Histology (Gastro) Lec 1 PDF

he digestive system consists of the digestive tract Absorption of the small molecules and water into the oral cavity, esophagus, stomach, small and large intes blood and lymph, t...

he digestive system consists of the digestive tract Absorption of the small molecules and water into the oral cavity, esophagus, stomach, small and large intes blood and lymph, tines, and anusand its associated glandssalivary Elimination of indigestible, unabsorbed components of glands, and pancreas (Figure 15-1). Also called the liver, food. gastrointestinal (Gl) tract oralimentary canal, its function is to obtain from ingestedfood the molecules necessary forthe maintenance, growth, and energy needs of the body. During )GENERAL STRUCTURE OF THE digestionproteins,complex carbohydrates, nucleic acids, and DIGESTIVE TRACT fats are broken down into their small molecule subunits that are easily absorbed through the small intestine lining. Most All regions of the GI tract have certain structural features in water and electrolytes are absorbed in the large intestine. In common. The GI tract is a hollow tube with lumen of vari a addition,the inner layer of the entire digestive tract forms an able diameter and a wall made up of four main layers: the important protective barrierbetween the content of the tract's mucosa, submucosa, muscularis, and serosa. Figure 15-2 lumen and the internal milieu of the body's connective tissue shows a general overview of these four layers; key features of and vasculature. each layer are summarized here. Structureswithin the digestive tract allow the following: The mucosa consists of anepithelial lining;an under Ingestion, or introduction of foodand liquid into the lying lamina propria of loose connective tissue rich in oral cavity, blood vessels, lymphatics,lymphocytes, smooth muscle Mastication, or chewing, which dividessolid food into cells, and often containing small glands; and a thin layer digestible pieces, of smooth muscle called the muscularis mucosae Motility,muscular movements of materials through the separatingmucosa from submucosa and allowing local tract, movements of the mucosa. The mucosa is also frequently Secretion of lubricatingand protective mucus, digestive mucous membrane. called a enzymes, acidic and alkaline fluids, and bile, The submucosa contains denser connective tissue with Hormone release for local control of motility and larger blood and lymph and the submucosal vessels secretion, (Meissner)plexus of autonomic nerves.It may also Chemical digestion or enzymatic degradation of large contain glands and significant lymphoid tissue. macromolecules in food to smaller molecules and their The thick muscularis (ormuscularisexterna)is composed subunits, of smooth muscle cells organized as two or more sublayers. 295 308/573 296 CHAPTER 15 Digestive Tract FIGURE 15-1 The digestive system. Accessory digestive organs Gastrointestinal tract (digestive organs) Parotid salivary gland Teeth Oral cavity Tongue Pharynx Sublingual salivary gland Submandibular salivary gland Esophagus Liver Stomach Gallbladder Pancreas Large intestine Small intestine Anus The digestive system consists of the tract from the mouth (oral this tract, primarily the salivary glands, liver, and pancreas. These cavity) to the anus, as well as the digestive glands emptying into accessory digestive glands are described in Chapter 16. In the internal sublayer (closer to the lumen), the filber luminal contents forward,are generated and coordinated orientation is generally circular; in the external sublayer by the myenteric plexus. it is longitudinal. The connectivetissue between the The serosa is a thin layer of loose connective tissue, rich in muscle sublayers contains blood and lymph vessels, as bloodvessels, lymphatics, and adipose tissue, with a simple well as the myenteric (Auerbach) nerve plexus of squamous covering epithelium or mesothelium. In the many autonomic neurons aggregated into small ganglia abdominalcavity, the serosa is continuous with mesenteries, and interconnected by pre- and postganglionic nerve thin membranes covered by mesothelium on both sides that fibers. This and the submucosal plexus together comprise support the intestines. Mesenteries are continuous with the enteric nervous system of the digestive tract. Con the peritoneum, a serous membrane that lines that cav tractions of the muscularis, which mix and propel the ity. In places where the digestive tract is not suspended in 309/573  General Structure of the Digestive Tract 297 FIGURE 15-2 Major layers and organization of thedigestive tract. P CHA TER Mucosa Epithelium Lamina propria 15 Muscularis mucosae Mesentery Digestive Tract Vein Artery Lymph vessel Submucosa Submucosal gland Blood vessel Lumen General Submucosal nerve plexus Muscularis Structure Inner circular layer of the Myenteric nerve plexus Dig Outer longitudinal layer < Serosa