Histology: Digestive System I (PDF)

16 Digestive System I: Oral Cavity and Associated Structures OVERVIEW OF THE DIGESTIVE SYSTEM / 526 Major Salivary Glands / 550 ORAL CAVITY / 527 Saliva / 550 TONGUE / 529...

16 Digestive System I: Oral Cavity and Associated Structures OVERVIEW OF THE DIGESTIVE SYSTEM / 526 Major Salivary Glands / 550 ORAL CAVITY / 527 Saliva / 550 TONGUE / 529 Folder 16.1 Clinical Correlation: The Genetic Basis TEETH AND SUPPORTING TISSUES / 533 of Taste / 535 Enamel / 534 Folder 16.2 Clinical Correlation: Classification Cementum / 541 of Permanent (Secondary) and Deciduous Dentin / 542 (Primary) Dentition / 538 Dental Pulp and Central Pulp Cavity Folder 16.3 Clinical Correlation: Dental Caries / 546 (Pulp Chamber) / 543 Folder 16.4 Clinical Correlation: Salivary Gland Supporting Tissues of the Teeth / 544 Tumors / 553 SALIVARY GLANDS / 545 HISTOLOGY 101 / 554 Secretory Gland Acini / 545 Salivary Ducts / 548 O VE RVI E W O F T H E DIGESTIVE glands, food passes rapidly through the pharynx to the esoph- agus. The rapid passage of food through the pharynx keeps SYST E M it clear for the passage of air. The food passes more slowly The digestive system consists of the alimentary canal through the gastrointestinal tract, aided by the secretion and its principal associated organs, namely, the tongue, of digestive juices that may amount to 7 L or so per day. teeth, salivary glands, pancreas, liver, and gallbladder. Major During food transit through the stomach and small intestine, functions of the digestive system include transport of ingested the major alterations associated with digestion, solubilization, water and food along the alimentary canal; secretion of fluids, and absorption occur. Most of these fluids and nutrients are electrolytes, and digestive enzymes; digestion and absorption absorbed chiefly through the wall of the small intestine, but a of digested products; and excretion of indigestible remains. small portion is absorbed in the large intestine (see Fig. 16.1). Undigested food and other substances within the alimen- The lumen of the alimentary canal is physically and tary canal, such as mucus, bacteria, desquamated cells, and functionally external to the body. bile pigments are excreted as solids (feces). As it passes through the alimentary canal, food is broken The alimentary mucosa is the surface across which most down physically and chemically so that the degraded prod- substances enter the body. ucts can be absorbed into the body. The various segments of the alimentary canal are morphologically specialized for The alimentary mucosa performs numerous functions in its specific aspects of digestion and absorption. role as an interface between the body and the environment. Approximately 2 L of water and food are ingested into These functions include the following: the body each day (Fig. 16.1). After preliminary maceration, Secretion. The lining of the alimentary canal secretes, at moistening, and formation into a bolus by the actions of the specific sites, digestive enzymes, hydrochloric acid, mucin, structures of the oral cavity and by secretion of the salivary and antibodies. 526  O R A L C AV ITY The oral cavity consists of the mouth and its structures, which 527 Ingest include the tongue, teeth and their supporting structures (periodontium), major and minor salivary glands, and tonsils. CHAPTER 16 1200 mL water Saliva 1500 mL 800 g food pH 6.8–7.0 The oral cavity is divided into a vestibule and the oral cavity proper. The vestibule is the space between the lips, cheeks, and teeth. The oral cavity proper lies behind the teeth and is bounded by the hard and soft palates superiorly, the tongue and the floor of the mouth inferiorly, and the entrance to the oropharynx posteriorly. The three major salivary glands are paired structures; Digestive System I Gastric secretions they include the following: 2000 mL pH 1.5–3.0 Parotid gland, the largest of the three glands, located in the infratemporal region of the head. Its excretory duct, the parotid (Stensen’s) duct, opens at the parotid papilla, a small elevation on the mucosal surface of the Bile 500 mL cheek opposite the second upper molar tooth. Small intestine absorbs pH 7.8–8.0 Submandibular gland, located in the submandibular triangle of the neck. Its excretory duct, the submandib- 8500 mL ular (Wharton’s) duct, opens at a small fleshy promi- nence (the sublingual caruncle) on each side of the Pancreatic secretions lingual frenulum on the floor of the oral cavity. 1500 mL Sublingual gland, lying inferior to the tongue within the O R A L C AV I T Y pH 8.0–8.4 sublingual folds at the floor of the oral cavity. It has a num- 500 mL Intestinal secretions ber of small excretory ducts; some enter the submandibular Large intestine 1500 mL duct, and others enter individually into the oral cavity. absorbs pH 7.8–8.0 400 mL The parotid and submandibular glands have relatively long ducts that extend from the secretory portion of the gland to the oral cavity. The sublingual ducts are relatively short. The minor salivary glands are located in the submu- cosa of the oral cavity. They empty directly into the cavity Excreted via short ducts and are named for their location (i.e., buccal, 100 mL water labial, lingual, and palatine). 50 g solids The tonsils consist of aggregations of lymphatic nodules FIGURE 16.1 ▲ The alimentary canal and its function in that are clustered around the posterior opening of the oral secretion and absorption of fluids. This schematic diagram shows and nasal cavities. regions of the alimentary canal with associated exocrine glands that contribute to secretion of digestive juices. Almost all of the absorption of Lymphatic tissue is organized into a tonsillar (Waldeyer’s) fluids, electrolytes, and nutrients occurs in the small intestine. ring of immunologic protection located at the shared entrance to the digestive and respiratory tracts. This lymphatic tissue sur- rounds the posterior orifice of the oral and nasal cavities and con- Absorption. The epithelium of the mucosa absorbs tains aggregates of lymphatic nodules that include the following: metabolic substrates (e.g., the breakdown products of di- gestion) as well as vitamins, water, electrolytes, recyclable Palatine tonsils, or simply the tonsils, which are located at either side of the entrance to the oropharynx materials such as bile components and cholesterol, and between the palatopharyngeal and palatoglossal arches other substances essential to the functions of the body. Barrier. The mucosa serves as a barrier to prevent the Tubal tonsils, which are located in the lateral walls of the nasopharynx posterior to the opening of the auditory tube entry of noxious substances, antigens, and pathogenic