The digestive system consists of the digestive tract-oral cavity, esophagus, stomach, small and large intes-tines, and anus-and its associated glands-salivary glands, liver, and pancreas (Figure 15-1). Also called the gastrointestinal (Gl) tract or alimentary canal, its function is to obtain molecules from the ingested food that are necessary for the maintenance, growth, and energy needs of the body. During digestion proteins, complex carbohydrates, nucleic acids, and fats are broken down into their small molecule subunits that are easily absorbed through the small intestine lining. Most water and electrolytes are absorbed in the large intestine. In addition, the inner layer of the entire digestive tract forms an important protective barrier between the content of the tract’s lumen and the internal milieu of the body’s connective tissue and vasculature.
Structures within the digestive tract allow the following:
- Ingestion, or introduction of food and liquid into the oral cavity;
- Mastication, or chewing, which divides solid food into digestible pieces;
- Motility, muscular movements of materials through the tract;
- Secretion of lubricating and protective mucus, digestive enzymes, acidic and alkaline fluids, and bile;
- Hormone release for local control of motility and secretion;
- Chemical digestion or enzymatic degradation of large macromolecules in food to smaller molecules and their subunits;
- Absorption of the small molecules and water into the blood and lymph; and
- Elimination of indigestible, unabsorbed components of food.
GENERAL STRUCTURE OF THE DIGESTIVE TRACT
All regions of the GI tract have certain structural features in common. The GI tract is a hollow tube with a lumen of variable diameter and a wall made up of four main layers: the mucosa, submucosa, muscularis, and serosa. Figure 15-2 shows a general overview of these four layers; key features of each laver are summarized here.
- The mucosa consists of an epithelial lining; an underlying lamina propria of loose connective tissue rich in blood vessels, lymphatics, lymphocytes, smooth muscle cells, and often containing small glands; and a thin layer of smooth muscle called the muscularis mucosa separating mucosa from submucosa and allowing local movements of the mucosa. The mucosa is also frequently called a mucous membrane.
- The submucosa contains denser connective tissue with larger blood and lymph vessels and the submucosal (Meissner) plexus of autonomic nerves. It may also contain glands and significant lymphoid tissue.
- The thick muscularis (or muscularis externa) is composed of smooth muscle cells organized as two or more sublayers.
In the internal sublayer (closer to the lumen), the fiber orientation is generally circular; in the external sublayer it is longitudinal. The connective tissue between the muscle sublayers contains blood and lymph vessels, as well as the myenteric (Auerbach) nerve plexus of many autonomic neurons aggregated into small ganglia and interconnected by pre- and postganglionic nerve fibers. This and the submucosal plexus together comprise the enteric nervous system of the digestive tract. Con- tractions of the muscularis, which mix and propel the luminal contents forward, are generated and coordinated by the myenteric plexus.
■ The serosa, a thin sheet of loose connective tissue, rich in blood vessels, lymphatics, and adipose tissue, and covered with a simple squamous covering epithelium or mesothelium, is the outermost layer of the digestive tract located within the abdominal cavity. The serosa of the small and large intestines is continuous with portions of the mesentery, a large fold of adipose connective tis- sue, covered on both sides by mesothelium, that suspends the intestines and is continuous with the peritoneum, the serous membrane lining the abdominal cavity. The esophagus is not suspended in a cavity but bound directly to adjacent structures (Figure 15–1) and therefore lacks a serosa, having instead a thick adventitia, a layer of con- nective tissue continuous with that of surrounding tissues.
The numerous free immune cells and lymphoid nodules in the mucosa and submucosa constitute the MALT described in Chapter 14. The digestive tract normally contains thou- sands of microbial species, including both useful inhabitants of the gut as well as potential pathogens ingested with food and drink. The mucosa-associated immune defense system provides an essential backup to the thin physical barrier of the epithelial lining. Located just below the epithelium, the lamina propria is rich with macrophages and lymphocytes, many for production of IgA antibodies. Such antibodies undergo transcytosis into the intestinal lumen bound to the secretory protein produced by the epithelial cells. This IgA complex resists proteolysis by digestive enzymes and provides important pro- tection against specific viral and bacterial pathogens.
