Digestive System I Oral Cavity PDF

16 Digestive System I: Oral Cavity and Associated Structures (Lip, Tooth, Tongue, Salivary Glands) Oral cavity...

16 Digestive System I: Oral Cavity and Associated Structures (Lip, Tooth, Tongue, Salivary Glands) Oral cavity – Tongue – Lip – Salivary glands – Tooth INTRODUCTION Digestive system performs functions of ingestion, digestion, and absorption of food and water. Digestive system consists of (Fig. 16.1): – Oral cavity and associated structures such as lips, teeth, tongue, and salivary glands – Gastrointestinal tract that includes esophagus, stomach, small and large intestines – Associated glands such as liver, gallbladder, and pancreas Oral cavity helps in ingestion of food and water with the help of saliva. Oral cavity is lined by nonkeratinized stratified squamous epithelium. This epithelium is supported by lamina propria. Lamina propria consists of loose areolar tissue. Part of oral cavity is lined by parakeratinized epithelium. This epithelium has superficial cells with nuclei. These cells get exfoliated. Lamina propria has few mucous and serous glands. Oral cavity is surrounded by salivary glands such as parotid, submandibular, and sublingual glands. Oral cavity also performs immunological functions with the help of palatine tonsil, lingual tonsil and other diffuse mucosa-associated lymphatic tissue. Fig.16.1: Upper part of oral cavity and pharynx. LIP Lip is externally lined by skin and internally by Lip is a mucomuscular fold having core made of mucous membrane. skeletal muscles (Fig. 16.2). Free margin of lip shows mucocutaneous junction. 183 16-Digestive System I.pmd 183 07/05/2019, 3:27 PM  184 Textbook of Human Histology Fig. 16.2: Histology of lower lip (sagittal section) (practice figure). Mucocutaneous junction is also called vermilion/red TEETH margin. Teeth are located in the alveolar sockets of maxilla Large number of blood vessels that are present and mandible. beneath epithelium give red color to the lip. In children there are 10 deciduous milk teeth in each Skin covering the lip is lined by stratified squamous jaw (2 incisors, 1 canine, and 2 molar teeth on each epithelium (epidermis), hair follicles, sweat glands, side of jaw), whereas in adults, there are 16 perma- and sebaceous glands. nent teeth in each jaw (2 incisors, 1 canine, 2 Mucous membrane of lip is lined by nonkeratinized premolar, and 3 molar teeth on each side of the jaw). stratified squamous epithelium. This epithelium is Alveolar processes of mandible and maxilla, and part thick and has prominent rete ridges. of teeth are covered by gum. Rete ridges are finger-like extensions of epithelium that projects into the underlying connective tissue. Histology of Tooth Long rete ridges help to withstand frictions. Tooth has three parts (Figs 16.3, 16.4 and Flowchart Parakeratinization: Mucosa of lip shows parakera- 16.1): tinization. 1. Crown: It lies superficial to gum and visible in Red margin is highly sensitive to the touch because oral cavity. of presence of more sensory receptors, such as 2. Root: It is covered by gum and bony alveolar Meissner's corpuscles. process. Clinical Correlation Fordyce' spots/granuloma: These are visible sebaceous glands seen in oral mucosa, lips, face, and genitals. These are physiologically seen in 70–80% people [John Fordyce, 1858–1925, American dermatologist]. Peutz-Jeghers syndrome is an autosomal dominant disorder that involves formation of polyps (abnormal growths) 16 in intestine and dark spots on lips and oral mucosa. Cheilitis is an inflammation of lips. Cheilitis most commonly involves angles of mouth. It results in red, cracked, and scaly lips. 16-Digestive System I.pmd 184 07/05/2019, 3:27 PM  Digestive System I: Oral Cavity and Associated Structures (Lip, Tooth, Tongue, Salivary Glands) 185 Flowchart 16.1: Structure and histology of tooth Fig. 16.3: Parts of tooth (schematic diagram). 3. Neck: It is a small part that lies at the junction of crown and neck. Tooth is made up of four specialized zones/coats as follows: 1. Enamel: It is the hardest substance of body. It covers crown of tooth. 2. Dentine: It forms a major portion of tooth. It lies deep to the enamel in crown and cementum in root. 3. Cementum: It is a bone-like calcified tissue. It is present in root of tooth and covers dentine. 