organisms. Pharyngeal tonsil, or adenoid, which is located in the roof of the nasopharynx Immunologic protection. Lymphatic tissue within the Lingual tonsil, which is located at the base of the tongue mucosa serves as the body’s first line of immune defense. on its superior surface The functions listed above are discussed at the beginning of the next chapter. The digestive system is considered in The oral cavity is lined by the oral mucosa that consists of three chapters that deal, respectively, with the oral cavity and masticatory mucosa, lining mucosa, and specialized mucosa. pharynx (this chapter); the esophagus and gastrointestinal The masticatory mucosa is found on the gingiva (gums) tract (Chapter 17); and the liver, gallbladder, and pancreas and the hard palate (Fig. 16.2). It has a keratinized and, (Chapter 18). in some areas, a parakeratinized stratified squamous  incisive papilla epithelium of the masticatory mucosa resembles that of the skin but lacks a stratum lucidum. The underlying lamina 528 propria consists of a thick papillary layer of loose connec- masticatory tive tissue that contains blood vessels and nerves, some of mucosa which send bare axon endings into the epithelium as sensory O R A L C AV I T Y fatty zone receptors, and some of which end in Meissner’s corpuscles. gingiva Deep to the lamina propria is a reticular layer of denser connective tissue. hard palate As in the skin, the depth and number of connective tissue glandular papillae contribute to the relative immobility of the masti- zone raphe catory mucosa, thus protecting it from frictional and shear- ing stress. At the midline of the hard palate, in the palatine raphe, the mucosa adheres firmly to the underlying bone. Digestive System I The reticular layer of the lamina propria blends with the soft palate periosteum, and thus, there is no submucosa. The same is true of the gingiva. Where there is a submucosa underlying the lamina propria on the hard palate (see Fig. 16.2), it con- tains adipose tissue anteriorly (fatty zone) and mucous glands posteriorly (glandular zone) that are continuous with those of FIGURE 16.2 ▲ Roof of oral cavity. The hard palate, which con- the soft palate. In the submucosal regions, thick collagenous tains bone, is bisected into right and left halves by a raphe. Anteriorly, in the fatty zone, the submucosa of the hard palate contains adipose tissue; bands extend from the mucosa to the bone. posteriorly, in the glandular zone, there are mucous glands within the Lining mucosa is found on the lips, cheeks, alveolar submucosa. Neither the raphe nor the gingiva contains a submucosa; mucosal surface, floor of the mouth, inferior surfaces of the instead, the mucosa is attached directly to the bone. The soft palate has tongue, and soft palate. At these sites, it covers striated muscle muscle instead of bone, and its glands are continuous with those of the (lips, cheeks, and tongue), bone (alveolar mucosa), and glands hard palate in the submucosa. CHAPTER 16 (soft palate, cheeks, inferior surface of the tongue). The lining mucosa has fewer and shorter papillae so that it can adjust to epithelium (Fig. 16.3). Parakeratinized epithelium is simi- the movement of its underlying muscles. lar to keratinized epithelium except that the superficial cells Generally, the epithelium of the lining mucosa is non- do not lose their nuclei and their cytoplasm does not stain keratinized, although in some places, it may be parakera- intensely with eosin (Plate 48, page 556). The nuclei of the tinized. The epithelium of the vermilion border of the lip parakeratinized cells are pyknotic (highly condensed) and re- (the reddish portion between the moist inner surface and main until the cell is exfoliated (see Fig. 16.3). The keratinized the facial skin) is keratinized. The nonkeratinized lining epi- thelium is thicker than keratinized epithelium. It consists of only three layers: parakeratinized keratinized epithelium epithelium Stratum basale, a single layer of cells resting on the basal lamina Stratum spinosum, which is several cells thick Stratum superficiale, the most superficial layer of cells, also referred to as the surface layer of the mucosa The cells of the mucosal epithelium are similar to those keratohyalin of the epidermis of the skin and include keratinocytes, granule–containing cells Langerhans’ cells, melanocytes, and Merkel’s cells. The lamina propria contains blood vessels, nerves that send bare axon endings into the basal layers of the epithelium, and encapsulated sensory endings in some papillae. The sharp contrast between the numerous deep papillae of the alveolar mucosa and the shallow papillae in the rest of the lining mu- cosa allows easy identification of the two different regions in a histologic section. A distinct submucosa underlies the lining mucosa ex- FIGURE 16.3 ▲ Stratified squamous epithelium of the hard cept on the inferior surface of the tongue. This layer con- palate. This photomicrograph shows a transition in the oral mucosa from a tains large bands of collagen and elastic fibers that bind stratified squamous epithelium (on the right) to a stratified squamous parake- the mucosa to the underlying muscle; it also contains the ratinized epithelium (on the left). The flattened surface cells of the keratinized many minor salivary glands of the lips, tongue, and cheeks. epithelium are devoid of nuclei. The layer of keratohyalin granule–containing Occasionally, sebaceous glands not associated with a hair cells is clearly visible in this type of epithelium. The flattened surface cells of the parakeratinized epithelium display the same characteristics as the keratinized follicle are found in the submucosa just lateral to the corner cells, except they retain their nuclei, that is, they are parakeratinized. In addition, of the mouth and in the cheeks opposite the molar teeth. note the paucity of keratohyalin granules present in the subsurface cells. !380. They are visible to the eye and are called Fordyce spots. The submucosa contains the larger blood vessels, nerves, and lymphatic vessels that supply the subepithelial neuro- vascular networks in the lamina propria throughout the oral 529 cavity. Specialized mucosa is associated with the sensation CHAPTER 16 of taste and is restricted to the dorsal surface of the tongue. palatine It contains papillae and taste buds responsible for generat- tonsil epiglottis ing the chemical sensation of taste. Oral mucosa forms an important protective barrier between the external environment of the oral cavity and in- lingual tonsil ternal environments of the surrounding tissues. It is resistant to the pathogenic organisms that enter the oral cavity and to indigenous microorganisms residing there as