MEDICAL APPLICATION
In diseases such as Hirschsprung disease (congenital agan- glionic megacolon) or Chagas disease (trypanosomiasis, infection with the protozoan Trypanosoma cruzi), plexuses
in the digestive tract’s enteric nervous system are absent or severely injured, respectively. This disturbs digestive tract motility and produces dilations in some areas. The rich auto- nomic innervation of the enteric nervous system also provides an anatomic explanation of the well-known actions of emo- tional stress on the stomach and other regions of the GI tract.
› ORAL CAVITY
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- sively with skin (see Chapter 18). Like the keratinized sur- face cells of epidermis, the flattened superficial cells of the oral epithelium undergo continuous desquamation, or loss at the surface. Unlike those of the epidermis, the shed cells of the nonkeratinized or parakeratinized oral epithelium retain their nuclei.
››MEDICALAPPLICATION
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 weakened by emotional stress, fever, illness, or local skin damage, allowing the virus, present in the local nerves, to move into the epithelial cells.
The keratinized cell layers resist damage from abrasion and are best developed in the masticatory mucosa on the gingiva (gum) and hard palate. The lamina propria in these 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 oral cavity leading to the esophagus. Lining mucosa over- lies a thick submucosa containing many minor salivary glands, which secrete continuously to keep the mucosal surface wet, and diffuse lymphoid tissue. Throughout the oral cavity, the epithelium contains transient antigen-presenting cells and rich sensory innervation.
The well-developed core of striated muscle in the lips, or labia, (Figure 15–3) makes these structures highly mobile for ingestion, speech, and other forms of communication. Both lips have three differently covered surfaces:
■ The internal mucous surface has lining mucosa with a thick, nonkeratinized epithelium and many minor labial salivary glands.
■ The red vermilion zone of each lip is covered by very thin keratinized stratified squamous epithelium and
is transitional between the oral mucosa and skin. This region lacks salivary or sweat glands and is kept moist with saliva from the tongue. The underlying connective tissue is very rich in both sensory innervation and capil- laries, which impart the pink color to this region.
■ The outer surface has thin skin, consisting of epidermal and dermal layers, sweat glands, and many hair follicles with sebaceous glands.
Tongue
The tongue is a mass of striated muscle covered by mucosa, which manipulates ingested material during mastication and swallowing. The muscle fibers are oriented in all directions, allowing a high level of mobility. Connective tissue between the small fascicles of muscle is penetrated by the lamina pro- pria, which makes the mucous membrane strongly adherent to the muscular core. The lower surface of the tongue is smooth, with typical lining mucosa. The dorsal surface is irregular, hav- ing hundreds of small protruding papillae of various types on its anterior two-thirds and the massed lingual tonsils on the posterior third, or root of the tongue (Figure 15–4). The papil- lary and tonsillar areas of the lingual surface are separated by a V-shaped groove called the sulcus terminalis.
The lingual papillae are elevations of the mucous mem- brane that assume various forms and functions. There are four types (Figure 15–4):J
■ Filiform papillae (Figure 15–5) are very numer- ■ ous, have an elongated conical shape, and are heavily keratinized, which gives their surface a gray or whitish appearance. They provide a rough surface that facilitates movement of food during chewing.
■ Fungiform papillae (Figure 15–5) are much less numerous, lightly keratinized, and interspersed among the filiform papillae. They are mushroom-shaped
with well-vascularized and innervated cores of lamina propria.
■ Foliate papillae consist of several parallel ridges on each side of the tongue, anterior to the sulcus termi- nalis, but are rudimentary in humans, especially older individuals.
