Gastric Gland

cells is different from that produced by the surface mucous cells as evidenced by the lighter magenta staining in this region of the gland. X320. b. Schematic diagram of a gastric gland, illustrating the relationship of the gland to the gastric pit. Note that the isthmus region contains dividing cells and undifferentiated cells; the neck region contains mucous neck cells, parietal cells, and enteroen-docrine cells, including amine precursor uptake and decarboxylation (APUD) cells. Parietal cells are large, pear-shaped acidophilic cells found throughout the gland. The fundus of the gland contains mainly chief cells, some parietal cells, and several types of en-teroendocrine cells.

known as the isthmus, the site of cell replication. Cells destined to become mucous surface cells migrate upward in the gastric pits to the stomach surface. Other cells migrate downward, maintaining the population of the fundic gland epithelium. Typically, several glands open into a single gastric pit. Each gland has a narrow, relatively long neck segment and a shorter and wider base or fundic segment. The base of the gland usually divides into two and sometimes three branches that become slightly coiled near the muscularis mucosae. The cells of the gastric glands produce gastric juice (about 2 L/day), which contains a variety of substances. In addition to water and electrolytes, gastric juice contains four major components:

• Hydrochloric acid (HCl) in a concentration ranging from 150 to 160 mmol/L, which gives the gastric juice a low pH (<1.0 to 2.0). It is produced by parietal cells and initiates digestion of dietary protein (it promotes acid hydrolysis of substrates). It also converts inactive pepsinogen into the active enzyme pepsin. Because HCl is bacteriostatic, most of the bacteria entering the stomach with the ingested food are destroyed.

• Pepsin, a potent proteolytic enzyme. It is converted from pepsinogen produced by the chief cells by HCl at a pH lower than 5. Pepsin hydrolyzes proteins into small peptides by splitting interior peptide bonds. Peptides are further digested into amino acids by enzymes in the small intestine.

• Mucus, an acid-protective coating for the stomach secreted by several types of mucus-producing cells. The mucus and bicarbonates trapped within the mucous layer maintain a neutral pH and contribute to the so-called physiologic gastric mucosa barrier. In addition, mucus serves as a physical barrier between the cells of the gastric mucosa and the ingested material in the lumen of the stomach.

• Intrinsic factor, a glycoprotein that binds to vitamin B12. It is essential for absorption of vitamin B12, which occurs in the distal part of the ileum.

In addition, gastrin and other hormones and hormonelike secretions are produced by enteroendocrine cells in the fundic glands and secreted into the lamina propria where they enter the circulation or act locally on other gastric epithelial cells.

Fundic glands are composed of four functionally different cell types

The cells that constitute the fundic glands are of four functional types. Each has a distinctive appearance. In addition, undifferentiated cells that give rise to these cells are also present. The various cells that constitute the gland are

• Mucous neck cells

• Parietal cells, also called oxyntic cells

• Enteroendocrine cells

• Undifferentiated cells

Mucous neck cells are localized in the neck region of the gland and are interspersed with parietal cells

As the name implies, the mucous neck cells are located in the neck region of the fundic gland. Parietal cells are usually interspersed between groups of these cells. The mucous neck cell is much shorter than the surface mucous cell and contains considerably less mucinogen in the apical cytoplasm. Consequently, these cells do not exhibit a prominent mucous cup. Also, the nucleus tends to be spherical compared with the more prominent, elongate nucleus of the surface mucous cell.

The mucous neck cells secrete a soluble mucus compared with the insoluble or cloudy mucus produced by the surface mucous cell. Release of mucinogen granules is induced by vagal stimulation; thus, secretion from these cells does not occur in the resting stomach.

Chief cells are located in the deeper part of the fundic glands

Chief cells are typical protein-secreting cells (Fig. 16.9). The abundant rER in the basal cytoplasm gives this part of the cell a basophilic appearance, whereas the apical cytoplasm is eosinophilic due to the presence of the secretory granules, also called zymogen granules because they contain enzyme precursors. The basophilia, in particular, allows easy identification of these cells in H&E sections. The eosino-philia may be faint or absent when the secretory granules are not adequately preserved. Chief cells secrete pepsinogen and a weak lipase. On contact with the acid gastric juice, pepsinogen is converted to pepsin, a proteolytic enzyme.

Parietal cells secrete HCl and intrinsic factor

Parietal (oxyntic) cells are found in the neck of the fundic glands, among the mucous neck cells, and in the deeper part of the gland. They tend to be most numerous in the upper and middle portions of the neck. They are large cells, sometimes binucleate, and appear somewhat triangular in sections, with the apex directed toward the lumen of the gland and the base resting on the basal lamina. The nucleus is spherical, and the cytoplasm stains with eosin and other acidic dyes. Their size and distinctive staining characteristics allow them to be easily distinguished from other cells in the fundic glands.

When examined with the transmission electron microscope (TEM), parietal cells (Fig. 16.10) are seen to have an extensive intracellular canalicular system that communicates with the lumen of the gland. Numerous microvilli project from the surface of the canaliculi, and an elaborate tubulovesicular membrane system is present in the cytoplasm adjacent to the canaliculi. In an actively secreting lumen junctional complex zymogen granules

apparatus

basal lamina

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