microbial flora. foramen Digestive System I Epithelial cells, migratory neutrophils, and saliva all contrib- cecum ute to maintaining the health of the oral cavity and protecting circumvallate the oral mucosa from bacterial, fungal, and viral infections. papillae The protective mechanisms include several salivary anti- microbial peptides, the "-defensins expressed in the epi- foliate thelium, the #-defensins expressed in neutrophils, and the papillae secretory immunoglobulin A (sIgA). However, in individu- als with immunodeficiency or those undergoing antibiotic therapy, in which the balance between microorganisms and protective mechanisms is disrupted, oral infections are rather fungiform common. papillae TONGUE TONGUE The tongue is a muscular organ projecting into the oral cavity from its inferior surface. Lingual muscles (i.e., mus- cles of the tongue) are both extrinsic (having one attachment outside of the tongue) and intrinsic (confined entirely to the FIGURE 16.4 ▲ Human tongue. Circumvallate papillae are po- tongue, without external attachment). The striated muscle of sitioned in a V configuration, separating the anterior two-thirds of the the tongue is arranged in bundles that generally run in three tongue from the posterior third. Fungiform and filiform papillae are on planes, with each arranged at right angles to the other two. the anterior portion of the dorsal tongue surface. The uneven contour of the posterior tongue surface is attributable to the lingual tonsils. The This arrangement of muscle fibers allows enormous flexibility palatine tonsil is at the junction between the oral cavity and the pharynx. and precision in the movements of the tongue, which are essential to human speech as well as to its role in digestion and swallowing. This form of muscle organization is found page 558). This epithelium does not contain taste buds. only in the tongue, which allows easy identification of this The papillae serve only a mechanical role. Filiform papil- tissue as lingual muscle. Variable amounts of adipose tissue lae are distributed over the entire anterior dorsal surface are found among the muscle fiber groups. of the tongue, with their tips pointing backward. They Grossly, the dorsal surface of the tongue is divided appear to form rows that diverge to the left and right into an anterior two-thirds and a posterior one-third by a from the midline and that parallel the arms of the sulcus V-shaped depression, the sulcus terminalis (Fig. 16.4). terminalis. The apex of the V points posteriorly and is the location of the foramen cecum, the remnant of the site from which an Fungiform papillae, as the name implies, are mush- room-shaped projections located on the dorsal surface of evagination of the floor of the embryonic pharynx occurred the tongue (Fig. 16.5b). They project above the filiform to form the thyroid gland. papillae, among which they are scattered, and are just vis- Papillae cover the dorsal surface of the tongue. ible to the unaided eye as small spots (see Fig. 16.4 and Plate 50, page 560). They tend to be more numerous near Numerous mucosal irregularities and elevations called the tip of the tongue. Taste buds are present in the strati- lingual papillae cover the dorsal surface of the tongue fied squamous epithelium on the dorsal surface of these anterior to the sulcus terminalis. The lingual papillae and papillae. their associated taste buds constitute the specialized mu- cosa of the oral cavity. Four types of papillae are described: Circumvallate papillae are the large, dome-shaped struc- tures that reside in the mucosa just anterior to the sulcus filiform, fungiform, circumvallate, and foliate. terminalis (see Fig. 16.4). The human tongue has 8 to 12 Filiform papillae are the smallest and most numerous of these papillae. Each papilla is surrounded by a moat-like in humans. They are conical, elongated projections of invagination lined with stratified squamous epithelium connective tissue that are covered with highly keratinized that contains numerous taste buds (Fig. 16.5d). Ducts of stratified squamous epithelium (Fig. 16.5a and Plate 49, lingual salivary (von Ebner’s) glands empty their  foliate papilla filiform papillae 530 TONGUE taste buds ducts Digestive System I serous glands striated a muscle c fungiform papilla taste buds circumvallate papilla CHAPTER 16 serous glands b d FIGURE 16.5 ▲ Lingual papillae. a. Structurally, the filiform papillae are posteriorly bent conical projections of the epithelium. These papil- lae do not possess taste buds and are composed of stratified squamous keratinized epithelium. !45. b. Fungiform papillae are slightly rounded, elevated structures situated among the filiform papillae. A highly vascularized connective tissue core forms the center of the fungiform papilla and projects into the base of the surface epithelium. Because of the deep penetration of connective tissue into the epithelium (arrows), combined with a very thin keratinized surface, the fungiform papillae appear as small red dots when the dorsal surface of the tongue is examined by gross inspection. !45. c. In a section, foliate papillae can be distinguished from fungiform papillae because they appear in rows separated by deep clefts (arrows). The foliate papillae are covered by stratified squamous nonkeratinized epithelium containing numerous taste buds on their lateral surfaces. The free surface epithelium of each papilla is thick and has a number of secondary connective tissue papillae projecting into its undersurface. The connec- tive tissue within and under the foliate papillae contains serous glands (von Ebner’s glands) that open via ducts into the cleft between neighboring papillae. !45. d. Circumvallate papillae are covered by stratified squamous epithelium that may be slightly keratinized. Each circumvallate papilla is surrounded by a trench or cleft. Numerous taste buds are on the lateral walls of the papillae. The dorsal surface of the papilla is smooth. The deep trench surrounding the circumvallate papillae and the presence of taste buds on the sides rather than on the free surface are features that distin- guish circumvallate from fungiform papillae. The connective tissue near the circumvallate papillae also contains many serous-type glands that open via ducts into the bottom of the trench. !25. serous secretion into the base of the moats. This secretion The dorsal surface of the base of the tongue exhibits presumably flushes material from the moat to enable the smooth bulges that reflect the presence of the lingual tonsil in taste buds to respond rapidly to changing stimuli. the lamina propria (see Fig. 16.4). Foliate papillae consist of parallel low ridges sepa- Taste buds are present on fungiform, foliate, and