■ Vallate (or circumvallate) papillae (Figure 15–5) are the largest papillae, with diameters of 1-3 mm. Eight to twelve vallate papillae are normally aligned just in front of the terminal sulcus. Ducts of several small, serous salivary (von Ebner) glands empty into the deep, moatlike groove surrounding each vallate papilla.
This provides a continuous flow of fluid over the taste buds that are abundant on the sides of these papillae, washing away food particles so that the taste buds can receive and process new gustatory stimuli. Secretions from these and other minor salivary glands associated with taste buds contain a lipase that prevents the formation of a hydropho- bic film on these structures that would hinder gustation.
Taste buds are ovoid structures within the stratified epithelium on the tongue’s surface, which sample the general chemical composition of ingested material (Figures 15–4 and 15–5). Approximately 250 taste buds are present on the lat- eral surface of each vallate papilla, with many others present on fungiform and foliate (but not the keratinized filiform) papillae. They are not restricted to papillae and are also widely scattered elsewhere on the dorsal and lateral surfaces of the tongue, where they are also continuously flushed by numerous minor salivary glands.
A taste bud has 50-100 cells, about half of which are elon- gated gustatory (taste) cells, which turn over with a 7- to 10-day life span. Other cells present are slender supportive cells, immature cells, and slowly dividing basal stem cells that give rise to the other cell types. The base of each bud rests on the basal lamina and is entered by afferent sensory axons that
form synapses with the gustatory cells. At the apical ends of the gustatory cells, microvilli project toward a 2-μm-wide open- ing in the structure called the taste pore. Molecules (tastants) dissolved in saliva contact the microvilli through the pore and interact with cell surface taste receptors (Figure 15–4).
Taste buds detect at least five broad categories of tastants: sodium ions (salty); hydrogen ions from acids (sour); sugars and related compounds (sweet); alkaloids and certain tox- ins (bitter); and amino acids such as glutamate and aspartate (umami; Jap. umami, savory). Salt and sour tastes are pro- duced by ion channels, and the other three taste categories are mediated by G-protein-coupled receptors. Receptor binding produces depolarization of the gustatory cells, stimulating the sensory nerve fibers that transmit information to the brain for processing. Conscious perception of tastes in food requires olfactory and other sensations in addition to taste bud activity.
Teeth
In the adult human there are normally 32 permanent teeth, arranged in two bilaterally symmetric arches in the maxillary and mandibular bones (Figure 15–6a). Each quadrant has eight teeth: two incisors, one canine, two premolars, and three per- manent molars. Twenty of the permanent teeth are preceded by primary teeth (deciduous or milk teeth) that are shed; the oth- ers are permanent molars with no deciduous precursors. Each tooth has a crown exposed above the gingiva, a constricted neck at the gum, and one or more roots that fit firmly into bony sockets in the jaws called dental alveoli (Figure 15–6b).
The crown is covered by very hard, acellular enamel and the roots by a bone-like tissue called cementum. These two coverings meet at the neck of the tooth. The bulk of a tooth is composed of another calcified material, dentin, which sur- rounds an internal pulp cavity (Figure 15–6b). Dental pulp is highly vascular and well-innervated and consists largely of loose, mesenchymal connective tissue with much ground substance, thin collagen fibers, fibroblasts, and mesenchymal stem cells. The pulp cavity narrows in each root as the root canal, which extends to an opening (apical foramen) at the tip of each root for the blood vessels, lymphatics, and nerves of the pulp cavity. The periodontal ligaments are fibrous con- nective tissue bundles of collagen fibers inserted into both the cementum and the alveolar bone.
Dentin
Dentin is a calcified tissue harder than bone, consisting of 70% hydroxyapatite. The organic matrix contains type I colla- gen and proteoglycans secreted from the apical ends of odon- toblasts, tall polarized cells derived from the cranial neural crest that line the tooth’s pulp cavity (Figure 15–7a). Miner- alization of the predentin matrix secreted by odontoblasts involves matrix vesicles in a process similar to that occurring in osteoid during bone formation (see Chapter 8).
