Oral Cavity Microanatomy Notes 2024-25 PDF

Microanatomy of the Oral Cavity Rod D. Braun Page 1 of 41 MICROANATOMY OF THE ORAL CAVITY Lecture Learning Objectives: 1. Describe the lining of the oral cavity. Describe t...

Microanatomy of the Oral Cavity Rod D. Braun Page 1 of 41 MICROANATOMY OF THE ORAL CAVITY Lecture Learning Objectives: 1. Describe the lining of the oral cavity. Describe the lining of the oral cavity in terms of its mucosa, submucosa, and minor salivary glands. Compare and contrast the three types of oral mucosa and describe where each might be found. 2. Describe the microanatomy of the lip. Compare and contrast the cutaneous surface and the mucosal surface of the lip Describe the histological organization of the free red (vermilion) border of the lip. 3. Describe the microanatomy of the tongue. Describe the histological organization of the tongue, including the skeletal muscles and its dorsal and ventral surfaces. Describe the form and histological organization of the 4 types of papillae found on the dorsal and lateral tongue surface, including their taste bud content and any special relationship with glands. Describe the cellular organization of taste buds, their renewal, and their relationship with nerve processes. 4. Describe the microanatomy and functions of the major salivary glands. Describe the functions and composition of saliva. Describe the organization of a secretory unit in major salivary glands. Describe the major salivary glands in terms of their lobular organization (lobes, lobules). Describe and identify (LM & TEM) the two types of secretory cells found in the secretory acini of major salivary glands. Describe the location and role of myoepithelial cells in acini. Describe and identify (LM & TEM) the 3 types of secretory acini in major salivary glands and describe their secretions. Know the roles of plasma cells and serous acinar cells in secretion of sIgA. Describe the histological organization of the duct system of these glands, their identifying characteristics (LM & TEM), and the functions performed by the ducts. Describe the production of saliva. Compare and contrast the histological appearance (H&E) of the three major salivary glands. 5. Describe the microanatomy of teeth. Compare and contrast the crown and root of a tooth. Contrast the anatomical and clinical crown of a tooth. Describe the location and contents of the pulp cavity and its relationship to the apical foramen. Describe the location and histological organization of the 3 bone-like components of teeth, noting their degree of mineralization and the cells that produce them. Describe the histological organization of the gingiva and its special relationship to the tooth. Describe the histological organization of the periodontal membrane (ligament) and its function. Describe the role of Sharpey’s fibers in the periodontal membrane. Microanatomy of the Oral Cavity Rod D. Braun Page 2 of 41 Lecture Content Outline Overview of digestive system I. Oral cavity A. Components B. Divisions of oral cavity C. Oral mucosa D. Three types of oral mucosa E. Submucosa II. Lip A. Characteristics B. Three distinct zones in lip III. Tongue A. Skeletal muscle of tongue B. Minor salivary glands C. Ventral and dorsal surfaces D. Lingual papillae E. Tate buds IV. Major salivary glands A. Overview B. Organization C. Cells of secretory acini D. Secretory acini types E. Surrounding stroma F. Ducts G. Summary of saliva production H. Distinguishing characteristics of 3 major salivary glands V. Teeth A. Divisions B. Dental pulp C. Bone-like components 1. Dentin 2. Enamel 3. Cementum D. Gingiva E. Periodontal membrane or ligament Microanatomy of the Oral Cavity Rod D. Braun Page 3 of 41 MICROANATOMY OF ORAL CAVITY OVERVIEW OF DIGESTIVE SYSTEM A. Components 1. Alimentary canal: Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and anal canal 2. Principal organs associated with alimentary canal: tongue, teeth, major salivary glands, pancreas, liver, and gallbladder B. Functions 1. Functions include mechanical fragmentation and propulsion, chemical digestion of foodstuffs, absorption of nutrients, protective barrier (stratified epithelium or tight junctions), immunological protection (lymphatic tissue and secretory IgA), and lubrication (mucus). 2. Many of these functions are initiated within the oral cavity. I. ORAL CAVITY A. Components 1. Mouth 2. Associated structures: tongue, teeth, major and minor salivary glands, and tonsils B. Divisions of Oral Cavity: The oral cavity is divided into two sections (Figure 1) 1. Vestibule: space between lips, cheeks, and teeth Microanatomy of the Oral Cavity Rod D. Braun Page 4 of 41 Figure 1. Divisions of the oral cavity. Adapted from Figure 13.3 in Wheater’s Functional Histology (Young & th Heath, 4 ed., 2000). 2. Oral cavity proper: space behind the teeth and bounded superiorly by hard and soft palates, inferiorly by floor of the mouth, and posteriorly by entrance to the oropharynx. C. Oral Mucosa: The oral cavity is lined by oral mucosa, a moist epithelium resting on an underlying layer of loose connective tissue, called the lamina propria (Figure 2). 