Tract Diagram showing the structure of the small intestine portion large intestine are suspended by mesenteries that are the sites ofthe digestive tract, with the four main layers and their major of nerves, blood vessels and lymphatics from the stomach and components listed on the left. The stomach, small intestine, and intestines. a cavity but bound directly to adjacent structures, such as protein produced by the epithelial cells. This IgA complex in the esophagus (Figure15-1),the serosa is replaced by a resists proteolysis by digestive enzymes and provides impor thick adventitia, a connective tisue layer that merges with tant protection against specific viral and bacterial pathogens. the surrounding tissues and lacks mesothelium. The numerousfree immune cells and lymphoid nodules >> MEDICAL APPLICATION in mucosaandsubmucosaconstitute the the MALT described In diseases such as Hirschsprung disease (congenital agan in Chapter 14. The digestive tract normally contains thou glionic megacolon) or Chagas disease (trypanosomiasis, sands of microbial species, including both useful inhabitants infection with the protozoan Trypanosoma cruz), plexuses of the gut as well as potential pathogens ingested with food in the digestive tract's enteric nervous system are absent or and drink. The mucosa-associated immune defense system severely injured, respectively. This disturbs digestive tract provides an essential backup tothe thin physical barrier ofthe motility and produces dilations in some areas. The rich auto epithelial lining. Located just below the epithelium, the lam nomic innervation ofthe enteric nervous system also provides ina propria is rich with macrophages and lymphocytes, many an anatomic explanation of the well-known actions of emo for production of IgA antibodies. Such antibodies undergo tional stress on the stomach and other regions of the GI tract. transcytosis into the intestinal lumen bound to the secretory  ORAL CAVITY FIGURE 15-3 Lip. > The oral cavity (Figure 15-1) is lined with stratified squamous epithelium, which may be keratinized, partially keratinized, or nonkeratinized depending on the location. Epithelial dif ferentiation, keratinization, and the interface between the epithelium and lamina propria are similar to those features in the epidermis and dermis and are discussed more exten M sively with skin (see Chapter 18). Like the keratinized sur face cells of epidermis, the flattened superficial cells of the oral epithelium undergocontinuous desquamation, or loss at the surface. Unlike those of the epidernmis, the shed cells of the nonkeratinized or parakeratinized oral epithelium retain their nuclei. OM >>MEDICAL APPLICATION Viral infections with herpes simplex 1 cause death of infected epithelial cells that can lead to vesicular or ulcerating lesions of the oral mucosa or skin near the mouth. In the oral cavity such areas are called canker sores,and on the skin they are usually called cold sores or fever blisters. Such lesions, often painful and clustered, occur when the immune defenses are Low-magnificationmicrograph of a lip section showing one side weakened by emotional stress, fever, illness, or local skin covered by typical oral mucosa (OM), the opposite side covered damage,allowing the virus, present in the local nerves,to by skin (S) containing hair follicles (F) and associated glands. move into the epithelial cells. Between the oral portion of the lips and normal skin is the vermil ion zone (V), where epidermis is very thin, lightly keratinized, and transparent to blood in the rich microvasculature of the underly ing connective tissue. Because this region lacks the glands for oil The keratinized resist damage from abrasion cell layers and sweat,it is prone to excessive dryness and chapping in cold, and are best developed masticatorymucosa on the in the dryweather. Internally, the lips contain much striated muscle (M) gingiva (gum) and hard palate. The lamina propria in these and many minor salivary glands (G). (X10;H&E) regions rests directly on the periosteum of underlying bone. Nonkeratinized squamous epithelium predominates in the lining mucosa over the soft palate, cheeks, the floor of the mouth,and the pharynx (or throat), the posterior region of The outer surface has thin skin, consisting of epidermal the oral cavity leading to the esophagus. Lining mucosa over and dermal layers,sweat glands, and many hair follicles lies a thick submucosa containing many minor salivary glands, with sebaceous glands. which secrete continuously to keep the mucosal surface wet, and diffuse lymphoid tissue. Throughout the oral cavity, the Tongue epithelium contains transient antigen-presenting cells and The tongue is a mass of striated muscle covered by mucosa, rich sensory innervation. which manipulates ingested material during mastication and The well-developed core of striated muscle in the lips, or swallowing. The