Long apical odontoblast processes extend from the odontoblasts within dentinal tubules (Figure 15–7b), which penetrate the full thickness of the dentin, gradually becoming longer as the dentin becomes thicker. Along their length, the processes extend fine branches into smaller lateral branches of the tubules (Figure 15–7c). The odontoblast processes are important for the maintenance of dentin matrix. Odontoblasts continue predentin production into adult life, gradually reduc- ing the size of the pulp cavity, and are stimulated to repair den- tin if the tooth is damaged.
Teeth are sensitive to stimuli such as cold, heat, and acidic pH, all of which can be perceived as pain. Pulp is highly innervated, and unmyelinated nerve fibers extend into the dental tubules along with odontoblast processes near the pulp cavity (Figure 15–8). Such stimuli can affect fluid inside the dentinal tubules, stimulat- ing these nerve fibers and producing tooth sensitivity.
››MEDICALAPPLICATION
Immune defenses in the oral cavity cannot protect against all infections. Pharyngitis and tonsillitis are often due to the bacterium Streptococcus pyogenes. White excrescences or leukoplakia on the sides of the tongue can be caused by Epstein-Barr virus. Oral thrush, a white exudate on the tongue’s dorsal surface, is due to a yeast (Candida albicans) infection and usually affects neonates or immunocompro- mised patients.
Enamel
Enamel is the hardest component of the human body, consist- ing of 96% calcium hydroxyapatite and only 2%-3% organic material including very few proteins and no collagen. Other ions, such as fluoride, can be incorporated or adsorbed by the hydroxyapatite crystals; enamel containing fluorapatite is more resistant to acidic dissolution caused by microorganisms, hence the addition of fluoride to toothpaste and water supplies.
Enamel consists of uniform, interlocking columns called enamel rods (or prisms), each about 5 μm in diameter and
surrounded by a thinner layer of other enamel. Each rod extends through the entire thickness of the enamel layer, which averages 2 mm. The precise, interlocked arrangement of the enamel rods is crucial for enamel’s hardness and resistance to great pressures during mastication.
In a developing tooth bud, the matrix for the enamel rods is secreted by tall, polarized cells, the ameloblasts (Figure 15–9a), which are part of a specialized epithelium in the tooth bud called the enamel organ. The apical ends of the ameloblasts face those of the odontoblasts producing predentine (Figure 15–10). An apical extension from each ameloblast, the ameloblast (or Tomes) process, contains numerous secretory granules with the proteins of the enamel matrix. The secreted matrix undergoes very rapid mineraliza- tion. Growth of the hydroxyapatite crystals to produce each elongating enamel rod is guided by a small (20 kDa) pro- tein amelogenin, the main structural protein of developing enamel.
Ameloblasts are derived from the ectodermal lining of the embryonic oral cavity, while odontoblasts and most tis- sues of the pulp cavity develop from neural crest cells and mesoderm, respectively. Together, these tissues produce a series of 52 tooth buds in the developing oral cavity, 20 for the primary teeth and 32 for the secondary or permanent teeth. Primary teeth complete development and begin to erupt about 6 months after birth. Development of the secondary tooth buds arrests at the “bell stage,” shown in Figure 15–10a, until about 6 years of age, when these teeth begin to erupt as the primary teeth are shed.
››MEDICALAPPLICATION
Periodontal diseases include gingivitis, inflammation of the gums, and periodontitis, which involves inflammation at deeper sites, both of which are caused most commonly
by bacterial infections with poor oral hygiene. Chronic peri- odontitis weakens the periodontal ligament and can lead to loosening of the teeth. The depth of the gingival sulcus, mea- sured during clinical dental examinations, is an important indicator of potential periodontal disease.