1. Stratified squamous epithelium a. Mostly non-keratinized (see Section ID) b. Cells include Figure 2. Right: Oral mucosa (epithelium with underlying keratinocytes, loose connective tissue of lamina propria). Left: 3 types of oral mucosa. Example on right is lining mucosa. Modified Langerhans cells, from Michigan Medical Histology (MacCallum, 2000). Merkel cells, and melanocytes. Microanatomy of the Oral Cavity Rod D. Braun Page 5 of 41 c. Layers: stratum basale, stratum spinosum, stratum superficiale (superficial cells - flattened, but nucleated). 2. Lamina propria: layer of loose connective tissue with blood vessels and nerves that underlies the epithelium. D. Three Types of Oral Mucosa (Figure 2) 1. Masticatory mucosa (Figure 3) a. Found on the gingiva (gums) and hard palate (Figure 2). b. Epithelium: keratinized and, Figure 3. LM of hard palate, showing masticatory mucosa, submucosa with palatine minor salivary glands, and underlying bone. Modified in some areas, from Michigan Medical Histology (MacCallum, 2000). parakeratinized stratified squamous epithelium. [Note: Parakeratinized epithelium is similar to keratinized, except superficial cells do not lose their nuclei. Instead the nuclei are pyknotic (highly condensed).] c. Lamina propria: loose connective tissue underlying the epithelium. 2. Lining mucosa a. Found on the lips, cheeks, floor of the mouth, inferior surfaces of the tongue, and soft palate (Figure 2). Microanatomy of the Oral Cavity Rod D. Braun Page 6 of 41 b. Epithelium: Generally nonkeratinized (Figure 2, right), but in some places it may be parakeratinized. c. Lamina propria: loose connective tissue underlying the epithelium. 3. Specialized mucosa a. Found on dorsal surface of tongue only (Figure 2). b. Epithelium: keratinized with lingual papillae and taste buds (see Section IV). c. Lamina propria: loose connective tissue underlying the epithelium. E. Submucosa (Figure 3) 1. Coarser dense irregular connective tissue underlying the mucosa. 2. Provides attachment of the mucosa to muscle (cheeks and lips) or bone (palate, dental arches). 3. Contains minor salivary glands a. Typically compound tubuloalveolar exocrine glands. b. May be mucous, serous, or mixed serous and mucous glands. c. Named for location: labial, buccal, lingual, palatine. Microanatomy of the Oral Cavity Rod D. Braun Page 7 of 41 II. LIP A. Characteristics 1. Entry point to alimentary canal (Figure 1). 2. Point where thin keratinized epidermis of facial skin changes to thick nonkeratinized Figure 4. Midline section through a lip. Modified from Figure 14-2A in Atlas epithelium of st of Histology (Cui et al., 1 ed., 2011). oral mucosa - a mucocutaneous junction (Figure 4). B. Three Distinct Zones in Lip (Figure 4) 1. Outer cutaneous surface - thin skin (stratified squamous keratinized epithelium) with hairs and sweat glands. 2. Vermilion (Red) border a. Transition zone between outer skin and inner oral mucosa (Figure 4). b. Keratinized epithelium with finger-like connective tissue projections (stromal papillae) from the lamina propria that Figure 5. Vermilion (red) border of lip. Note bring capillaries near the connective tissue stromal papillae that bring capillaries close to surface. From Michigan surface (Figure 5). Medical Histology (MacCallum, 2000). Microanatomy of the Oral Cavity Rod D. Braun Page 8 of 41 3. Inner oral mucosal surface a. Covered with a lining mucosa: Moist stratified squamous nonkeratinized epithelium with underlying lamina propria. b. Underneath mucosa is a submucosa that is bound to the underlying skeletal muscle, the orbicularis oris muscle (Figure 4). c. Minor salivary glands in submucosa: labial salivary glands (Figure 4). III. TONGUE Muscular organ projecting into oral cavity from its inferior surface. A. Skeletal muscle of tongue (Figure 6) 1. Arranged in bundles that run in 3 planes: longitudinal (SL, IL), vertical (V), and horizontal (oriented in and out of page Figure 6. Longitudinal section through tongue, showing skeletal muscle and dorsal (top) and ventral (bottom) surfaces. Plate 13- in Figure 6). th 5-1 from Color Atlas of Histology (Gartner & Hiatt, 6 ed., 2014). 2. Each arranged at right angles to the other two, i.e., muscles are arranged orthogonally. 3. Arrangement allows flexibility and precise movements essential for speech. Microanatomy of the Oral Cavity Rod D. Braun Page 9 of 41 B. Minor salivary glands: Lingual glands 1. Mucous, serous, or mixed 2. Usually embedded in muscle. C. Ventral and dorsal surfaces: Two surfaces of tongue are different (Figure 6) 1. Ventral surface: thin mucosa with smooth, non-keratinized epithelium (lining mucosa). Figure 7. Diagram of dorsal surface of tongue, showing location of 4 types of lingual papillae. Adapted from Figure 15-1 st in Histology and Cell Biology (Kierszenbaum, 1 ed., 2002). 