muscle fibers are oriented in all directions, labia,(Figure 15-3) makes these structures highly mobile for allowing a high level of mobility. Connective tissue between ingestion, speech, and other forms of communication. Both the small fascicles of muscle is penetrated by the lamina pro lips have three differently covered surfaces: pria, which makes the mucous membrane strongly adherent to The internal mucous surface has lining mucosa with a the muscular core. The lower surface of the tongue is smooth, thick, nonkeratinized epithelium and many minor labial with typical lining mucosa. The dorsal surface is irregular, hav salivary glands. ing hundreds of small protruding papillae of various types on The red vermilion zone of each lip is covered by very its anterior two-thirds and the massed lingual tonsils on the thin keratinized stratified squamous epithelium and posterior third, or root of thetongue (Figure 15-4). The papil is transitional between the oral mucosa and skin. This lary and tonsillar areas of the lingual surface are separated by a region lacks salivary or sweat glands and is kept moist V-shaped groove called the sulcus terminalis. with saliva from the tongue. The underlying connective The lingual papillae are elevations of themucous mem tissue is very rich in both sensory innervation and capil brane that assumevarious forms and functions. There are four laries, which impart the pink color to this region. types (Figure 15-4):  Oral Cavity 299 FIGURE 15-4 Tongue,lingual papillae, and taste buds. CHAP TER Stratified squamous epithelium of tongue surface Lingual 15 tonsil Epithelium -Gustatory Sulcus cell terminalis -Gustatory microvillus -Taste pore Digestive Tract -Supporting cell Basal cell Sensory Oral nerve (b)Vallate papilla (c) Taste bud Nuclei of Cavity Taste gustatory cells pore Apex of tongue Epithelium Taste bud Taste bud Epithelium Epithelium Nuclei of supporting cells Filiform papilla Fungiform papilla Foliate papilla (a) Dorsal surface of tongue (d)Histologyof taste bud On its dorsal surface (a), the posterior thirdofthe tongue has the (c)Diagram of a single taste bud shows the gustatory (taste) cells, lingual tonsils and the anterior portion has numerous lingual the supporting cells whose function is not well understood, and papillae of four types. Pointed filiform papillae provide friction the basal stem cells. Microvilli at the ends of the gustatory cells tohelp move food during chewing. Ridge-like foliate papillae project throughan opening in the epithelium, the taste pore. Affer on the sides ofthe tongue are best developed in young children. ent sensory axons enter the basal end of taste buds and synapse Fungiform papillae are scattered across the dorsal surface, and with the gustatory cells. In the stratified squamous epithelium of 8-12 large vallate papillae (b) are present in a V-shaped line the tongue surface, taste buds form as distinct clusters of cells that near the terminal sulcus. Taste buds are present on fungiform are recognizable histologically even at low magnification (d). At and foliate papillae but are much more abundant on vallate higher power the taste pore may be visible, as wellas the elongated papillae. nuclei of gustatory and supporting cells. (140Xand 500X; H&E) Filiform papillae (Figure 15-5) are very numer with well-vascularizedand innervated cores of lamina ous, have an elongated conical shape, and are heavily propria. keratinized, which gives their surface a gray or whitish Foliate papillae consist of several parallel ridges on appearance. They provide a rough surface that facilitates each side of the tongue, anterior to the sulcus termi movement of food during chewing. nalis, but are rudimentary in humans,especially older Fungiform papillae (Figure 15-5) are much less individuals. numerous, lightly keratinized, and interspersed among Vallate (or circumvallate) papillae (Figure 15-5) are the filifornm papillae. They are mushroom-shaped the largest papillae, with diameters of 1-3 mm. Eight to 300 CHAPTER 15 Digestive Tract FIGURE 15-5 Lingual papillae. FI SS F FI SS SS TB CT TB ss SS GL GL b (a)Section of the dorsal surface of tongue showing both filiform (b)Micrograph shows large vallate papilla with two a single very (FI) and fungiform papillae (F). Both types are elevations ofthe distinctive features: many taste buds (TB)around the sides and connective tissue (CT)covered by stratified squamous epithelium several small salivary glands (GL)emptying into the cleft or moat (SS), but the filiform type is pointed and heavily keratinized while formed by the elevated mucosa surroundingthe papilla. These the fungiform type is mushroom-shaped, lightly keratinized, and glands continuously flush the cleft, renewing the fluid in contact has a few taste buds. with the taste buds. (BothX20; H&E) twelve vallate papillae are normally aligned just in front give rise to the other cell types. The