circum- rated by deep mucosal clefts (see Fig. 16.5c and Plate 50, vallate papillae. page 560), which are aligned at right angles to the long axis of the tongue. They occur on the lateral edge of the In histologic sections, taste buds appear as oval, pale- tongue. In aged individuals, the foliate papillae may not be staining bodies that extend through the thickness of the recognized; in younger individuals, they are easily found epithelium (Fig. 16.6). A small opening onto the epi- on the posterior lateral surface of the tongue and contain thelial surface at the apex of the taste bud is called the many taste buds in the epithelium of the facing walls taste pore. of neighboring papillae (Fig. 16.4). Small serous glands Three principal cell types are found in taste buds: empty into the clefts. In some animals, such as the rabbit, Neuroepithelial (sensory) cells are the most numer- foliate papillae constitute the principal site of aggregation ous cells in the taste bud. These elongated cells extend of taste buds. from the basal lamina of the epithelium to the taste  taste pore taste pore microvilli 531 CHAPTER 16 surface epithelial microvilli cells supporting cells sensory cells sensory cells Digestive System I supporting cells afferent basal basal cell cell nerve fibers nerve nerve basal cells TONGUE a b FIGURE 16.6 ▲ Diagram and photomicrograph of a taste bud. a. This diagram of a taste bud shows the neuroepithelial (sensory), sup- porting, and basal cells. One of the basal cells is shown in the process of dividing. Nerve fibers have synapses with the neuroepithelial cells. (Based on Warwick R, Williams PL, eds. Gray’s Anatomy, 35th ed. Edinburgh: Churchill Livingstone, 1973.) b. This high-magnification photomicrograph shows the organization of the cells within the taste bud. The sensory and supporting cells extend through the full length of the taste bud. The apical surface of these cells contains microvilli. The basal cells are located at the bottom of the taste bud. Note that the taste bud opens at the surface by means of a taste pore. !1,100. pore, through which the tapered apical surface of each Taste is a chemical sensation in which various chemicals cell extends microvilli (see Fig. 16.6). Near their apical elicit stimuli from neuroepithelial cells of taste buds. surface, they are connected to neighboring neuroepithe- Taste is characterized as a chemical sensation in which lial or supporting cells by tight junctions. At their base, various tastants (taste-stimulating substances) contained they form a synapse with the processes of afferent sensory in food or beverages interact with taste receptors located neurons of the facial (cranial nerve VII), glossopha- at the apical surface of the neuroepithelial cells. These cells ryngeal (cranial nerve IX), or vagus (cranial nerve X) react to five basic stimuli: sweet, salty, bitter, sour, and nerves. The turnover time of neuroepithelial cells is umami [Jap. delicious]. The molecular action of tastants about 10 days. can involve opening and passing through ion channels (i.e., Supporting cells are less numerous. They are also elon- salt and sour), closing ion channels (sour), or acting on a gated cells that extend from the basal lamina to the taste specific G protein–coupled taste receptor (i.e., bitter, sweet, pore. Like neuroepithelial cells, they contain microvilli on and umami). their apical surface and possess tight junctions, but they do not synapse with the nerve cells. The turnover time of Stimulation of bitter, sweet, and umami receptors activates supporting cells is also about 10 days. G protein–coupled taste receptors that belong to T1R and Basal cells are small cells located in the basal portion T2R chemosensory receptor families. of the taste bud, near the basal lamina. They are the stem Bitter, sweet, and umami tastes are detected by a variety of cells for the two other cell types. receptor proteins encoded by the two taste receptor genes (T1R and T2R). Their products are all characterized as being In addition to those associated with the papillae, taste G protein–coupled taste receptors. buds are also present on the glossopalatine arch, the soft palate, the posterior surface of the epiglottis, and the pos- Bitter taste is detected by about 30 different types of T2R terior wall of the pharynx down to the level of the cricoid chemosensory receptors. Each receptor represents a cartilage. single transmembrane protein coupled to its own G protein.  After receptor activation by the tastant, the G protein stim- they are also composed of two subunits. One subunit, ulates the enzyme phospholipase C, leading to increased T1R3, is identical to that in the sweet receptor, but the 532 intracellular production of inositol 1,4,5-trisphosphate second subunit formed by the T1R1 protein is unique (IP3), a second messenger molecule. IP3 in turn activates for umami receptors (see Fig. 16.7a). The transduction taste-specific Na! channels causing influx of Na$ process is identical to that described previously for bitter TONGUE ions, thus depolarizing the neuroepithelial cell. Depolariza- taste pathways. Monosodium glutamate, added to many tion of the plasma membrane causes voltage-gated Ca2! foods to enhance their taste (and the main ingredient of channels in neuroepithelial cells to open. Increasing the soy sauce), stimulates the umami receptors. concentration of intracellular Ca2$ levels either by influx of extracellular Ca2$ into the cell (the effect of depolarization) The mechanism of transduction can be similar to several or by its release from intracellular stores (direct IP3 stimula- tastes (i.e., bitter or sweet); however, it is important to Digestive System I tion) results in the release of neurotransmitter molecules, remember that neuroepithelial cells selectively express only which generate nerve impulses along the gustatory afferent one class of receptor proteins. Therefore, the messages about nerve fiber (Fig. 16.7a). bitterness or sweetness from eating food are transferred to the CNS along different nerve fibers. Sweet taste receptors are also G protein–coupled recep- tors. In contrast to the bitter taste receptors, they have Sodium ions and hydrogen protons, which are responsible for two protein subunits, T1R2 and T1R3. The sweet tastants salty and sour taste, respectively, act directly on ion channels. bound to these receptors activate the same second mes- Signaling mechanisms, in the case of sour and salty tastes, are senger system cascade of reactions that the bitter receptors similar to other signaling mechanisms found in synapses and do (see Fig. 16.7a). neuromuscular junctions. Umami taste is linked to certain amino acids (e.g., L-glutamate, aspartate, and related compounds) and