2. Dorsal surface a. Covered by specialized oral mucosa: thick mucosa with keratinized epithelium and lingual papillae (mucosal irregularities and elevations – Figure 7). Microanatomy of the Oral Cavity Rod D. Braun Page 10 of 41 b. Divided into an anterior two-thirds and a posterior 1/3 by a V-shaped depression, the sulcus terminalis. c. Apex of V points posteriorly and is location of the foramen cecum (remnant of site at which embryonic formation of thyroid gland occurred). d. Anterior portion Figure 8. SEM of anterior portion of dorsal surface of covered with lingual tongue; tip at bottom. Filiform papillae (CP) and fungiform papillae (arrows - white spots) scattered across surface. MS: median sulcus. From Tissues and papillae (Figure 8). Organs (Kessel and Kardon, 1979). D. Lingual papillae: Four types of lingual papillae cover anterior 2/3 of dorsal surface (Figure 7) 1. Filiform (Figures 8, 9, and 10) a. Pointed mucosal projections distributed over entire anterior dorsal surface, with tips pointing backward Figure 9. SEM of filiform papillae (Fi) on dorsal tongue surface. White arrows: keratinized cells. From Tissues and (Figures 8 and 9). Organs (Kessel and Kardon, 1979). Microanatomy of the Oral Cavity Rod D. Braun Page 11 of 41 b. Smallest and most numerous papillae in humans. c. Serve only a mechanical role to increase friction between tongue and food; no taste buds! d. In H&E-stained sections, they appear as mucosal projections with keratinized epithelium and a connective tissue core (lamina propria) (Figure 10, top, FL). 2. Fungiform (Figures 8, 10, and 11) a. Mushroom-shaped mucosal projections. b. About 200 are scattered singly across dorsal surface among filiform papillae, but mainly Figure 10. Four types of lingual papillae. Top: Fg: concentrated at the tip and fungiform, FL: filiform, Center: CV: circumvallate. TB: taste buds. VE: serous glands of von Ebner, Bottom: lateral margins of the foliate, Gl: serous glands, M: skeletal muscle, D: ducts. Figures 13.11 and 13.12a in Wheater’s Functional Histology (Young & Heath, 4th ed., 2000) tongue (Figure 7). and Plate 46, Figure 1 in Histology: A Text and Atlas th (Ross et al., 4 ed., 2003). c. Associated with taste buds. i. Fungiform papillae contain from 0-22 taste buds on the superior surface (Figure 11). Microanatomy of the Oral Cavity Rod D. Braun Page 12 of 41 ii. Average of about 7 taste buds/papilla (Segovia et al., Dev Brain Res 138: 135–146, 2002). d. In H&E-stained sections, they appear as mushroom-shaped mucosal projections with keratinized epithelium and a connective tissue core (lamina propria) Figure 11. SEM of fungiform papillae among filiform (Figure 10, top, Fg). papillae. Top: No taste bud evident. Bottom: Arrow shows location of taste pore of taste bud on superior surface. From Tissues and Organs (Kessel and Kardon, 1979). e. Vascularity of lamina propria shows through; visible to eye as small red spots. 3. Circumvallate (vallate) a. 10-12 large (1-3 mm diameter) dome-shaped Figure 12. SEM of circumvallate papilla. CP: central papilla, W: wall, white arrows: moat, OTB: taste bud on papillae surrounded by superior surface (most are on lateral surface). From Tissues and Organs (Kessel and Kardon, 1979). moat-like invaginations (Figure 10, center, CV and Figure 12). Microanatomy of the Oral Cavity Rod D. Braun Page 13 of 41 b. Located just in front of (anterior to) the sulcus terminalis, the V-shaped boundary between anterior and posterior regions of the tongue (Figure 7). c. Contain many taste buds on lateral surface that faces moat (Figure 10, center, CV). May have a few taste buds on upper surface. d. Surrounded by a circular trench into which the serous (posterior lingual) glands of von Ebner (VE, Figure 10) empty. They function to: i. Flush material from moat to enable the taste buds to respond rapidly to changing stimuli. ii. Secrete lingual lipase - begins process of lipid hydrolysis in the mouth. 4. Foliate (Figure 10, bottom) a. Deep mucosal folds on lateral surface of tongue. b. Contain a dozen to hundreds of taste buds on lateral surfaces of folds. c. Less prominent in human than in other species. In humans: i. Localized to the posterior lateral edge of the tongue (Figure 7). ii. Prominent at birth, but rudimentary in adults. Microanatomy of the Oral Cavity Rod D. Braun Page 14 of 41 d. Serous glands of von Ebner (Gl, Figure 10, bottom) are also located around foliate papillae. As in circumvallate papillae, these glands secrete lingual lipase and function to rinse the cleft. E. Taste buds 1. Oval groups of sensory cells found primarily within the epithelium of papillae (Figures 10 and 13). 2. They open to the surface via taste pores (Figures 11, 14, and 15). 3. 