base of each bud rests on of the terminal sulcus. Ducts of severalsmall, serous the basal lamina and is entered by afferent sensory axons that salivary (von Ebner)glandsempty into the deep, form synapses with the gustatory cells. At the apical ends ofthe moatlike groove surrounding each vallate papilla. This gustatory cells, microvilli project toward a 2-um-wide open provides a continuous flow of fluid over the taste buds ing in the structure called the taste pore. Molecules (tastants) that are abundant on the sides of these papillae, washing dissolved in saliva contact the microvilli through the pore and away food particles so that the taste buds can receive and interact with cell surface taste receptors (Figure 15-4). process new gustatory stimuli. Secretions from these and Taste buds detect at least five broad categoriesof tastants: other minor salivary glands associatedwith taste buds sodium ions (salty); hydrogen ions from acids (sour);sugars contain a lipase that prevents the formation of a hydropho and related compounds (sweet); alkaloids and certain tox bic film on these structures that would hinder gustation. ins (bitter); and amino acids such as glutamate and aspartate Taste buds are ovoid structures within the stratified (umami; Jap. umami, savory). Salt and sour tastes are pro duced by ion channels and the other three taste categories are epithelium on the tongue's surface,which sample the general mediated by G-protein-coupled receptors. Receptor binding chemical composition of ingested material (Figures 15-4 and produces depolarization of the gustatory cells, stimulating the 15-5). Approximately 250 taste buds are present on the lat sensory nerve fibers that send information to the brain for eral surface of each vallate papilla, with many others present processing. Conscious perception of tastes in food requires on fungiform and foliate (but not the keratinized filiform) olfactory and other sensations in addition to taste bud activity. papillae.They are not restricted to papillae and are also widely scattered elsewhere on the dorsal and lateral surfaces of the Teeth tongue, wherethey are also continuously flushed by numerous minor salivary glands. In the adult human there are normally 32 permanent teeth, A taste bud has 50-100 cells, about half of which are elon arranged in two bilaterally symmetric arches in the maxillary gated gustatory(taste) cells, which turn over with a 7- to and mandibular bones (Figure 15-6a). Each quadrant has eight 10-day life span. Other cells present are slender supportive teeth: two incisors, one canine, two premolars, and three per cells, immaturecells, and slowly dividing basal stem cells that manent molars. Twenty of the permanent teeth are preceded  Oral Cavity 301 FIGURE 15-6 Teeth. CHA Central incisor (7-8 y) Lateral incisor (8-9 y) Canine (11-12 y) Crown PTER 1st premolar (10-11 y) Enamel 2nd premolar (10-12 y)S Gingiva Upper teeth 1st molar (6-7 y) Neck 15 Dentin 2nd molar (12-13 y) Pulp cavity 3rd molar (17-25 y) Hard palate Digestive Root canal Root 3rd molar (17-25 y) Tract 2nd molar (11-13 y) Cementum 1st molar (6-7 y) Lower teeth Periodontal Oral 2nd premolar (11-12 y) ligaments 1st premolar (10-12 y) Dental alveolus Cavity Canine (9-10 y) Lateral incisor (7-8 y) Central incisor (6-7 y) (a) Permanent teeth (b)Molar All teeth are similar embryologically and histologically. constricted neck where the enamel and cementum coverings meet at the gingiva. Most ofthe roots and neck consists of dentin. (a)The dentition of the permanent teeth is shown, as well as the A pulp cavity extends into the neck and is filled with well approximate age at eruption for each tooth. vascularized, well-innervated mesenchymal connective tissue. Blood (b)Diagram ofa molar's internal structure is similar to that of all vessels and nerves enter the tooth through apical foramina at the teeth, with an enamel-covered crown, cementum-covered roots root tips. Periodontal ligaments hold the tooth to bone of the jaw. anchoringthe tooth to alveolar bone of the jaw, and a slightly by primaryteeth (deciduous or milk teeth) that are shed; the Dentin others are permanent molars with no deciduous precursors. Dentin is a calcified tissue harder than bone, consisting of Each tooth has a crown exposed above the gingiva,a constricted 70% hydroxyapatite. The organic matrix contains type I col neck at the gum,and one or more roots that fit firmly into bony lagen and proteoglycans secreted from the apical ends of sockets in the jaws called dental alveoli (Figure 15-6b). odontoblasts, tall polarized cells derived from the cranial The crown is covered byvery hard, acellular enamel and neural crest that line the tooth's pulp cavity (Figure 15-7a). the roots by a bone-like tissue called cementum. These two Mineralization of the predentin matrix secreted by odon coverings meet at the neck of the tooth. The bulk of a tooth is