Periodontium
The periodontium comprises the structures responsible for maintaining the teeth in the maxillary and mandibular bones, and includes the cementum, the periodontal liga- ment, and the alveolar bone with the associated gingiva (Figures 15–6b and 15–11).
Cementum covers the dentin of the root and resembles bone, but it is avascular. It is thickest around the root tip where cementocytes reside in lacunae with processes in canaliculi, especially near the cementum surface. Although less labile
than bone, cementocytes maintain their surrounding matrix and react to stresses by gradually remodeling.
The periodontal ligament is fibrous connective tis- sue with bundled collagen fibers (Sharpey fibers) binding the cementum and the alveolar bone (Figure 15–11). Unlike typi- cal ligaments, it is highly cellular and has a rich supply of blood vessels and nerves, giving the periodontal ligament sensory and nutritive functions in addition to its role in supporting the tooth. It permits limited movement of the tooth within the alveolus and helps protect the alveolus from the recurrent pres- sure exerted during mastication. Its thickness (150-350 μm) is fairly uniform along the root but decreases with aging.
The alveolar bone lacks the typical lamellar pattern of adult bone but has osteoblasts and osteocytes engaging in con- tinuous remodeling of the bony matrix. It is surrounded by the periodontal ligament, which serves as its periosteum. Col- lagen fiber bundles of the periodontal ligament penetrate this bone, binding it to the cementum (Figure 15–11c).
Around the peridontium the keratinized oral mucosa of the gingiva is firmly bound to the periosteum of the maxillary and mandibular bones (Figure 15–11). Between the enamel and the gingival epithelium is the gingival sulcus, a groove up to 3 mm deep surrounding the neck (Figure 15–11a). A specialized part of this epithelium, the junctional epithe- lium, is bound to the tooth enamel by means of a cuticle, which resembles a thick basal lamina to which the epithelial cells are attached by numerous hemidesmosomes.
› ESOPHAGUS
The esophagus is a muscular tube, about 25-cm long in adults, which transports swallowed material from the pharynx to the stomach. The four layers of the GI tract (Figure 15–12) first become well-established and clearly seen in the esophagus. The esophageal mucosa has nonkeratinized stratified squa- mous epithelium, and the submucosa contains small mucus- secreting glands, the esophageal glands, which lubricate
and protect the mucosa (Figure 15–13a). Near the stomach the mucosa also contains groups of glands, the esophageal cardiac glands, which secrete additional mucus.
››MEDICALAPPLICATION
The lubricating mucus produced in the esophagus offers little protection against acid that may move there from the stomach. Such movement can produce heartburn or reflux esophagitis. An incompetent inferior esophageal sphincter may result in chronic heartburn, which can lead to erosion of the esophageal mucosa or gastroesophageal reflux disease (GERD). Untreated GERD can produce metaplastic changes
in the stratified squamous epithelium of the esophageal mucosa, a condition called Barrett esophagus.
Swallowing begins with voluntary muscle action but fin- ishes with involuntary peristalsis. In approximately the upper one-third of the esophagus, the muscularis is exclusively skel- etal muscle like that of the tongue. The middle portion of the esophagus has a combination of skeletal and smooth muscle fibers (Figure 15–13b), and in the lower third the muscularis is exclusively smooth muscle. Only the distal 1-2 cm of the esophagus, in the peritoneal cavity, is covered by serosa; the rest is enclosed by the loose connective tissue of the adventitia, w›hich blends into the surrounding tissue.
STOMACH
The stomach is a greatly dilated segment of the digestive tract whose main functions are
■ to continue the digestion of carbohydrates initiated by the amylase of saliva,
■ to add an acidic fluid to the ingested food and mixing its contents into a viscous mass called chyme by the churn- ing activity of the muscularis,
■ tobegindigestionoftriglyceridesbyasecretedlipase,and
■ to promote the initial digestion of proteins with the enzyme pepsin.