3 cell types in taste buds (Figures Figure 13. SEM of taste bud. NE: neuroepithelial cell. From Tissues and Organs (Kessel and Kardon, 14 and 15): 1979). a. Neuroepithelial (sensory) cells i. Have apical microvilli (taste hairs). Figure 14. Diagram of a taste bud. Figure 16.5a Figure 15. LM of a taste bud. Figure 15.5a from from Histology: A Text and Atlas (Ross & Histology: A Text and Atlas (Ross & Pawlina, 6 th th Pawlina, 6 ed., 2011). ed., 2011). Microanatomy of the Oral Cavity Rod D. Braun Page 15 of 41 ii. Synapse on afferent nerve terminals. iii. Transduce taste impulses. b. Supporting cells i. Contain secretory granules. ii. Also have apical microvilli. iii. Less numerous than neuroepithelial cells. c. Peripheral or basal stem-type cell i. Small cell located at base of taste bud ii. Cells undergo rapid renewal and replace other cell types every 10-14 days. 4. Perception of taste includes a variety of transduction mechanisms including direct action on ion channels and interaction with G proteins and second messenger systems (Table 1). This topic will be covered in more detail in the Gustatory and Olfaction lecture in the HBF-III course. Taste Molecule(s) Receptor Type Salty Na+ Channels Sour HCl, H+ Channels Sweet Glucose G-protein coupled receptors Bitter Quinine G-protein coupled receptors Mono-sodium Umami G-protein coupled receptors glutamate Table 1. Tastes and receptor types used by taste buds. Microanatomy of the Oral Cavity Rod D. Braun Page 16 of 41 IV. MAJOR SALIVARY GLANDS A. Overview 1. Major salivary glands are large paired multicellular exocrine glands that produce saliva. 2. There are 3 sets of these multicellular exocrine glands in the oral cavity (Figure 16). a. Parotid glands i. Largest of the major salivary glands. ii. Located below and in Figure 16. Location of the 3 pairs of major salivary glands in oral cavity. Figure 16-1 from Atlas of Histology (Cui et front of ear. st al., 1 ed., 2011). iii. Ducts enter oral cavity opposite 2nd upper molar. b. Submandibular glands i. Located under either side of the floor of the mouth. ii. Ducts run along floor of mouth and empty just lateral to frenulum of tongue (midline fold from bottom of tongue to floor). Microanatomy of the Oral Cavity Rod D. Braun Page 17 of 41 c. Sublingual glands i. Smallest of the major salivary glands. ii. Located in floor of mouth anterior to submandibular glands. iii. Ducts empty into submandibular ducts as well as directly onto floor of mouth. 3. Compound tubuloalveolar exocrine glands with serous and/or mucous secretory acini surrounded by basal myoepithelial cells. 4. Glands secrete saliva in response to mechanical, chemical, or psychic stimuli. Salivary secretion is exclusively under neural control by the autonomic nervous system. You will learn more about this in the Regulation of the Alimentary Canal lecture. 5. Produce approximately 1 liter of saliva/day. 6. Functions of saliva a. Initial starch and triglyceride digestion. b. Lubrication of ingested food by mucus. c. Protection of the mouth and esophagus by dilution and buffering of ingested foods. d. Antimicrobial properties Microanatomy of the Oral Cavity Rod D. Braun Page 18 of 41 7. Composition of saliva a. Characterized by high concentrations of K+ and HCO3- and low concentrations of Na+ and Cl-. b. Digestive enzymes i. α-amylase (ptyalin): produced by major salivary glands and responsible for initial starch digestion ii. Lingual lipase: produced by lingual minor salivary glands and responsible for initial triglyceride digestion c. Mucins i. Sialomucin and sulfomucin (MUC5B) ii. Serve as lubricant and may have some protective functions d. Antimicrobial components i. Lysozyme: enzyme responsible for hydrolysis of bacterial walls ii. Histatins: histidine-rich antimicrobial and antifungal peptides iii. Cystatins: cysteine protease inhibitors that block the action of microbial proteases Microanatomy of the Oral Cavity Rod D. Braun Page 19 of 41 iv. Lactoferrin: iron-binding cationic glycoprotein that binds and destroys microbial cell membranes and chelates iron necessary for bacterial growth. v. Peroxidases: catalyze production of hypothiocyanite (OSCN-), a bactericidal and fungicidal agent vi. Secretory IgA (sIgA): neutralizes bacteria and viruses Figure 17. Components of a secretory unit in a major salivary gland. Adapted from Graphic 2-2 in Color Atlas of Histology (Gartner th & Hiatt, 6 ed., 2014). B. Organization of Major Salivary Glands 1. Secretory unit (Figure 17) a. Secretory cells (serous and/or mucous cells) are organized into spherical or tubular clusters called acini (see Section VC). Microanatomy of the Oral Cavity Rod D. Braun Page 20 of 41 b. Secretory product is collected from acini by intercalated ducts. c. Intercalated ducts are drained by striated ducts. d. Intercalated ducts and striated ducts modify the original product to form saliva. e. The acini, intercalated ducts, and striated ducts make up a secretory unit. f. Multiple secretory units combine to form a lobule (Figure 18). Figure 18. Lobular organization of a major salivary gland. Modified and adapted from Figure 17-1 Histology and Cell Biology st (Kierszenbaum, 1 ed., 2002). Microanatomy of the Oral Cavity Rod D. Braun Page 21 of 41 2. Lobular organization (Figure 18) a. Multiple secretory units composed of acini, intercalated ducts, and striated ducts are organized into