toblasts involves matrix vesicles in a process similar to that composed of another calcified material, dentin, which sur occurring in osteoid during bone formation (see Chapter 8). rounds an internal pulp cavity (Figure 15-6b). Dental pulp Long apical odontoblast processes extend from the is highly vascular and well-innervated and consists largely odontoblasts within dentinal tubules (Figure 15-7b) that of loose, mesenchymal connective tissue with much ground penetrate the full thickness of the dentin, gradually becoming substance, thin collagen fibers, fibroblasts, and mesenchymal longer as the dentin becomes thicker. Along their length, the stem cells. The pulp cavity narrows in each root as the root processes extend fine branches into smaller lateral branches canal, which extends to an opening (apical foramen) at the of the tubules (Figure 15-7c). The odontoblast processes are tip of each root for the blood vessels, lymphatics, and nerves of important for the maintenance of dentin matrix. Odontoblasts the pulp cavity. The periodontal ligamentsare fibrous con continue predentin production into adult life, gradually reduc nective tissue bundles of collagen fibers inserted into both the ing the size of the pulp cavity, and are stimulated to repair den cementum and the alveolar bone. tin if the tooth is damaged. 302 CHAPTER 15 Digestive Tract FIGURE 15-7 Dentin and odontoblasts. E b PC OP OP P D a d (a) Odontoblasts(0) are long polarized derivedfrom mesen cells their processes are maintained in canals called dentinaltubules chyme ofthe developing pulp cavity (PC). Odontoblasts are spe that run through the full thickness of the dentin. (X400;Mallory cialized for collagen and GAG synthesis and are bound together trichrome) by junctional complexes as a layer, with no basal lamina, so that (b)Odontoblast processes can be silver-stained and shown to a collagen-rich matrix called predentin only from(P) is secreted branch near the junction of dentin with enamel (E) and along their apical ends at the dentinal surface. Within approximately 1 their length closer to their source (c),with the lateral branches day of secretion, predentin mineralizes to become dentin (D)as occupying smaller canaliculi within dentin. (Both X400; Silver) hydroxyapatite crystals form in a process similar to that occurring (d)These odontoblast process (OP) connections to the in osteoid of developing bones (see Chapter 8). In this processthe odontoblasts (0), shownwith stained nuclei here, are collagen is masked, and calcified matrix becomes much more aci important for the maintenance of dentin in adult teeth. dophilic and stains quite differently than that of predentin. When (X400; Mallorytrichrome) predentin secretion begins, an apical extension from each cell, the (Figure 15-76, c, and d,used with permission from M. F.Santos, odontoblast process (OP), forms and is surrounded by new matrix. Department of Histology and Embryology,Institute of Biomedical As the dentin-predentin these processes lengthen. layer thickens, Sciences, University of São Paulo, Brazil.) When tooth formationis complete,odontoblastspersist and  Oral Cavity 303 Teeth are sensitive to stimuli such as cold, heat, and acidic surrounded by a thinner layer of other enamel. Each rod pH, all of which can be perceived as pain. Pulp is highly inner extends through the entire thickness of the enamel layer, vated, and unmyelinated nerve fibers extend into the dental which averages 2 mm. The precise, interlocked arrangement of CHA tubules along with odontoblast processes near the pulp cavity the enamel rods is crucial for enamel's hardness and resistance (Figure 15-8). Such stimuli can affect fluid inside the dentinal to great pressures during mastication. tubules, stimulating these nerve fibers and producing tooth In a developing tooth bud, the matrix for the enamel PTER sensitivity. rods is secreted by tall, polarized cells, the ameloblasts (Figure 15-9a), which are part of a specialized epithelium >MEDICAL APPLICATION in the tooth bud called the enamel organ.The apical ends 15 of the ameloblasts face those of the odontoblasts producing Immune defenses in the oral cavity cannot protectagainst predentine (Figure 15-10). An apical extension from each all infections. Pharyngitis and tonsillitis are often due to ameloblast, the ameloblast (or Tomes) process, contains the bacterium Streptococcuspyrogenes. White excrescences numerous secretory granules with the proteins of the enamel or leukoplakia on the sides ofthe tongue can be caused Digestive matrix. The secreted matrix undergoes very rapid mineral by Epstein-Barr virus. Oral thrush, a white exudate on the ization. Growth of the hydroxyapatite crystals to produce tongue's dorsal surface, is due to a yeast (Candida albicans) Tract each elongating enamel rod is guided by a small (20kDa) infection and usually affects neonates or immunocompro protein amelogenin, the