is Sour taste is generated by H$ protons that are formed by common to asparagus, tomatoes, cheese, and meat. hydrolysis of acidic compounds. The H$ proton primary CHAPTER 16 Umami taste receptors are very similar to sweet receptors; blocks K! channels that are responsible for generating bitter sweet umami sour salt taste receptor H+ H+ Ca2+ Na+ Na+ Ca2+ T1R2 T1R3 T1R1 T1R3 G protein T2Rs taste-specific H+ H+ Ca2+ Na+ Na+ Ca2+ Na+ channel K+ channel PLC IP2 + + taste-specific Na Na IP3 H+ channels Ca2+ stores synaptic synaptic vesicles vesicles Ca2+ Ca2+ synaptic vesicles a b c gustatory afferent nerve fiber Ca2+ H+ Na+ tastants voltage-gated amiloride-sensitive voltage-sensitive Ca2+ channel Na+ channel Na+ channel FIGURE 16.7 ▲ Diagram of taste receptors and their signaling mechanism. a. This diagram shows the signaling mechanism of bitter, sweet, and umami receptors in the neuroepithelial cells. These cells selectively express only one class of receptor proteins; for simplicity, all three taste recep- tors are depicted in the apical cell membrane. See text for details. PLC, phospholipase C; IP2, inositol-1,4-diphosphate; IP3, inositol 1,4,5-trisphosphate. b. Signaling mechanism in sour sensation is generated by H$ protons that primarily block K$ channels. The H$ protons enter the cell via amiloride- sensitive Na$ channels and through taste-specific H$ channels (PKD1L3 and PKD2L1) exclusively expressed in cells involved in sour taste transduction. c. Salty sensation derives from Na$ ions that enter the neuroepithelial cells through the amiloride-sensitive Na$ channels. Intracellular Na$ causes a depolarization of membrane and activation of additional voltage-sensitive Na$ and Ca2$ channels. Calcium-mediated release of neurotransmitters from synaptic vesicles results in stimulating gustatory nerve fiber.  the cell membrane potential that causes depolarization (cranial nerve V). General sensation for the posterior one- of the cell membrane. In addition, H$ protons enter third of the tongue is carried in the glossopharyngeal the cell through amiloride-sensitive Na! channels nerve (cranial nerve IX) and the vagus nerve (cranial 533 and through specification channels, called PKD1L3 and nerve X). PKD2L1, found in neuroepithelial cells exclusively in- Taste sensation is carried by the chorda tympani, CHAPTER 16 volved in sour taste transduction. The entry of H$ into a branch of the facial nerve (cranial nerve VII) anterior receptor cell activates the voltage-sensitive Ca2! to the sulcus terminalis, and by the glossopharyngeal channels. Influx of Ca2$ triggers migration of synaptic nerve (cranial nerve IX) and vagus nerve (cranial vesicles, their fusion, and transmitter release, which results nerve X) posterior to the sulcus. in generating action potentials in apposed sensory nerve Motor innervation for the musculature of the tongue is fiber (Fig. 16.7b). supplied by the hypoglossal nerve (cranial nerve XII). Salty taste that is stimulated by table salt (NaCl) is es- Vascular and glandular innervation is provided by the sentially derived from the taste of the sodium ions. The sympathetic and parasympathetic nerves. They sup- Digestive System I Na$ enters the neuroepithelial cells through the specific ply blood vessels and small salivary glands of the tongue. amiloride-sensitive Na! channels (the same that are Ganglion cells are often seen within the tongue. These cells involved in sour taste transmission). These channels are belong to postsynaptic parasympathetic neurons and are different from voltage-sensitive Na$ channels that gener- destined for the minor salivary glands within the tongue. ate action potentials in nerve or muscle cells. The entry of The cell bodies of sympathetic postsynaptic neurons are Na$ into a receptor cell causes a depolarization of its mem- located in the superior cervical ganglion. brane and activation of additional voltage-sensitive Na! channels and voltage-sensitive Ca2! channels. TEETH AND SUPPO RTI NG As previously described, influx of Ca2$ triggers mi- TIS S U ES gration and release of neurotransmitter from synaptic vesicles, which results in stimulating gustatory nerve fiber Teeth are a major component of the oral cavity and are (Fig. 16.7c). essential for the beginning of the digestive process. Teeth TEETH AND SUPPORTING TISSUES are embedded in and attached to the alveolar processes of Some areas of the tongue are more responsive to certain the maxilla and mandible. Children have 10 deciduous tastes than others. (primary, milk) teeth in each jaw, on each side: In general, taste buds at the tip of the tongue detect sweet stimuli, those immediately posterolateral to the tip detect A medial (central) incisor, the first tooth to erupt (usually in the mandible) at approximately 6 months of salty stimuli, and those more posterolateral detect sour-tasting age (in some infants, the first teeth may not erupt until 12 stimuli. Taste buds on the circumvallate papillae detect bitter or 13 months of age) and umami stimuli. However, studies with thermal stimu- A lateral incisor, which erupts at approximately 8 months lation of the tongue have shown that the classic taste maps A canine tooth, which erupts at approximately 15 months as described above represent an oversimplified view of the distribution of taste receptors. Sensitivity to all tastes is dis- Two molar teeth, the first of which erupts at 10 to 19 months and the second of which erupts at 20 to 31 months tributed across the entire tongue, but some areas are indeed more responsive to certain tastes than others. During a period of years, usually beginning at about age 6 and ending at about age 12 or 13, deciduous teeth are gradu- The lingual tonsil consists of accumulations of lymphatic ally replaced by 16 permanent (secondary) teeth in each tissue at the base of the tongue. jaw (Folder 16.2). Each side of both upper and lower jaws The lingual tonsil is located in the lamina propria of the consists of the following: root or base of the tongue. It is found posterior to the sulcus terminalis (see Fig. 16.4). The lingual tonsil contains diffuse A medial (central) incisor, which erupts at age 7 or 8 lymphatic tissue with lymphatic nodules containing germi- A lateral incisor, which erupts at age 8 or 9 nal centers. These structures are discussed in Chapter 14, A canine tooth, which erupts at age 10 to 12 Lymphatic System. Two premolar teeth, which erupt between ages 10 and 12 Epithelial crypts usually invaginate into the lingual ton- Three molar teeth, which erupt at different times; the first molar usually erupts at age 6, the second molar in the sil. However, the structure of the epithelium may be diffi- early teens, and the third molar (wisdom teeth) during cult to distinguish