends of ganglionic primordia—for trigeminal (V) nerve as well as for cranial nerves VII, IX, and X. They contribute presumptive neuroblasts that migrate into previously purely crest cell ganglionic primordia. Distribution of crest and placodal neurons is illustrated in E and F (Adapted from Johnston MC and Hazelton RD: Embryonic origins of facial structures related to oral sensory and motor functions. From Bosma JB, editor: Third symposium on oral sensation and perception, Springfield, IL, 1972, Charles C Thomas Publisher). plate (Fig. 2.2F). Experimental evidence indicates that DEVELOPMENT OF FACIAL PROMINENCES the lateral edges of the placodes actively curl forward, which enhance the initial development of the lateral nasal On the completion of the initial crest cell migration and the prominence (LNP, sometimes called the nasal wing—see vascularization of the derived mesenchyme, a series of out- Fig. 2.7A). This morphogenetic movement combined with growths or swellings termed “facial prominences” initiates the persisting high rates of cell proliferation rapidly brings the next stages of facial development (Figs. 2.7 and 2.8). The LNP forward so that it catches up with the medial nasal prom- growth and fusion of upper facial prominences produce the pri- inence (MNP), which was situated in a more forward position mary and secondary palates. As will be described below, other at the beginning of its development (Fig. 2.7A and C). prominences developing from the first two visceral arches However, before that contact is made, the maxillary promi- considerably alter the nature of these arches. nence (MxP) has already grown forward from its origin at the proximal end of the first visceral arch (Figs. 2.7A and 2.13) to merge with the LNP and make early contact with the MNP Development of the frontonasal region: olfactory (Fig. 2.7G). With development of the lateral nasal promi- placode, primary palate, and nose nence—medial nasal prominence contact, all three promi- After the crest cells arrive in the future location of the upper nences contribute to the initial separation of the developing face and midface, this area often is referred to as the frontonasal oral cavity and nasal pit (Fig. 2.7C). This separation is usually region. The first structures to become evident are the olfactory called the primary palate (Fig. 2.9A to C). The combined right placodes. These are thickenings of the ectoderm that appear to and left maxillary prominences are sometimes called the inter- be derived at least partly from the anterior rim of the neural maxillary segment. Chapter-02.indd 9 6/16/2011 4:04:16 PM  10 Orban’s Oral Histology and Embryology Fig. 2.5 Development of arterial system serving facial region with emphasis on its relation to visceral arches. In 3-week human embryo visceral arches are little more than conduits for blood traveling through aortic arch vessels (indicated by Roman numerals according to the visceral arch containing them) from heart to dorsal aorta. Other structures indicated are eye (broken circle) and ophthalmic artery. In 6-week embryo first two aortic arch vessels have regressed almost entirely, and distal portions of arches have separated from heart. Portion of third aortic arch vessel adjacent to dorsal aorta persists and eventually forms stem of external carotid artery by fusing with stapedial artery. Stapedial artery, which develops from second aortic arch vessel, temporarily (in humans) provides arterial supply for embryonic face. After fusion with external carotid artery proximal portion of stapedial artery regresses. Aortic arch vessel of fourth visceral arch persists as arch of aorta. By 9 weeks primordium of definitive vascular system