main structural protein of develop mised patients. ing enamel. Oral Ameloblasts are derived from the ectodermal lining of Enamel the embryonic oral cavity, while odontoblasts and most tis Enamel component of the human body, consist from and Cavity is the hardest sues of the pulp cavity develop neural crest cells ing of 96% calcium hydroxyapatite and only 2%-3% organic mesoderm, respectively. Together, these tissues produce a material including very few proteins and no collagen. Other series of 52 tooth buds in the developing oral cavity, 20 for the ions, such as fluoride, can be incorporated or adsorbed by primary teeth and 32 for the secondary or permanent teeth. the hydroxyapatite crystals; enamel containing fluorapatite is Primary teeth complete development and begin to erupt about more resistant to acidic dissolution caused by microorganisms, 6 months after birth. Development of the secondary tooth hence the addition of fluoride to toothpaste and water supplies. buds arrests at the "bell stage," shown in Figure 15-10a, until Enamel consists of uniform, interlocking columns called about 6 years of age, when these teeth begin to erupt as the enamel rods (or prisms), each about5 um in diameter and primary teeth are shed. FIGURE 15-8 Ultrastructure of dentinaltubule. OP OP P a b (a) TEM shows the calcification ofdentin (D)at its border with (b)TEM Cross section of an odontoblast process (OP) near pre not-yet calcified An odontoblast process (OP) with predentin (P). dentin (P) shows its close association with an unmyelinated nerve microtubulesand a few secretory vesicles is seen in the fluid-filled fiber (N)extending there from fibers in the pulp cavity. These space (S) in the derntinal tubule. A process extends from each nerves respond to various stimuli, such as cold temperatures, odontoblast, and the tubulescontinue completely across the den reaching the nerve fibers through the dentinal tubules. (X61,000) tin layer. (X32,000) 304 CHAPTER 15 Digestive Tract FIGURE 15-9 Ameloblasts and enamel. E |CT A D D b (a) In a of tooth bud ameloblasts section (A) are tall polarized cells lost during tooth eruption. Teeth that have been decalcified for whose apicalends initially contact dentin (D). Ameloblasts are histologic sectioning typically lose their enamel layer completely. joined to form a cell layer surrounded basally by connective tissue (X400; H&E) (CT). As odontoblasts secrete predentin, ameloblasts secrete a (b) Micrograph a thin preparation of a tooth prepared by grind of matrix lacking collagens, but rich in proteins such as amelogenin ing. Fine, long can be observed in the dentin (D), and tubules that quickly initiate calcium to make hydroxyapatite formation rods aligned the same way can be very faintly observed (arrows) enamel (E),the hardest the body. Enamel forms a layer material in in the enamel (E).The more prominent lines that cross enamel but consists of enamel rods or prisms, solidly fused together by diagonally represent incremental growth lines produced as the more enamel.Each enamel rod represents the product of one enamel matrix is secreted cyclically by the ameloblast layer. (X400; ameloblast. No cellular processes occur in enamel, and the layer Unstained) of ameloblasts surrounding the developing crown is completely >> MEDICAL APPLICATION than bone, cementocytes maintain their surrounding matrix and react to stresses by graduallyremodeling. Periodontal diseases include gingivitis, inflammation of The periodontal ligament is fibrous connective tis the gums,and periodontitis, which involves inflammation sue with bundled collagen fibers (Sharpey fibers) binding at deeper sites, both of which are caused most commonly the cementum and the alveolar bone (Figure 15-11). Unlike by bacterial infections with poor oral hygiene. Chronic peri typical ligaments,it is highly cellular and has a rich supply of odontitis weakens the periodontal ligament and can lead to blood vesels and nerves,giving the periodontal ligament sen loosening ofthe teeth. The depth of the gingival sulcus, mea sory and nutritive functions in addition to its role in support sured during clinical dental examinations, is an important ing the tooth. It permits limited movementof the tooth within indicator of potential periodontal disease. the alveolus and helps protect the alveolus from the recurrent pressure exerted during mastication. Its thickness (150-350 um) is fairly uniform along the root but decreaseswith aging. Periodontium The alveolar bone lacks the typical lamellar pattern of The periodontium comprises the structures responsible for adult bone but has osteoblasts and osteocytes engaging in con maintaining the teeth in the maxillaryand mandibular bones, tinuous remodeling of the bony matrix. It is surrounded by and includesthe cementum, the periodontal ligament, and the periodontal ligament,which serves as its periosteum. Col the alveolar