Figure 169

Diagram of a chief cell. The large amount of rER in the basal portion of the cell accounts for the intense basophilic staining seen in this region. Zymogen granules containing pepsinogen and a weak lipase are not always adequately preserved, and thus the staining in the apical region of the cell is somewhat variable. This ceil produces and secretes the precursor enzyme of the gastric secretion. (Based on Lentz TL. Cell Fine Structure: An Atlas of Drawings of Whole-Cell Structure. Philadelphia: WB Saunders, 1971.)

cell, the number of microvilli in the canaliculi increases, and the tubulovesicular system is reduced significantly or disappears. The membranes of the tubulovesicular system serve as a reservoir of plasma membrane containing active proton pumps. This membrane material can be inserted into the plasma membrane of the canaliculi to increase their surface area and the number of proton pumps available for acid production. Numerous mitochondria with complex cristae and many matrix granules supply the high levels of energy necessary for acid secretion.

HCl is produced in the lumen of the intracellular canaliculi

Parietal cells have three different types of membrane receptors for substances that activate HCl secretion: gastrin, histamine H2, and acetylcholine M3 receptors. Activation of the gastrin receptor by gastrin, a gastrointestinal peptide hormone, is the major path for parietal cell stimulation. Following stimulation, several steps occur in the production of HCl (Fig. 16.11):

• Production of H+ ions in the parietal cell cytoplasm by the enzyme carbonic anhydrase. This enzyme hydrolyzes lumen junctional complex tubulovesicular system intracellular canaliculus lumen junctional complex tubulovesicular system intracellular canaliculus

lysosomes basal lamina

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