because of the extremely large number of the late teens or early twenties lymphocytes that normally invade it. Between nodules, the lingual epithelium has the characteristics of lining epithe- Incisors, canines, and premolars have one root each, except lium. Mucous lingual salivary glands may be seen within the for the first premolar of the maxilla, which has two roots. lingual tonsil and may extend into the muscle of the base of Molars have either two roots (lower jaw) or three (upper jaw) the tongue. and, on rare occasions, four roots. All teeth have the same The complex nerve supply of the tongue is provided by basic structure, however. cranial nerves and the autonomic nervous system. Teeth consist of several layers of specialized tissues. Teeth are made up of three specialized tissues: General sensation for the anterior two-thirds of the tongue (anterior to the sulcus terminalis) is carried in Enamel, a hard, thin, translucent layer of acellular miner- the mandibular division of the trigeminal nerve alized tissue that covers the crown of the tooth.  Dentin, the most abundant dental tissue; it lies deep to thickness over the crown and may be as thick as 2.5 mm the enamel in the crown and cementum in the root. Its on the cusps (biting and grinding surfaces) of some teeth. 534 unique tubular structure and biochemical composition The enamel layer ends at the neck, or cervix, of the tooth support the more rigid enamel and cementum overlying at the cementoenamel junction (Fig. 16.8); the root the surface of the tooth. of the tooth is then covered by cementum, a bone-like TEETH AND SUPPORTING TISSUES Cementum, a thin, pale-yellowish layer of bone-like cal- material. cified tissue covering the dentin of the root of the teeth. Enamel is composed of enamel rods that span the entire Cementum is softer and more permeable than dentin and thickness of the enamel layer. is easily removed by abrasion when the root surface is exposed to the oral environment. The nonstoichiometric carbonated calcium hydroxyapatite enamel crystals that form the enamel are arranged as rods Enamel that measure 4 %m wide and 8 %m high. Each enamel rod spans the full thickness of the enamel layer from the Enamel is the hardest substance in the body; it consists of dentinoenamel junction to the enamel surface. When exam- 96% to 98% calcium hydroxyapatite. ined in cross-section at higher magnification, the rods reveal Enamel is an acellular mineralized tissue that covers a keyhole shape (Fig. 16.9); the ballooned part, or head, is the crown of the tooth. Once formed, it cannot be re- oriented superiorly, and the tail is directed inferiorly toward placed. Enamel is a unique tissue because, unlike bone, the root of the tooth. The enamel crystals are primarily which is formed from connective tissue, it is a mineralized oriented parallel to the long axis of the rod within the head, material derived from epithelium. Enamel is more highly and in the tail, they are oriented more obliquely (Figs. 16.9 mineralized and harder than any other mineralized tissue and 16.10). The limited spaces between the rods are also filled in the body; it consists of 96% to 98% of calcium hy- with enamel crystals. Striations observed on enamel rods droxyapatite. The enamel that is exposed and visible above (contour lines of Retzius) may represent evidence of rhyth- Digestive System I the gum line is called the clinical crown; the anatomic mic growth of the enamel in the developing tooth. A wider crown describes all of the tooth that is covered by enamel, line of hypomineralization is observed in the enamel of the some of which is below the gum line. Enamel varies in deciduous teeth. This line, called the neonatal line, marks enamel lines of Retzius clinical crown anatomic crown dentin showing dentinal tubules interglobular spaces crown odontoblasts CHAPTER 16 gingival sulcus epithelium of gingiva cementoenamel junction epithelial attachment pulp chamber granular layer of Tomes fibers of periodontal membrane root noncellular cementum alveolar bone with marrow FIGURE 16.8 ▲ Diagram of a section pulp canal of an incisor tooth and surrounding bony cellular cementum and mucosal structures. The three miner- alized components of the tooth are dentin, apical foramen enamel, and cementum. The central soft core of the tooth is the pulp. The periodontal ligament (membrane) contains bundles of collagenous fibers that bind the tooth to the surrounding alveolar bone. The clinical crown of the tooth is the portion that projects into the oral cavity. The anatomic crown is the en- tire portion of the tooth covered by enamel.  FOLDER 16.1 Clinical Correlation: The Genetic Basis of Taste 535 The general ability to taste as well as the ability to sense cavity; mucosal disorders, including radiation-induced inflam- CHAPTER 16 specific tastes are genetically determined. Studies in large mation of the lingual mucosa; nutritional deficiencies; endo- populations demonstrate that taste variation is common. crine disorders such as diabetes mellitus, hypogonadism, and About 25% of the population, referred to as “supertasters,” pseudohypoparathyroidism; and hormonal fluctuations during have more than the normal number of lingual papillae and menstruation and pregnancy. Some rare genetic disorders a high density of taste buds. Rare individuals in this group, also affect taste sensation. Type I familial dysautonomia such as wine, brandy, coffee, or tea tasters, have prodigious (Riley-Day syndrome) causes severe hypogeusia (de- taste discrimination and taste memory. These individuals are creased ability to detect taste) because of the developmental characterized by their extreme sensitivity to the chemical absence of taste buds and fungiform papillae. This sensory Digestive System I phenylthiocarbamide (PTC) and its derivative 6-N-propylthio- and autonomic neuropathy is an autosomal recessive disor- uracil (PROP); they typically report an intensely bitter taste der caused by a mutation in the DYS gene (also referred to after a drop of PTC/PROP solution is placed on the tip of their as the IKBKAP gene) located on chromosome 9. In addition tongue. On the other side of the spectrum (about 25% of to hypogeusia, these individuals experience other symptoms the population) are individuals known as “nontasters,” with related to developmental defects in the peripheral and auto- a smaller than normal number of lingual papillae and a lower nomic nervous systems, including diminished lacrimation, density of taste buds. When tested with PTC/PROP solution, defective thermoregulation, orthostatic hypotension, exces- these individuals are unaware of its bitter taste. sive sweating, loss of pain and temperature sensation, and Many clinical conditions