bone with the associated gingiva (Figure15-6b; lagen fiber bundles of the periodontal ligament penetratethis Figure 15-11). bone, binding it to the cementum (Figure 15-1lc). Cementum covers the dentin of the root and resembles Around the peridontium the keratinized oral mucosa of bone, but it is avascular. It is thickest around the root tip where the gingiva is firmly bound to the periosteum ofthe maxillary cementocytes reside in lacunae with processes in canaliculi, and mandibular bones (Figure 15-11). Between the enamel especially near the cementum surface. Although less labile and the gingival epithelium is the gingival sulcus, a groove  Esophagus 305 FIGURE 15-10 Tooth formation. CH OEE DP: APTER PD 15 PD D A DP Digestive Tract B B b Esophagus Tooth formation begins in the embryo when ectodermal epithe odontoblasts (0)facing the ameloblasts. These two cell layers lium lining the oral cavity grows into the underlyingmesenchyme begin to move apart as the odontoblasts begin to produce the of the developing jaws. At a series of sites corresponding to each layer of predentin (PD). Contact with dentin induces each amelo future tooth, these epithelial cells proliferate extensively and blast to begin secretion of an enamel rod. More slowly, calcifying become organized as enamel organs,each shaped like a wine interprismatic enamel fuses all the enamel rods into a very strong, glasswith its stem initially still attached to the oral lining. Amelo solid mass.(X20; H&E) blasts form from the innermost layer of cells in the enamel organ. (b)Detail an enamel organ showing the layers of predentin of Mesenchymal cells inside the concave portion of the enamel (PD) and (D)and a layer of enamel (E), along with the orga dentin organ include neural crest cells that differentiate as the layer of nized cell layers that produced this material. Odontoblasts (0)are odontoblasts with their apical ends in contact with the apical in contact with the very cellular mesenchyme of the dental papilla ends of the ameloblasts. (DP)that will become the pulp cavity. Ameloblasts (A) are promi (a) A section of enamel organ in which production of dentin and nent in the now much thinner enamel organ, which is very close enamel has begun. The ameloblast layer (A) is separated from to developing bone (B). Enamel formation continues until shortly the outer enamel epithelium (0EE) by a thick intervening region before tooth eruption; formation of dentin continues after erup rich in GAGs but with few, widely separated cells. Surrounding tion until the tooth is fully formed. Odontoblasts persist around the enamel organ is mesenchyme, someparts of which begin the pulpcavity, with processes penetrating the dental layer, to undergo intramembranous bone formation (B). Inside the producingfactors to help maintain dentin. Mesenchymal cells cavity of each enamel organ, mesenchymal cells comprise the immediatelyaround the enamel organ differentiate into the cells dental papilla (DP), in which the outermost cells are the layer of of cementum and other periodontal tissues. (X120; H&E) up to 3 mm deep surrounding the neck (Figure 15-1la). A spe and protect the mucosa (Figure 15-13a). Near the stomach cialized part of this epithelium, the junctional epithelium, the mucosa also contains groups of glands, the esophageal is bound to the tooth enamel by means of a cuticle, which cardiac glands, which secrete additional mucus. resembles a thick basal lamina to which the epithelial cells are attached by numeroushemidesmosomes. >> MEDICAL APPLICATION The lubricating mucus produced in the esophagus offers >ESOPHAGUS little protection against acid that mnay move there from the stomach. Such movementcan produce heartburn or reflux The esophagus is a muscular tube, about 25-cm long in esophagitis.An incompetent inferior esophageal sphincter adults, which transports swallowed material from the pharynx may result in chronic heartburn,which can lead to erosion of to the stomach. The four layers of the GI tract (Figure 15-12) the esophageal mucosa or gastroesophageal reflux disease become well-established first and clearly seen in the esophagus. (GERD). Untreated GERD can produce metaplasticchanges The esophageal mucosa has nonkeratinized stratified squa in the stratified squamousepithelium ofthe esophageal mous epithelium, and the submucosacontains small mucus mucosa, a condition called Barrett esophagus. secreting glands, the esophageal glands, which lubricate 306 CHAPTER 15 Digestive Tract FIGURE 15-11 Periodontium. FG D B LP P B PL a b The periodontium of each tooth consists of the cementum, (b) Micrograph shows the periodontal ligament (L)with its many periodontal ligament, alveolar bone, and gingiva. blood vessels (V) and insertions into the alveolar bone (B).This liga ment serves as the periosteum of the alveolar in tooth sockets and (a) Micrograph of decalcified tooth shows the gingiva. The