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
FIGURE 17.9 • 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. Secretory vesicles (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 cell produces and secretes the precursor enzyme of the gastric secretion.
allow them to be easily distinguished from other cells in the fundic glands.
When examined with the transmission electron microscope (TEM), parietal cells (Fig. 17.10) 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 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 receptors, histamine H2 receptors, and acetylcholine M3 receptor. Activation of the gastrin receptor by gastrin, a gastrointestinal peptide hormone, is the major path for parietal cell stimulation (Folder 17.2). After stimulation, several steps occur in the production of HCl (Fig. 17.11):
FIGURE 17.10 • Diagram of a parietal cell. The cytoplasm of the parietal cell stains with eosin largely because of the extensive amount of membrane comprising the intracellular canaliculus, tubulovesicular system, mitochondria, and the relatively small number of ribosomes. This cell produces HCl and intrinsic factor.
 Production of H ions in the parietal cell cytoplasm by the enzyme carbonic anhydrase. This enzyme hydrolyzes carbonic acid (H2CO3) to H and HCO3  . Carbon dioxide (CO2), necessary for synthesis of carbonic acid, diffuses across the basement membrane into the cell from the blood capillaries in the lamina propria.
 Transport of H ions from the cytoplasm, across the membrane to the lumen of the canaliculus by the H/ K-ATPase proton pump. Simultaneously, K from the canaliculus is transported into the cell cytoplasm in exchange for the H ions.
 Transport of K and Cl ions from the parietal cell cytoplasm into the lumen of the canaliculus through activation of K and Cl channels (uniporters) in the plasma membrane.
 Formation of HCl from the H and Cl that were transported into the lumen of the canaliculus.