lobules. i. Lobules are surrounded by connective tissue. ii. The saliva from each lobule is drained by a striated duct and collected by an interlobular duct (“duct between lobules”; a type of excretory duct). b. Multiple lobules are collected into a lobe, and the saliva from each lobe is collected by a lobar duct. c. Multiple lobes combine to form the gland, which delivers the collected saliva to the oral cavity via the main duct. C. Cells of secretory acini 1. Secretory cells: two types; one or both may be present in gland. a. Serous cells (Figure 19) i. Protein-producing Figure 19. Diagram and TEM of serous acinar cell. Adapted from Figure 17-4 in Histology and cells with prominent Cell Biology (Kierszenbaum, 1st ed., 2002) and Figure 16.25a from Histology: A Text and Atlas th rER and Golgi. (Ross & Pawlina., 6 ed., 2011). Microanatomy of the Oral Cavity Rod D. Braun Page 22 of 41 ii. Produce a watery secretion containing α- amylase (ptyalin) that breaks 1-4 glycoside bonds to begin the digestion of carbohydrates. iii. Also produce antimicrobial agents including lysozyme, histatins, cystatins, lactoferrin, peroxidases, and secretory IgA (sIgA) (Figure 19). b. Mucous cells i. Mucus-secreting cells. ii. Produce sialomucin and sulfomucin (glycoproteins) (Figure 20). 2. Myoepithelial cells a. Contractile cells that surround the base of acini and intercalated ducts Figure 20. Diagram and TEM of mucous acinar cell. Adapted from Figure 17-4 in Histology and Cell Biology (Kierszenbaum, 1st ed., 2002) and (Figures 17 and 18). Figure 15.25 from Histology: A Text and Atlas th (Ross et al., 4 ed., 2003). b. Stain eosinophilically, because of actin content (Figure 21, top). c. Enclosed within the basal lamina of the epithelial cells (Figure 21, bottom). Microanatomy of the Oral Cavity Rod D. Braun Page 23 of 41 d. Help move secretory product out of the acinus and through the intercalated duct toward the striated duct. D. Secretory acini types: There are three types of secretory acini, based on the types of secretory cells present (Figures 22 and 23). 1. Serous acini a. Contain only serous cells: protein- producing cells with prominent rER and Golgi (Figures 22 and 23). b. Acini generally spherical. Figure 21. LM (top, arrows) and TEM c. In H&E-stained sections, cells in (right) of myoepithelial cell (mi) in serous acinus of salivary gland. l: lumen. acinus appear stained due to staining of secretory granules and rER in cytoplasm. 2. Mucous acini a. Contain only mucous cells: mucus-secreting cells (Figures 22 and 23). Figure 22. 3 types of acini that can be found in the major salivary glands. Adapted from Figure 17-2 Histology and st Cell Biology (Kierszenbaum, 1 ed., 2002). Microanatomy of the Oral Cavity Rod D. Braun Page 24 of 41 b. Acini usually more tubular. c. In H&E-stained sections, cells in acinus appear pale since mucinogen granules do not stain. Figure 23. The three types of secretory acini found in major salivary glands. 3. Mixed acini a. Contain both serous and mucous cells (Figures 22 and 23). b. In H&E-stained material, the serous cells are seen as serous demilunes, crescent-shaped groups of serous cells that surround mucous acini (Figure 23). c. Note: Their product may flow via intercellular canaliculi (channels) into the central lumen, although recent studies suggest that both cell types may border similarly on the lumen. Microanatomy of the Oral Cavity Rod D. Braun Page 25 of 41 E. Surrounding stroma 1. Loose connective tissue surrounds the acini and ducts. 2. Plasma cells (Figure 22) a. Produce IgA, which is modified by serous acinar cells to form secretory IgA (sIgA), which is then released into the lumen (Figure 19). b. This immunologic protection is similar to the production of sIgA in the more distal GI tract (discussed in more detail in the Microanatomy of the Intestines lecture). F. Ducts 1. Intralobular ducts: modify salivary fluid and collect all of the fluid from the lobule (Figure 17). a. Intercalated ducts (Figure 24) i. Initial portion Figure 24. LM of parotid gland, showing longitudinal cut of duct through intercalated duct at lower right and a cross section through striated duct at left. ii. Small with simple low cuboidal epithelium. iii. Cells have carbonic anhydrase. They modify salivary fluid by secreting HCO3- into the fluid and reabsorbing Cl- out of the fluid (Figure 17). Microanatomy of the Oral Cavity Rod D. Braun Page 26 of 41 b. Striated (salivary) ducts i. Striated ducts are lined by simple cuboidal to simple columnar epithelium. ii. Cells modify salivary fluid by secreting HCO3- and K+ into the fluid and reabsorbing Na+ and Cl- out of the fluid (Figures 17 and 25, left). iii. Cells have carbonic anhydrase and express Na+-K+ ATPase in the basolateral plasma membrane (Figure 25). iv. The ATPase in cooperation with several cation channels function to secrete K+ into the salivary fluid and reabsorb Na+ from the fluid (Figure 25). Figure 25. Diagram of saliva production. Modified from Figure 8.12 in Physiology (Costanzo, 