can affect taste perception. They absent reflexes. A test that detects the causative mutation include lesions in the nerves that transmit the taste sensa- in the DYS gene has recently been developed to confirm the tion to the central nervous system; inflammations of the oral diagnosis of familial dysautonomia. TEETH AND SUPPORTING TISSUES the nutritional changes that take place between prenatal and postnatal life. Although the enamel of an erupted tooth lacks cells and cell processes, it is not a static tissue. It is influenced by the secretion of the salivary glands, which are essential to its maintenance. The substances in saliva that affect teeth in- clude digestive enzymes, secreted antibodies, and a variety of inorganic (mineral) components. Mature enamel contains very little organic material. Despite its hardness, enamel can be decalcified by acid- HEAD producing bacteria acting on food products trapped on the enamel surface. This is the basis of the initiation of dental 8 !m caries. Fluoride added to the hydroxyapatite complex makes the enamel more resistant to acid demineralization. The wide- TAIL spread use of fluoride in drinking water, toothpaste, pediatric vitamin supplements, and mouthwashes significantly reduces the incidence of dental caries. Enamel is produced by ameloblasts of the enamel organ, 4 and dentin is produced by neural crest–derived odonto- !m rod blasts of the adjacent mesenchyme. length 2000 !m The enamel organ is an epithelial formation that is de- rived from ectodermal epithelial cells of the oral cavity. The onset of tooth development is marked by proliferation of oral epithelium to form a horseshoe-shaped cellular band of tissue, the dental lamina, in the adjacent mesenchyme where the upper and lower jaws will develop. At the site of FIGURE 16.9 ▲ Diagram showing the basic organization and each future tooth, there is a further proliferation of cells that structure of enamel rods. The enamel rod is a thin structure extending arise from the dental lamina, resulting in a rounded, cel- from the dentinoenamel junction to the surface of the enamel. Where the lular, bud-like outgrowth, one for each tooth, that projects enamel is thickest, at the tip of the crown, the rods are longest, measuring into the underlying mesenchymal tissue. This outgrowth, re- up to 2,000 %m. On cross-section, the rods reveal a keyhole shape. The ferred to as the bud stage, represents the early enamel organ upper, ballooned part of the rod, called the head, is oriented superiorly, and the lower part of the rod, called the tail, is directed inferiorly. Within (Fig. 16.11a). Gradually, the rounded cell mass enlarges and the head, most of the enamel crystals are oriented parallel to the long then develops a concavity at the site opposite where it arose axis of each rod. Within the tail, the crystals are oriented more obliquely. from the dental lamina. The enamel organ is now referred to  536 TEETH AND SUPPORTING TISSUES enamel rod Digestive System I enamel rod a b FIGURE 16.10 ▲ Structure of young enamel. a. This electron micrograph shows enamel rods cut obliquely. Arrows indicate the boundaries between adjacent rods. !14,700. b. Parts of two adjacent rods are seen at higher magnification. Arrows mark the boundary between the two rods. The dark needle-like objects are young hydroxyapatite crystals; the substance between the hydroxyapatite crystals is the organic matrix of the developing enamel. As the enamel matures, the hydroxyapatite crystals grow, and the bulk of the organic matrix is removed. !60,000. CHAPTER 16 as being in the cap stage (Fig. 16.11b). Further growth and Inner enamel epithelium, made up of a cell layer that development of the enamel organ results in the bell stage forms the concave surface (Fig. 16.11c and d). At this stage, the enamel organ consists Stratum intermedium, a cell layer that develops inter- of four recognizable cellular components: nal to the inner enamel epithelium Outer enamel epithelium, made up of a cell layer that Stellate reticulum, made up of cells that have a stellate ap- pearance and occupy the inner portion of the enamel organ forms the convex surface FIGURE 16.11 ▲ Diagrams and photomicrographs of a developing tooth. a. In this bud stage, the oral epithelium invaginates into the underly- ing mesenchyme, giving origin to the enamel organ (primordium of enamel). Mesenchymal cells adjacent to the tooth bud begin to differentiate, forming the dental papilla that protrudes into the tooth bud. b. Tooth bud in cap stage. In this stage, cells located in the concavity of the cap differentiate into tall, columnar cells (ameloblasts) forming the inner enamel epithelium. The condensed mesenchyme invaginates into the inner enamel epithelium, forming the dental papilla, which gives rise to the dentin and the pulp. c. In this bell stage, the connection with the oral epithelium is almost cut off. The enamel organ consists of a narrow line of outer enamel epithelium, an inner enamel epithelium formed by ameloblasts, several condensed layers of cells that form the stra- tum intermedium, and the widely spaced stellate reticulum. The dental papilla is deeply invaginated against the enamel organ. d. In this appositional dentin and enamel stage, the tooth bud is completely differentiated and independent from the oral epithelium. The relationship of the two mineralized tissues of the dental crown, enamel and dentin, is clearly visible. The surrounding mesenchyme has developed into bony tissue. e. In this stage of tooth eruption, the apex of the tooth emerges through the surface of the oral epithelium. The odontoblastic layer lines the pulp cavity. Note the developed periodontal ligaments that fasten the root of the tooth to the surrounding bone. The apex of the root is still open, but after eruption occurs, it becomes narrower. f. Functional tooth stage. Note the distribution of enamel and dentin. The tooth is embedded in surrounding bone and gingiva. g. This photomicrograph of the developing tooth in the cap stage (comparable to b) shows its connection with the oral epithelium. The enamel organ consists of a single layer of cuboidal cells forming the outer enamel epithelium; the inner enamel epithelium has differentiated into columnar ameloblasts, and the layer of cells adjacent to the inner enamel epithelium has formed the stratum intermedium. The remainder of the structure is occupied by the stellate reticulum. The mesenchyme of the dental papilla has prolifer- ated and pushed into the enamel organ. At this stage, the forming tooth is surrounded by condensed mesenchyme, called the dental sac, which gives rise to periodontal structures. !300. h. This photomicrograph shows the developing crown of an incisor, which is surrounded by the outer enamel epithelium and remnants of the stellate reticulum. It is comparable to d. The underlying lighter stained layer of dentin is a product of the odontoblasts. These tall columnar odontoblasts have differentiated from cells of the dental papilla. The pulp cavity is filled with dental pulp, and blood vessels permeate the pulp tissue. !40.  