The fundus of the stomach is relatively thin-walled and can be expanded with ingested food (see Fig. 26.24). The main body (corpus) of the empty stomach is composed of many longitudinal folds called rugae gastricae. The stomach's mu-cosal lining, the glandular gastric mucosa, contains three main types of glands: cardiac, pyloric, and oxyntic. These glands contain mucous cells that secrete mucus and HCO3~ ions, which protect the stomach from the acid in the stom-
Gastric pit a)
Gastric pit a)
i Oxyntic ^ gland
Neck i Oxyntic ^ gland
A simplified diagram of the oxyntic gland in the corpus of a mammalian stomach.
One to several glands may open into a common gastric pit. (Modified from Ito S. Functional gastric morphology. In: Johnson LR, Christensen J, Jackson MJ, et al. eds. Physiology of the Gastrointestinal Tract. New York: Raven, 1987.)
ach lumen. The cardiac glands are located in a small area adjacent to the esophagus and are lined by mucus-producing columnar cells. The pyloric glands are located in a larger area adjacent to the duodenum. They contain cells similar to mucous neck cells but differ from cardiac and oxyntic glands in having many gastrin-producing cells called G cells. The oxyntic glands, the most abundant glands in the stomach, are found in the fundus and the corpus.
The oxyntic glands contain parietal (oxyntic) cells, chief cells, mucous neck cells, and some endocrine cells (Fig. 27.4). Surface mucous cells occupy the gastric pit (foveola); in the gland, most mucous cells are located in the neck region. The base of the oxyntic gland contains mostly chief cells, along with some parietal and endocrine cells. Mucous neck cells secrete mucus, parietal cells principally secrete hydrochloric acid (HCl) and intrinsic factor, and chief cells secrete pepsinogen. (Intrinsic factor and pepsinogen are discussed later in the chapter.)
Parietal cells are the most distinctive cells in the stomach. The structure of resting parietal cells is unique in that they have intracellular canaliculi as well as an abundance of mitochondria (Fig. 27.5A). This network consists of clefts and canals that are continuous with the lumen of the oxyn-tic gland. There is also an extensive smooth ER referred to as the tubulovesicular membranes. In active parietal cells (Fig. 27.5B), the tubulovesicular system is greatly diminished with a concomitant increase in the intracellular canaliculi. The mechanism for these morphological changes is not well understood. Hydrochloric acid is secreted across the parietal cell microvillar membrane and flows out of the intracellular canaliculi into the oxyntic gland lumen. As mentioned, surface mucous cells line the entire surface of the gastric mucosa and the openings of the cardiac, pyloric, and oxyntic glands. These cells secrete mucus and HCO3~ to protect the gastric surface from the
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