6th ed., 2018). Microanatomy of the Oral Cavity Rod D. Braun Page 27 of 41 v. Cl-/HCO3- exchangers and Cl- channels function to secrete HCO3- into the salivary fluid and reabsorb Cl- out of the fluid (Figures 25). vi. Striated duct cells, especially within basal infoldings, contain many mitochondria to supply ATP to Na+-K+ ATPase for pumping Na+ (Figure Figure 26. Diagram and TEM of striated 26). The many duct cell. Modified from Figures 17-4 and 17-5 in Histology and Cell Biology mitochondria make the st (Kierszenbaum, 1 ed., 2002). cells acidophilic (eosinophilic or pink/red in H&E) (Figure 27). iv. These features are characteristic of cells with ion-pumping activity. Figure 27. LM of striated duct. Note simple columnar epithelial cells with clear basal striations. 2. Excretory ducts: interlobular and larger a. These are extralobular ducts, i.e., ducts located outside of lobules, which are surrounded by connective tissue (Figure 28). Microanatomy of the Oral Cavity Rod D. Braun Page 28 of 41 b. Drain to primary or main ducts that enter oral cavity. c. Epithelium increases in height from simple columnar through pseudostratified columnar to stratified columnar Figure 28. LM of interlobular or excretory ducts. Note pseudostratified or stratified columnar (Figures 18 and 28). epithelium and surrounding connective tissue. NOTE: For a summary of the ducts in major salivary glands, please see Table 2 at the end of these notes. G. Summary of saliva production (Figure 25) 1. Initial saliva produced in acinus a. Initial saliva has a composition similar to blood plasma. b. This initial saliva is isotonic, i.e., has the same Na+, K+, Cl- and HCO3- concentrations as plasma (Fig. 25, step 1). 2. The intercalated ducts modify the initial saliva by recovering Cl- from the saliva and adding HCO3- to the saliva (Fig. 25, step 2). 3. The striated ducts modify the saliva by the following process (Fig. 25, step 3): a. The striated ducts reabsorb Na+ and Cl-. Therefore, the concentrations of these ions are lower than their plasma concentrations. Microanatomy of the Oral Cavity Rod D. Braun Page 29 of 41 b. The ducts secrete K+ and HCO3-. Therefore, their concentrations are higher than those in plasma. c. Aldosterone, a hormone secreted by the zona glomerulosa of the adrenal gland, acts on the ductal cells to increase the reabsorption of Na+ and the secretion of K+. d. Saliva becomes hypotonic in the ducts. i. More NaCl is absorbed than KHCO3 is secreted, i.e., there is net absorption of solute. ii. The ducts are relatively impermeable to water. iii. Since more solute than water is reabsorbed by the ducts, the saliva becomes dilute relative to plasma, i.e., hypotonic. e. The effect of flow rate on saliva composition is explained primarily by the changes in the contact time available for reabsorption and secretion processes to occur in the ducts. i. Thus, at high flow rates, saliva is most like the initial secretion from the acinus. It has the highest Na+ and Cl- concentrations and the lowest K+ concentration. Microanatomy of the Oral Cavity Rod D. Braun Page 30 of 41 ii. At low flow rates, saliva is least like the initial secretion from the acinus. It has the lowest Na+ and Cl- concentrations and the highest K+ concentration. iii. The only ion that does not “fit” this contact- time explanation is HCO3-. HCO3- secretion is selectively stimulated when saliva secretion is stimulated. 4. Salivary secretion is exclusively under neural control by the autonomic nervous system (See Regulation of the Alimentary Canal lecture). G. Distinguishing characteristics of 3 major salivary glands 1. Parotid gland (Figures 29 and 30) a. All serous: serous acini only. b. Often has adipose cell infiltration among secretory acini in lobules. c. Long intercalated ducts and many striated ducts. 2. Submandibular gland (Figures 31 and 32) a. Mixed secretion i. Mostly serous acini, with some mixed acini (mucous with serous demilunes) ii. Few purely mucous acini present b. Intercalated ducts are shorter than in the parotid.  Microanatomy of the Oral Cavity Rod D. Braun Page 31 of 41 Figure 29. Low mag LM of parotid gland. Note all serous acini. Figure 30. Higher mag LM of parotid gland. Note all serous acini. Figure 31. Low mag LM of submandibular gland. Mostly serous acini. Figure 32. Higher mag LM of submandibular gland. Mostly serous acini. Figure 33. Low mag LM of sublingual gland. Mostly mucous acini. Figure 34. Higher mag LM of sublingual gland. Mostly mucous acini. Microanatomy of the Oral Cavity Rod D. Braun Page 32 of 41 3. Sublingual gland (Figures 33 and 34) a. Mixed secretion i. Mostly mucous acini ii. Some mixed acini (serous demilunes) iii. Few purely serous acini present b. Intralobular ducts are poorly developed and are not as prominent. NOTE: For a summary of the distinguishing characteristics of the 3 types of major salivary glands, please see Table 3 at the end of these notes. Continued on next page. Microanatomy of the Oral Cavity Rod D. Braun Page 33 of 41 Figure 35. Schematic diagram of a section of an incisor tooth and surrounding bony and mucosal structures. Modified from Figure 16.7 from Histology: A Text and Atlas (Ross & Pawlina., 6th ed., 2011). V. TEETH A. Divisions 1. Crown (Figure 35) a. Anatomical crown: part of tooth covered by enamel. b. Clinical crown: part of the tooth that extends above the gums (gingiva). 