Mills Ch22 Stomach

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SECTION VII : AlimentaryTract

The muscularis mucosae consist of two layers, the inner circular and outer longitudinal, together with some elastic fibers. Thin bundles of smooth muscles also penetrate into the lamina propria, where they terminate in the basement membrane of the epithelium. This is most obvious in the antral area. ULTRASTRUCTURE The surface and foveolar lining epithelial cells are ultra- structurally similar. They are characterized by multiple rounded, electron-dense mucous vacuoles in the superficial cytoplasm and stubby microvilli projecting from the luminal surface. The basal cytoplasm contains moderate amounts of rough endoplasmic reticulum and some mitochondria. Adjacent epithelial cells are joined by tight junctions (zonula occludens) at their luminal aspect and by adherence junc- tions along the rest of the cell interfaces. These tight junc- tions are considered to play an important role in maintaining mucosal integrity and the gastric mucosal barrier. Parietal cells are unique ultrastructurally (Fig. 22.14) (26). In the unstimulated state, the cytoplasm contains an apical crescent-shaped canaliculus lined by stubby micro- villi (Fig. 22.14). Between the microvilli are elongated membrane invaginations termed microtubules. Upon stimu- lation, the microtubules disappear, to be replaced by a dense meshwork of intracellular canaliculi (27). The canalicular system is considered essential for the formation of hydro- chloric acid. This is achieved by active transport of hydrogen ions across the canalicular membrane. Since this process

has high energy requirements, most of the remainder of the parietal cell cytoplasm is occupied by mitochondria. The zymogenic cells (chief cells) are similar to protein- secreting exocrine cells elsewhere in the body. They have rough-surfaced vesicles in the superficial cytoplasm and abundant rough endoplasmic reticulum in the remainder of the cell. GASTRIC FUNCTION The function of the stomach is to act as a reservoir and mixer of food and to initiate the digestive process. Gastric secretion of acid, pepsin, and electrolytes is partly under nervous control by the vagus and partly under the control of gastrin, produced by G cells in the antrum. Gastrin release from the G cells may occur either as a result of distention of the antrum or by direct stimulation from ingested food, particularly amino acids and peptides. Hydrochloric acid is produced by the active transport of hydrogen ions across the cell membrane. High concentrations of hydrochloric acid are achieved so that most ingested microorganisms are killed and the contents of the stomach are normally sterile. Gastric mucus is secreted in two forms: a soluble frac- tion produced by the gastric glands and an insoluble form produced by the surface and foveolar lining cells. Bio- chemically, the mucus is a complex glycoprotein consisting of a protein core with branched carbohydrate side chains. Histochemically, gastric mucin is almost entirely neutral, although the mucous neck cells secrete small amounts of sulfomucin and sialomucin (17). By immunohistochemistry,

FIGURE 22.14  Ultrastructural appear- ances of the parietal cell canaliculus ( C ). Note the fingerlike microvilli ( MV ) and the microtubular invaginations ( MT ). (Original magnifications: left , × 9,000; right , × 41,000.)