4. Pulp cavity: Dentine encloses a cavity called pulp cavity. This cavity is filled with pulp that consists of connective tissue, blood vessels, and nerves. Pulp cavity continues as root canal in alveolar process. Periodontal ligament anchors cementum to alveolar socket. Enamel It is the hardest substance in the body It is made up of calcium hydroxyapatite (96–98%). Clinical crown is a part of enamel that is visible in the oral cavity. Enamel consists of rod-shaped prism of crystals of hydroxyapatite. Prisms radiate from dentinoenamel junction to enamel surface. Prismatic rod sheath: It is a thin layer of organic matrix that covers prism. 16 Ameloblasts: These cells form enamel. On eruption Fig. 16.4: Longitudinal section of dry tooth (practice figure). of tooth, ameloblasts are lost and there is no further Note: Draw this figure using HB pencil. deposition of enamel. 16-Digestive System I.pmd 185 09/05/2019, 3:39 PM  186 Textbook of Human Histology Contour lines of Retzius: Enamel is deposited in waves Cementum by ameloblasts. This results in growth lines at right Cementum covers dentine in the root of tooth. angle to the direction of enamel rods. These lies are It is a thin layer of bone-like material called contour lines/striae of Retzius [Anders Cementoblasts: These are large cuboidal cells that form Adolph Retzius, 1796–1860, Swedish anatomist]. cementum. Lines of Schreger: Alternating light and dark lines seen Periodontal ligament: It anchors cementum to the bony in enamel of the tooth that begin at the dentoenamel alveolar socket. junction and end before they reach the enamel surface; they may represent areas of enamel rods cut Cementocytes: These are mature cementoblasts. in cross-sections dispersed between areas of rods cut Cementocytes are similar to osteocytes of bone and longitudinally [John Hunter, 1728–1793, Scottish get trapped in lacunae of mineralized matrix of surgeon, anatomist]. cementum. Processes of cementocytes are connected Neonatal line: It is an incremental growth line that with that of adjacent cementocytes through represents the junction of enamel formed before and canaliculi. after birth. Sharpey's fibers: Collagen fibers of periodontal Enamel tufts: These are projections entering the ligament extend into cementum as Sharpey's fibers. enamel from dentoenamel junction. Pulp Enamel lamellae: These are projections entering from free surface of enamel. Pulp cavity of the tooth contains pulp. Enamel spindles: Dentineal tubules that extend into It consists of fibroblasts, macrophages, mast cells, enamel form enamel spindles. collagen fibers, and ground substance. Pulp receives neurovascular bundle through apical Dentine foramen of tooth. Dentine forms the wall of pulp cavity. Pulp cavity is lined by odontoblasts. It consists of 80% of inorganic salts and 20% organic Neurovascular bundle forms network deep to the material. odontoblast layer Hardness of dentine is similar to the bone. Dentine contains hydroxyapatite salts that provide Clinical Correlation hardness. Dental caries/tooth decay Odontoblasts: These are columnar cells that form It is the breakdown of tooth. dentine. Dental caries occurs in the form of pits or fissures. Dentineal tubules: Dentineal tubules radiate outward It occurs because of lack of proper oral hygiene. form pulp cavity to the outer wall of tooth. If the dental caries reaches up to pulp cavity, it Dentineal tubules are occupied by processes of becomes painful (because of sensory nerve endings). odontoblasts in living state. These tubules lie parallel to each other. Fibers of Tomes: Protoplasmic processes of odonto- TONGUE blasts that occupy the dentineal tubule are called fibers of Tomes. Q. Draw a well-labelled diagram of histology of tongue. Predentine: Newly formed layer of dentine by odontoblast is called predentine. In later stage, Q. Draw a well-labelled diagram of histology of predentine gets mineralized. circumvallate papilla. Incremental lines of von Ebner: These are less minera- Tongue is a mucomuscular organ having core of lized lines that separate layers of dentine. These lines skeletal muscles covered with mucous membrane. indicate rhythmic growth pattern of dentine. Tongue helps for taste, speech, mastication, and Granular layer of Tomes: It is a thin layer of dentine deglutination. adjacent to cementum. This layer appears granular on histological section [Sir John Tomes, 1815-1895, Gross Features of Tongue English dentist and anatomist]. Tongue is a conical-shaped organ having root, tip, Mantle dentine: It is less mineralized zone of dentine and body (Fig. 16.5). 