2. Root: part of tooth that is embedded in an alveolus in the alveolar bone and is covered with cementum (Figure 35). Microanatomy of the Oral Cavity Rod D. Braun Page 34 of 41 B. Dental pulp 1. Loose connective tissue inside pulp cavity (pulp chamber, Figure 35). 2. Contains branches of nerves and blood vessels that enter the tooth at the apical foramen (Figure 35). 3. Outer edge lined by odontoblasts, which produce dentin (see Section C.1). C. Bone-like components: The dental pulp is almost completely covered by two layers of bone-like material. The innermost layer is a continuous covering of dentin, surrounding the pulp cavity. Over the dentin is a layer of either enamel or Figure 36. Top: Ground section of whole tooth. Lower left: Crown of tooth, showing enamel (1), dentin (2), pulp cavity (3), and cementum. The only gap in enamel-cementum junction (4). Lower right: Root of tooth, showing dentin (1), pulp cavity (2), apical foramen (3), and cementum (4). these layers is at the apical From Histology: A Text and Atlas (Rhodin, 1974). foramen (Figures 35 and 36). 1. Dentin: yellowish, semi-transparent material which surrounds the pulp cavity and forms the bulk of the tooth (Figures 35 and 36). a. Components i. Slightly harder than bone: 70% inorganic. ii. Most of the organic material consists of type I collagen fibers surrounded by ground substance and is masked after calcification. Microanatomy of the Oral Cavity Rod D. Braun Page 35 of 41 b. Dentin is produced by odontoblasts lining pulp cavity (Figures 37 and 38). c. Layer of odontoblasts retreats into pulp cavity as dentin is laid down (Figure 37), leaving processes embedded in spaces in dentin (dentinal tubules, Figure 38). i. Since dentinal tubules are channels Figure 37. Developing tooth. A: ameloblasts; O: in dentin, they are odontoblasts; DP: dental papilla (future pulp cavity); DM: dentin matrix; e: enamel; d: dentin. Arrows point to processes of odontoblasts. visible in ground Modified from Plate 13-4-4 from Color Atlas of th Histology (Gartner & Hiatt, 6 ed., 2014). sections (Figure 39). Figure 38. LM of dentin-pulp border. D: dentin with dentinal tubules, P: predentin (dentin matrix), O: odontoblasts, W: cell-free zone in pulp cavity. Figure 39. Dentin (right) and enamel (left) in Figure 13.6a in Wheater’s Functional Histology ground tooth section. From Michigan Medical th (Young & Heath, 4 ed., 2000). Histology. (MacCallum, 2000). ii. Dentinal tubules extend to the junction of dentin with enamel (dentinoenamel junction, Figure 39) or to the junction of dentin with cementum (Figure 36). Microanatomy of the Oral Cavity Rod D. Braun Page 36 of 41 d. Since dentin is produced by odontoblasts throughout the life of the tooth, the volume of the pulp cavity decreases with age. 2. Enamel: covers the anatomic crown (Figures 35 and 36) a. Components i. Hardest substance in body: 96-98% inorganic. ii. The small amount of organic material consists primarily of a unique glycoprotein called enamelin. iii. Acellular b. Comprised of 4 μm-diameter Figure 40. LM of enamel in ground tooth prisms (enamel rods) that run showing enamel rods (small horizontal arrows) and an incremental line of Retzius perpendicular to tooth surface (large diagonal arrow). From Michigan Medical Histology (MacCallum, 2000). (Figure 40). i. Enamel rods are made of calcium hydroxyapatite phosphate crystallite (0.1 μm diameter) Figure 41. TEM of hydroxyapatite crystals in enamel. (Figure 41). From Michigan Medical Histology (MacCallum, 2000). Microanatomy of the Oral Cavity Rod D. Braun Page 37 of 41 ii. Rods are surrounded by interprismatic substance, fit together in a key-hole pattern (Figure 41). iii. Rods are produced in daily increments during development by cells called ameloblasts (Figure 37), giving rise to incremental lines of Retzius (Figure 40). c. Ameloblasts produce enamel. i. Ameloblasts are part of the enamel organ (Figure 37), which consists of three layers. The innermost layer (inner epithelium) forms the ameloblasts. ii. Ameloblasts undergo apoptosis after enamel is fully formed (about time of tooth eruption). 