Figure 1611

Diagram of parietal cell HCl synthesis. Following parietal cell stimulation, several steps occur leading to the production of HCl. Carbon dioxide (C02) from the blood diffuses across the basement membrane into the cell to form H2C03. The H2C03 dissociates Into H+ and HC03_. The reaction is catalyzed by carbonic anhydrase, which leads to the production of H+ ions in the cytoplasm, which are then transported across the membrane to the lumen of the intracellular canaliculus by a H7(C-ATPase proton pump. Simultaneously, K' within the canaliculus is transported into the cell in exchange for the H~ ions. CI- ions are also transported from the cytoplasm of the parietal cell into the lumen of the canaliculus by CI- channels in the membrane. HCl is then formed from H" and CI". The HC03~/CI_ anion channels maintain the normal concentration of both ions in the cell, as well as NaVK'-AT-Pase on the basolateral cell membrane.

plexes with vitamin Bl2 in the stomach and duodenum, a step necessary for subsequent absorption of the vitamin in the ileum.

Enteroendocrine cells secrete their products into the lamina propria

Enteroendocrine cells are found at every level of the fundic gland, although they tend to be somewhat more prevalent in the base. They are small cells that rest on the basal lamina and do not always reach the lumen (Fig. 16.12). Some, however, have a thin cytoplasmic extension bearing microvilli that are exposed to the gland lumen. It is thought that these cells sample the contents of the gland lumen and release hormones on the basis of the information from those samples.

Electron micrographs reveal small membrane-bounded secretory granules throughout the cytoplasm; however, the granules are typically lost in H&E preparations, and the cytoplasm appears clear because of the lack of sufficient stainable material. Although these cells are often difficult to identify because of their small size and lack of distinctive staining, the clear cytoplasm of the cell sometimes stands out in contrast to adjacent chief or parietal cells, thus allowing their easy recognition.

The names given to the enteroendocrine cells in the older literature were based on their staining with salts of silver and chromium, i.e., enterochromaffin cells, argentaffin cells, and argyropbil cells. Such cells are currently identified and characterized by immunochemical staining for the more than 20 peptide and polypeptide hormones and hormone-like regulating agents that they secrete (a list of many of these agents and their actions is given in Fig. 16.13 and in Tables 16.1 and 16.2). With the aid of the TEM, at least 17 different types of enteroendocrine cells lumen secretory granules

Golgi apparatus lumen secretory granules

Golgi apparatus


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