››MEDICALAPPLICATION
For various reasons, including autoimmunity, parietal cells may be damaged to the extent that insufficient quantities of intrinsic factor are secreted and vitamin B12 is not absorbed adequately. This vitamin is a cofactor required for DNA syn- thesis; low levels of vitamin B12 can reduce proliferation of erythroblasts, producing pernicious anemia.
Four major regions make up the stomach: the cardia, fundus, body, and pylorus (Figure 15–14a). The cardia is a nar- row transitional zone, 1.5-3 cm wide, between the esophagus and the stomach; the pylorus is the funnel-shaped region that opens into the small intestine. Both these regions are primarily involved with mucus production and are histogically similar. The much larger fundus and body regions are identical in microscopic structure and are the sites of gastric glands releas- ing acidic gastric juice. The mucosa and submucosa of the empty stomach have large, longitudinally directed folds called rugae, which flatten when the stomach fills with food. The wall in all regions of the stomach is made up of all four major layers (Figures 15–14c and 15–15).
››MEDICALAPPLICATION
Gastric and duodenal ulcers are painful erosive lesions of
the mucosa that may extend to deeper layers. Such ulcers can occur anywhere between the lower esophagus and the jejunum, and their causes include bacterial infections with Helicobacter pylori, effects of nonsteroidal anti-inflammatory drugs, overproduction of HCl or pepsin, and lowered produc- tion or secretion of mucus or bicarbonate.
Mucosa
Changing abruptly at the esophagogastric junction (Figure 15–14b), the mucosal surface of the stomach is a sim- ple columnar epithelium that invaginates deeply into the lam- ina propria. The invaginations form millions of gastric pits, each with an opening to the stomach lumen (see Figures 15–14 and 15–16). The surface mucous cells that line the lumen and gastric pits secrete a thick, adherent, and highly viscous mucous layer, which is rich in bicarbonate ions and protects the mucosa from both abrasive effects of intraluminal food and the corrosive effects of stomach acid.
The gastric pits lead to long, branched, tubular glands that extend through the full thickness of the lamina propria. Stem cells for the epithelium lining the glands, pits, and stomach lumen are found in a narrow segment (isthmus) between each gastric pit and the gastric glands. The pluripotent stem cells divide asymmetrically, producing progenitor cells for all the other epithelial cells. Some of these move upward to replace surface mucous cells, which have a turnover time of 4-7 days.
Other progenitor cells migrate more deeply and differentiate into the secretory cells of the glands that turn over much more slowly than the surface mucous cells.
The vascularized lamina propria surrounding and sup- porting the gastric pits and glands contains smooth muscle fibers, lymphoid cells, capillaries, and lymphatics. Separat- ing the mucosa from the underlying submucosa is a layer of smooth muscle, the muscularis mucosae (Figure 15–15).
In the fundus and body the gastric glands themselves
fill most of the mucosa, with several such glands formed by branching at the isthmus or neck of each gastric pit. Secretory epithelial cells of the gastric glands are distributed unevenly ■ and release products that are key to the stomach’s functions.
Junqueira’s Basic Histology: Text and Atlas, Fifteenth Edition
These cells are of four major types and important properties of each are as follows:
■ Mucous neck cells are present mainly clustered but also occur singly among the other cells in the necks of gas- tric glands and include many progenitor and immature surface mucous cells (Figure 15–17). Less columnar than the surface mucous cells lining the gastric pits, mucous neck cells are often distorted by neighboring cells, but they have round nuclei and apical secretory granules. Their mucus secretion is less alkaline than that of the surface epithelial mucous cells.