free gingiva (FG) is against the dentin (D), with little of the gingival is also continuous with developing layers of cementum (C) that cov ers the dentin. Cementum forms a thin layer of bone-like material sulcus apparent. Gingiva stratified squamous epithelium over con nective tissue of the lamina propria (LP). The connective tissue is secreted by large, elongated cells called cementoblasts. (X100; H&E) continuous with that of the periosteum (P) covering the alveolar (c) Polarizing light micrograph shows the continuity of collagen bone (B) and with the periodontal ligament (PL). (X10; H&E) fibers in alveolar bone (B),with the bundles in the periodontal ligament (L). (X200; Picrosirius in polarized light) FIGURE 15-12 Esophagus. Mucosa Submucosa Muscularis Adventitia MM a b (a) In cross section the four major layers of the Gl tract are clearly (b)Higher magnification of the mucosa shows the stratified seen.The esophageal mucosa is folded longitudinally, with the squamous epithelium (E),the lamina propria (LP) with scattered lumen largely closed. (X10; H&E) and strands of smooth muscle lymphocytes, in the muscularis mucosae (MM). (X65; H&E) 319/573  Stomach 307 FIGURE 15-13 Esophagus. CHAPTER LP St 15 MM Sm MM Digestive GL St Sm D Tract (a) Longitudinal esophagusshows mucosa consisting section of of (b) Transverse section showing the muscularis halfway along nonkeratinized squamous epithelium (SS),lamina pro stratified the esophagus reveals a combination of large skeletal or striated pria (LP), and smooth muscles of the muscularis mucosae (MM). muscle fibers (St) and smooth muscle fibers (Sm) in the outer Stomach Beneath the mucosa is the submucosa containing esophageal layer, which is cut transversely here. This transition from muscles mucous glands (GL) that empty via ducts (D) onto the luminal under voluntary control to the type controlled autonomically is surface. (X40; H&E) important in the swallowing mechanism.(X200; H&E) Swallowing begins with voluntarymuscle action but fin Four major regions make up the stomach: the cardia, fun ishes with involuntary peristalsis. In approximately the upper dus, body, and pylorus (Figure 15-14a). The cardia is a nar one-third of the esophagus,the muscularis is exclusively skel row transitional zone, 1.5-3 cm wide, between the esophagus etal muscle like that of the tongue.The middle portion of the and the stomach;the pylorus is the funnel-shaped region that esophagus has a combination of skeletal and smooth muscle opens into the small intestine. Both these regions are primarily fibers (Figure 15-13b), and in the lower third the muscularis involved with mucus production and are similar histogically. is exclusively smooth muscle. Only the distal 1-2 cm of the The much larger fundus and body regions are identical in esophagus, in the peritoneal covered by serosa; the cavity, is microscopic structure and are the sites of gastric glands releas rest is enclosed bythe loose connectivetissue of the adventitia, ing acidic gastric juice. The mucosa and submucosa of the which blends into the surrounding tissue. empty stomach have large, longitudinally directed folds called rugae, which flatten when the stomach fills with food. The > STOMACH wall in all regions of the stomach is made up of all four major layers (Figures 15-14c and 15-15). The stomach is a greatly dilated segment of the digestive tract whose main functions are: >> MEDICAL APPLICATION To continue the digestion of carbohydrates initiated by the amylase of saliva, Gastric and duodenal ulcers are painful erosive lesions of To add an acidic fluid to the ingested food and mixing its the mucosa that may extend to deeper layers. Such ulcers contents into a viscous mass called chyme by the churn can occur anywhere between the lower esophagus and the ing activity of the muscularis, jejunum,and their causes include bacterial infections with To begin digestion of triglycerides by a secreted lipase Helicobacter pylori, effects of nonsteroidal anti-inflammatory To promote the initial digestion of proteins with the drugs, overproduction of HClor pepsin, and lowered produc enzyme pepsin. tion or secretion of mucus or bicarbonate. > MEDICAL APPLICATION Mucosa For various reasons, including autoimmunity, parietal cells may be damaged to the extent that insufficient quantities of Changing abruptly at the esophagogastric junction (Figures 15-14b), the mucosal surface of thestomach is a sim intrinsic factor are secreted and vitamin B,, is not absorbed adequately. This vitamin is a cofactor required for DNA syn ple columnar epithelium that invaginates deeply into the lam thesis; low levels of vitamin B,, can reduce proliferation of ina propria. The invaginations form millions of gastric pits, erythroblasts, producing pernicious anemia. each with an opening to the stomach lumen (see Figure 15-14; Figure 15-16). The surface mucous cells that line the lumen 320/573