The neural crest–derived preodontoblasts lined up organ will become ameloblasts. Along with the cells of the within the “bell” adjacent to the inner enamel epithelial cells stratum intermedium, they will be responsible for enamel become columnar and have an epithelial-type appearance. production. At the early stage, just before dentinogenesis 537 They will become odontoblasts and form the dentin of and amelogenesis, the dental lamina degenerates, leaving the the tooth. The inner enamel epithelial cells of the enamel developing tooth primordium detached from its site of origin. CHAPTER 16 stratum intermedium inner enamel epithelium primordium of enamel stellate outer enamel enamel oral epithelium inner enamel epithelium reticulum epithelium dentin Digestive System I a b c d primordium of pulp dental papilla dental papilla dental pulp bone oral epithelium enamel dentin odontoblastic layer gingiva TEETH AND SUPPORTING TISSUES dental pulp periodontal ligament bone vessels and nerves e f ameloblasts enamel outer enamel epithelium oral stratum dentin epithelium intermedium odontoblasts stellate outer dental reticulum enamel sac epithelium dental dental papilla papilla stellate reticulum inner dental enamel papilla epithelium g g h  ameloblasts enamel 538 enamel dentin dentinoenamel junction predentin TEETH AND SUPPORTING TISSUES dentin odontoblasts secretory-stage ameloblasts dental sac odontoblasts stratum intermedium stellate reticulum dental pulp a b FIGURE 16.12 ▲ Diagram and photomicrograph showing the cellular relationships during enamel formation. a. In the initial secretory stage, dentin is produced first by odontoblasts. Enamel matrix is then deposited directly on the surface of the previously formed dentin by secretory-stage Digestive System I ameloblasts. The secretory-stage ameloblasts continue to produce enamel matrix until the full thickness of the future enamel is achieved. b. This photomi- crograph of an H&E–stained section of a developing human tooth shows an early stage of enamel formation (amelogenesis). The secretory-stage amelo- blasts lie directly adjacent to the developing enamel, which is being deposited on the layer of dentin. The beginning of enamel deposition is indicated by the arrow. As the first increment of enamel is formed, ameloblasts move away from the dentin surface. Basal domains of secretory-stage ameloblasts are adjacent to cells in the stratum intermedium (a part of the enamel organ). Dentin is secreted by odontoblasts. Note the lightly stained layer of the newly secreted organic matrix (predentin) is in close apposition to apical surfaces of odontoblasts. Predentin further undergoes mineralization to mature dentin (dark-stained layer). The layer of odontoblasts separates enamel from the dental pulp. !240. (Courtesy of Dr. Arthur R. Hand.) Dental enamel is formed by a matrix-mediated biomin- produced first. Then, partially mineralized enamel matrix eralization process known as amelogenesis. These are the (Fig. 16.12) is deposited directly on the surface of the pre- major stages of amelogenesis: viously formed dentin. The cells producing this partially Matrix production or secretory stage. In the for- mineralized organic matrix are called secretory-stage mation of mineralized tissues of the tooth, dentin is ameloblasts. As do osteoblasts in bone, these cells CHAPTER 16 Clinical Correlation: Classification of Permanent FOLDER 16.2 (Secondary) and Deciduous (Primary) Dentition Three systems are currently used to classify permanent the permanent dentition is designated by Arabic numer- and deciduous teeth (Fig. F16.2.1): als, and the deciduous dentition is designated with uppercase letters. For permanent dentition, numbering Palmer system, which is the most commonly used nota- begins in the UR quadrant, with the UR third molar desig- tion worldwide. In this system, uppercase letters are used nated number 1. Numbering continues across the maxil- for the deciduous teeth, and Arabic numerals are used for lary arch to the UL third molar, designated tooth number the permanent teeth. Each quadrant in this system is des- 16. Tooth number 17 is the third molar located in the LL ignated by angled lines: for upper right (UR), for upper quadrant inferior and opposite to tooth number 16. Then, left (UL), for lower right (LR), and for lower left (LL). For the numbering progresses across the mandibular arch example, permanent canines are called number 3 in each and terminates with tooth number 32, the LR third molar. quadrant, and the quadrant is designated by its angled line. In this system, the sum of the numbers of opposing International system, which uses two Arabic numerals to teeth adds up to 33. For the deciduous dentition, the designate the individual tooth. In this system, the first nu- same pattern is followed, but the letters A to T are used meral indicates the location of the tooth in a specific quad- to designate the individual teeth. Thus, in this system, rant. The permanent quadrants are designated UR & 1, UL the permanent canines are designated 6, 11, 22, and 27, & 2, LL & 3, and LR & 4; the deciduous quadrants are des- and the deciduous canines, C, H, M, and R. ignated UR & 5, UL & 6, LL & 7, and LR & 8. The second numeral designates the individual tooth, which is numbered Also note that in Figure F16.2.1, the color outline dem- beginning from the dental midline. For example, in this sys- onstrates the relationship of the deciduous and permanent tem, the permanent canines are named 13, 23, 33, and 43, dentitions. Examination of the table reveals that deciduous and the deciduous canines would be 53, 63, 73, and 83. molars are replaced with permanent premolars after exfo- American (universal) system, which is the most com- liation and that the permanent molars have no deciduous monly used notation in North America. In this system, precursors.  Clinical Correlation: Classification of Permanent FOLDER 16.2 (Secondary) and Deciduous (Primary) Dentition (continued) 539 upper right quadrant (UR) upper left quadrant (UL) CHAPTER 16 second first lateral medial medial lateral first second canine canine molar molar incisor incisor incisor incisor molar molar decidous (primary) dentition maxillary arcade (arch) A B C D E F G H I J 55 54 53 52 51 61 62 63 64 65 E D C B A A B C D E Digestive System I third second first second first lateral medial medial lateral first s