Parietal (oxyntic) cells produce hydrochloric acid (HCl) and are present among the mucous neck cells and throughout deeper parts of the gland. They are large cells, usually appearing rounded or pyramidal, each with one (sometimes two) central round nucleus. The cyto- plasm is intensely eosinophilic due to the high density
of mitochondria (Figure 15–17). A striking ultrastruc- tural feature of an active parietal cell is a deep, circular invagination of the apical plasma membrane to form an intracellular canaliculus with a large surface area produced by thousands of microvilli (Figure 15–18). As shown in Figure 15–19, carbonic anhydrase cata- lyzes the conversion of cytoplasmic water and CO2 into HCO3– and H+. The HCO3– is transported from the basal side of the cell and H+ is pumped from the cell apically, along with Cl–. In the lumen the H+ and Cl– ions com- bine to form HCl. While the gastric secretion becomes highly acidic, the mucosa itself remains at a more neutral pH partly because of the bicarbonate released into the lamina propria. The abundant mitochondria provide energy primarily for operating the cells’ ion pumps.
Parietal cells also secrete intrinsic factor, a glycoprotein required for uptake of vitamin B12 in the small intestine.
Parietal cell secretory activity is stimulated both by parasympathetic innervation and by paracrine release of
histamine and the polypeptide gastrin from enteroendo- crine cells.
■ Chief (zymogenic) cells predominate in the lower regions of the gastric glands (Figure 15–17) and have all the characteristics of active protein-secreting cells. Ultra- structurally chief cells show abundant RER and numerous apical secretory granules (Figure 15–20). The granules contain inactive enzyme pepsinogens, precursors which are converted in the acid environment of the stomach into active pepsins (Gr. peptein, to digest). Pepsins are endoproteinaseswithbroadspecificityandmaximal activityatapHbetween1.8and3.5.Pepsinsinitiatethe hydrolysis of ingested protein in the stomach. Chief cells also produce gastric lipase, which digests many lipids.
■ Enteroendocrine cells are scattered epithelial cells in the gastric mucosa with endocrine or paracrine func- tions. In the fundus small enteroendocrine cells secreting serotonin (5-hydroxytryptamine) are found at the basal lamina of the gastric glands (Figure 15–20). In the pylo- rus other enteroendocrine cells are located in contact with the glandular lumens, including G cells producing the peptide gastrin.
Various enteroendocrine cells secreting different hor- mones, usually peptides, are also found in the intestinal mucosa and are of major importance for function of the digestive tract. Important examples are summarized in Table 15–1. Seldom seen by routine light microscopy, these cells can be visualized by TEM tissue treatment with chromium or silver salts. This provided the alternative names enterochromaffin (EC) cells and argentaffin cells, respectively. Now usually visualized immunohistochemically using antibodies against their product, they are named with the initial letter of the main hormone they produce (Table 15–1). Most of these cells process amines and are also collectively called APUD cells for their “amine precur- sor uptake and decarboxylation” activity. All such cells are more generally considered part of the diffuse neuroendocrine sys- tem (DNES), which is discussed further in Chapter 20.
››MEDICALAPPLICATION
Tumors called carcinoids, which arise from enteroendocrine EC cells, are responsible for the clinical symptoms caused by overproduction of serotonin. Serotonin increases gut motil- ity, and chronic high levels of this hormone/neurotrans- mitter can produce mucosal vasoconstriction and tissue damage.
Upon stimulation, these cells release their hormone prod- ucts that then exert paracrine (local) or endocrine (systemic) effects via the vasculature. Cells of the digestive tract DNES fall into two classes: a “closed” type, in which the cellular apex is covered by neighboring epithelial cells (Figure 15–20), and an “open” type, in which the constricted apical end of the cell contacts the lumen and bears chemoreceptors that sample the lumen’s contents. Effects of the hormones include regulation of peristalsis and tract motility; secretion of digestive enzymes, water, and electrolytes; and the sense of being satiated after eating.
In the cardia and pylorus regions of the stomach, the mucosa also contains tubular glands, with long pits, branch- ing into coiled secretory portions, called cardiac glands and pyloric glands (Figure 15–21). These glands lack both pari- etal and chief cells, primarily secreting abundant mucus.
コメント