16 near dentoenamel junction. Root of tongue is attached to mandible and hyoid Circumpulpal dentine: It forms the major part of bone. dentine and lies between predentine and mantle Tip of tongue is free and comes in contact with dentine. central incisors. 16-Digestive System I.pmd 186 07/05/2019, 3:27 PM  Digestive System I: Oral Cavity and Associated Structures (Lip, Tooth, Tongue, Salivary Glands) 187 Lingual papillae are projections of lamina propria covered by mucous membrane. There are four types of lingual papillae as follows: 1. Filiform papillae: These are most numerous and smallest papillae. They are minute conical projec- tions that are covered by keratinized stratified squamous epithelium. They do not contain taste buds (Fig. 16.7). 2. Fungiform papillae: These are mushroom-shaped projections projecting above filiform papillae. They are present mostly near the tip and can be easily identified as small pink spots. They have taste buds in their epithelium (Fig. 16.7). 3. Circumvallate papillae: They are large, dome- shaped, 8–12 papillae. They are located just anterior to the sulcus terminalis. Each papilla is surrounded by a circular sulcus/trench. Base of the trench has openings of ducts of serous glands of von Ebner (Fig. 16.8). Secretions of these glands rinse the sulcus. Wall of sulcus (moat-like invagination) has taste buds. Fig. 16.5: Features of dorsal surface of tongue. Core of the circumvallate papillae consists of connective tissue, vessels, and nerves. Body has dorsal and ventral surfaces. 4. Foliate papillae: They consist of parallel rows of Dorsal surface is divided into anterior two-third or ridges near the margin of tongue in front of sulcus oral part and posterior one-third or dorsal part by terminalis. Foliate papillae are rudimentary in V-shaped sulcus terminalis. humans (well developed in rabbits). Apex of the sulcus terminalis shows a small depres- sion called foramen cecum (point of embryonic thyroglossal duct). Box 16.1: Taste buds Dorsal surface of oral part is rough because of Taste buds are neurosensory epithelial structures (Fig. 16.9). number of surface elevations called papillae. Taste buds are present on fungiform and circumvallate Pharyngeal part shows elevation of lingual tonsils papillae of tongue, oropharynx, soft palate, and (aggregation of lymphatic follicles covered by epiglottis. mucosa). Structure Papillae of Tongue Taste buds extend through the thickness of epithelium. H&E staining: Taste buds appear as onion-like, oval, Dorsal surface of the body of tongue shows pale staining epithelial structures. projections called lingual papillae (Fig. 16.6). Fig. 16.6: Tongue papillae (practice figure). 16 16-Digestive System I.pmd 187 07/05/2019, 3:27 PM  188 Textbook of Human Histology Fig. 16.8: Photomicrograph. Circumvallate papilla (high magni- fication, H&E stain). Fig. 16.7: Photomicrograph. Filiform and fungiform papillae high Muscles of Tongue magnification, H&E stain). Core of tongue is formed by intrinsic and extrinsic striated muscles. Length: 50–80 m, width: 30–50 m Intrinsic muscles of tongue are arranged as Taste/gustatory pore: It is a small opening on the longitudinal, transverse, and vertical bundles. surface of taste bud. Along with these, extrinsic muscles also enter the Histologically, taste bud consists of three cells. tongue and run in various directions. · 1. Neuroepithelial/sensory cell: These are elongated cells Muscle bundles are supported by loose connective that extend from basal lamina of epithelium to the tissue, adipose tissue, mucous lingual glands, von taste pore. Their apical surface shows few microvilli. Ebner's serous glands, vessels, nerves, and lymphatics. Basal portion of these cells synapse with afferent sensory processes of facial, glossopharyngeal, and vagus nerves. Life span of these cell is 10 days. 2. Supporting cells: These are elongated cells that extend from basal lamina to the taste pore. They support receptor cells and secrete amorphous dense material at taste pore. 3. Basal/stem cells: These are small dark cells situated near the basal lamina. They multiply and replace supporting and receptor cells. Mechanism – Chemicals (tastants) stimulate receptor cells and generate signals for sweet, sour, salty, bitter, and delicious tastes. – Bitter, sweet, and delicious tastants activate G-protein coupled taste receptors. – Sour and salty tastants activate ion channels. Tip of tongue detect sweet taste, posterolateral to tip 16 detect salty atste, followed by sour taste. Circumvallate papillae detect bitter and delicious taste. Fig. 16.9: Taste bud. 16-Digestive System I.pmd 188 07/05/2019, 3:27 PM  Digestive System I: Oral Cavity and Associated Structures (Lip, Tooth, Tongue, Salivary Glands) 189 Flowchart 16.2: Histology of tongue Summary (Examination Guide) Tongue consists of mucous membrane that covers core of skeletal muscle fibers (Figs16.10, 16.11 and Flowchart 16.2). Mucous membrane has: – Lining epithelium – nonkeratinized stratified squamous epitheliumIdentification feature – Lamina propria – loose connective tissue Mucosa shows projecting papillae such as:Identification feature 1. Filiform: Small, conical projections with keratinized tips 2. Fungiform: Mushroom-shaped projections 3. Circumvallate: Dome-shaped papillae surrounded by circular sulcus 4. Foliate: Rudimentary papillae Taste buds: Fungiform and circumvallate papillae have – Minor salivary glands: These are small salivary taste buds that open as taste pores. Taste buds have glands located in lamina propria of lips, cheeks, receptor, supporting, and basal cells. soft palate, and tongue. Skeletal muscle fibers in tongue are arranged longi- Each salivary gland has duct and secretory unit/acini. tudinally, transversely, and vertically. Tongue also shows mucous lingual salivary glands and Histology of Salivary Gland von Ebner's serous salivary glands. Salivary gland consists of highly branched duct system and well-developed secretory acinar unit SALIVARY GLANDS (Fig. 16.12 and Flowchart 16.3). Salivary glands secrete saliva in the oral cavity. Acini are made up of pyramidal cells. Salivary glands include: Based on the nature of secretory cells, acini are classi- – Major salivary glands such as parotid, submandi- fied as serous acini, mucous acini, and mixed acini. bular, and sublingual glands. These glands open Glandular tissue is surrounded by connective tissue into oral cavity with the help of long ducts. capsule. Fig. 16.10: Histology of anterior part of tongue (longitudinal section, practice figure). 16 16-Digestive System I.pmd 189 07/05/2019, 3:27 PM  190 Textbook of Human Histology Fig. 16.11: Photomicrograph. Longitudinal section of anterior part of tongue (low magnification, H&E stain). Intercalated duct These are lined by low cuboidal epithelium. These ducts connect secretory acini with larger ducts. These are predominant in serous glands. Functions: Absorption of CI– ions and secretion of HCO3– ions in saliva. Striated ducts These ducts connect intercalated ducts with interlobular ducts (Fig. 16.13). These are lined by simple columnar epithelium. Basal portion of these epithelial cells shows numerous infoldings of plasma membrane. It helps in accommodation of numerous mitochondria and increasing surface area. On H&E staining, basal infoldings give striated appearance; hence, called striated duct. Striated ducts are predominantly present in serous glands. Functions: Resorption of Na+ and secretion of K+ and HCO3– in saliva. Fig. 16.12: Schematic representation of mixed salivary gland. Striated ducts are absent in mucous glands. Septa arising from capsule divide the gland into Excretory ducts number of lobes and lobules. They include interlobular, lobar, and main excretory ducts. Salivary Ducts These duct shows gradual epithelial transformation 16 Main salivary duct of the gland divides to form lobar, interlobular, and intralobular (striated and intercalated) ducts. from simple columnar to stratified cuboidal or columnar epithelium. Main excretory ducts open into the oral cavity. 16-Digestive System I.pmd 190 09/05/2019, 1:08 PM  Digestive System I: Oral Cavity and Associated Structures (Lip, Tooth, Tongue, Salivary Glands) 191 Flowchart 16.3: Histology of salivary glands. Fig. 16.13: Ultrastructure of a cell from striated duct. Secretory Acini Salivary glands are tubuloalveolar glands salivary gland have serous, mucous, and mixed acini. Serous acinus Serous acini secrete protein-rich, watery secretions. These acini are lined by pyramidal-shaped simple columnar epithelium. Serous cells have (Fig. 16.14): – Rounded basal nucleus – Large amount of rough endoplasmic reticulum 16 that gives basal basophilia – Well-developed Golgi complex and secretory vesicles that gives apical eosinophilia Fig. 16.14: Ultrastructure of serous and mucous cells. 