3. Cementum: covers the root (Figure 36) a. Components i. Composition similar to bone: 50-65% inorganic salts ii. Contains type I collagen and mineralized ground substance (hydroxyapatite crystals). b. Secreted by cells called cementoblasts. i. Some become cementocytes when they are entrapped in cementum. They resemble osteocytes. Microanatomy of the Oral Cavity Rod D. Braun Page 38 of 41 ii. Other cementoblasts line up at the outer cemental surface along the length of the outer covering of the periodontal ligament. c. Cementum can be laid down throughout the life of the tooth. The growth is only appositional via the cementoblasts bordering the periodontal ligament. d. Unlike bone, cementum is avascular and aneural. e. Two types of cementum i. Acellular: Portion near junction with Figure 42. Acellular cementum near enamel- cementum junction (top) and cellular cementum near enamel is acellular apical foramen (bottom). From Michigan Medical Histology (MacCallum, 2000). (Figure 42, top). ii. Cellular cementum: Found in lower part of root near apical foramen (Figure 42, Figure 43. Ground section of tooth root, showing lacunae bottom); contains in cellular cementum. Dentin is at bottom. Inset: high magnification view of lacunae. From Michigan Medical cementocytes in Histology (MacCallum, 2000). lacunae with canaliculi, but there are no Haversian systems (Figure 43). Microanatomy of the Oral Cavity Rod D. Braun Page 39 of 41 D. Gingiva 1. The gingiva is masticatory mucosa surrounding the teeth (Figure 44). 2. Components a. Stratified squamous keratinized epithelium b. Underlying lamina propria (loose connective tissue) Figure 44. LM of gingiva in demineralized section. Note that enamel is missing, since 3. Functions it is almost completely mineral, while dentin (D, right half of slide) is still evident. FG: free gingiva, B: alveolar bone, C: cementum, a. Forms gingival attachment to D: dentin, E: enamel space, CEJ: cementum-enamel junction, PM: periodontal enamel via hemidesmosomes membrane (ligament), CE: crevicular epithelium (forms hemidesmosomes with (Figure 44). enamel). Modified from Figure 13.9 in Wheater’s Functional Histology (Young & th b. Functions as seal to prevent entrance of foreign materials into region between root and periodontal membrane (periodontal ligament, PM in Figure 44). E. Periodontal membrane or ligament 1. Dense irregular connective tissue between cementum of tooth and alveolar bone (Figures 35, 44, and Figure 45. The periodontal ligament (PL) links the 45). cementum (c) to the alveolar bone (A). d = dentin, SF = Sharpey’s fibers. From Plate 13-3-1 in Color th Atlas of Histology (Gartner & Hiatt, 6 ed., 2014). Microanatomy of the Oral Cavity Rod D. Braun Page 40 of 41 2. Functions a. Helps attach tooth to the bone (Figures 44 and 45). b. Serves as a suspensory ligament and prevents crushing of soft tissue near apex of tooth, i.e., near apical foramen. 3. Contains blood vessels, lymphatics, and nerves, especially proprioceptive nerves. 4. Sharpey's fibers i. The Type I collagen fibers that are embedded in cementum or bone are called Sharpey's fibers (Figure 45). ii. They serve to attach the ligament into the cementum or bone. References: Costanzo, LS. Physiology, 6th ed., Elsevier: Philadelphia, PA, 2018. Cui, D., Daley, W., Fratkin, J.D., Haines, D.E., Lynch, J.C. Atlas of Histology: With Functional and Clinical Correlations, 1st ed., Lippincott, Williams & Wilkins: Baltimore, 2011, Ch. 16. Gartner, L.P. and Hiatt, J.L. Color Atlas and Text of Histology, 6th ed., Lippincott, Williams and Wilkins: Baltimore, 2014, Ch. 4. Kessel, R.G. and Kardon, R.H., Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, 1979. Kierszenbaum, A.L., Histology and Cell Biology: An Introduction to Pathology, 1st ed., Mosby: St. Louis, 2002, Ch. 15. MacCallum, D.K., Michigan Medical Histology, University of Michigan, Ann Arbor, 2000. Rhodin, J.A.G., Histology: A Text and Atlas, Oxford University Press: New York, 1974. Microanatomy of the Oral Cavity Rod D. Braun Page 41 of 41 Ross, M.H., Kaye, G.I., and Pawlina, W., Histology: A Text and Atlas, 4th ed., Lippincott Williams & Wilkins: Philadelphia, 2003, Ch. 15. Ross, M.H. and Pawlina, W., Histology: A Text and Atlas, 6th ed., Lippincott Williams & Wilkins: Philadelphia, 2011, Ch. 16. Ross, M.H. and Pawlina, W., Histology: A Text and Atlas, 7th ed., Lippincott, Williams, & Wilkins, WoltersKluwer Health: Philadelphia, 2016, Ch. 16. (major source) Young, B. and Heath, J.W., Wheater’s Functional Histology: A Text and Colour Atlas, 4th ed., Churchill Livingstone: Edinburgh, 2000, Ch. 13. Ducts of the Major Salivary Glands Duct Location Epithelium Function Collects secretions from acini and delivers to striated ducts Intercalated Intralobular Low, simple cuboidal Cl- out of fluid HCO3- into fluid Simple cuboidal to Collects secretions from intercalated ducts Striated simple columnar Modifies saliva: Intralobular (salivary) (basal striations); Na+ & Cl- out of fluid acidophilic HCO3- & K+ into fluid Simple columnar to Extralobular pseudostratified Excretory (in connective Transports saliva to surface columnar to stratified tissue) columnar Table 2. Comparison of three types of ducts found in major salivary glands. Distinguishing the Three Major Salivary Glands Secretory Acini Intralobular Ducts Serous Interacalated Striated Adipose Gland Serous Mucous Demilunes? Ducts Ducts Tissue Large amounts Long and Large and Parotid All None No often present numerous conspicuous within lobules Shorter & less Not as Large and Submandibular Most (>) Some (