16-Digestive System I.pmd 191 07/05/2019, 3:27 PM  192 Textbook of Human Histology Mucous acinus They have actin filaments in their cytoplasm. Mucous acini secrete thick mucoid secretions. Their contractions help the acini to expel the These acini are lined by pyramidal-shaped simple secretions. columnar epithelium. Differences between serous and mucous cells are Mucous cells have (Fig. 16.14): listed in Table 6.1. Differences between serous and – Flat, elongated and basally placed nucleus mucous acini are listed in Table 6.2. – Foamy cytoplasm (mucus removed during tissue processing) Parotid Gland – Mucous acini have wider lumen than serous Parotid gland is the largest salivary gland. acini It is covered by thick capsule and divided into Mixed acini number of lobes and lobules (Figs 16.15 and 16.16). Mixed acini secrete both mucus and enzyme-rich Main secretory duct/Stenson's duct opens into the watery secretions. vestibule of mouth opposite to the upper second Mixed gland has mucous acini with serous molar tooth. demilunes. Acini: Parotid gland mainly consists of serous Mixed acini are lined by mucous cells and these cells acini. These acini are surrounded by myoepithelial are capped by demilune of serous cells. All serous cells.Identification feature cells open into the lumen of acinus by fine canaliculi. Ducts: Parotid gland has number of intralobular New concept: Serous demilunes are preparation (intercalated and striated) ducts, and interlobular artifact that are formed during tissue processing and lobar ducts.Identification feature because of swelling of mucous cells. Intercalated ducts are lined by low cuboidal These swollen cells push serous cells to produce epithelium. serous demilune. Striated ducts are lined by simple columnar Myoepithelial cells epithelium with basal infoldings. These are also called basket cells or myoepithelium. Other ducts are lined by stratified cuboidal to These are star-shaped cells that are present between stratified squamous epithelium. epithelium and basal lamina of acinus. In between acini, adipose tissue is also present. 16 Fig. 16.15: Histology of parotid (serous) salivary gland (practice figure). 16-Digestive System I.pmd 192 07/05/2019, 3:27 PM  Digestive System I: Oral Cavity and Associated Structures (Lip, Tooth, Tongue, Salivary Glands) 193 Fig. 16.16: Photomicrograph. Histology of parotid (serous) gland (low magnification, H&E stain). Submandibular Gland Acini: Gland shows presence of both serous and Submandibular gland is mixed salivary gland. mucous acini. Few mucous acini also show serous demilunes.Identification feature It has a well-defined capsule. Septa arising from capsule divide the gland into number of lobes and Ducts: Gland has lobar, interlobular, and intralobular lobules (Figs 16.17 and 16.18). (striated and intercalated) ducts.Identification feature Fig. 16.17: Histology of submandibular (mixed) salivary gland (practice figure). 16 16-Digestive System I.pmd 193 07/05/2019, 3:27 PM  194 Textbook of Human Histology Fig. 16.18: Photomicrograph. Histology of submandibular (mixed) gland (low magnification, H&E stain). 16 Fig. 16.19: Histology of sublingual (predominantly mucous) salivary gland (practice figure). 16-Digestive System I.pmd 194 07/05/2019, 3:27 PM  Digestive System I: Oral Cavity and Associated Structures (Lip, Tooth, Tongue, Salivary Glands) 195 Fig. 16.20: Photomicrograph. Sublingual (predominantly mucous) salivary gland (low magnification, H&E stain. Intercalated ducts are less in number than in the Acini: Sublingual gland predominantly has mucous parotid gland. acini. Few serous and seromucous acini are also present. Sublingual Gland Mucous secretory units are tubular (not purely These are small masses of glandular tissue that lie in acinar). the substance of tongue. Ducts: Sublingual glands do not have intercalated Sublingual gland is not surrounded by well- defined duct (rarely present) and striated ducts are also very capsule (Figs 16.19 and 16.20). few. Interlobular and lobar ducts are present. 16 16-Digestive System I.pmd 195 07/05/2019, 3:27 PM