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Gastric mucosa is a layer of the stomach composed of epithelium, lamina propria, and muscularis mucosae. The epithelium covers the surface of the gastric mucosa and consists of a single layer of columnar cells that secrete a thick, slippery, clear fluid called mucus as well as an alkaline fluid. Mucus and alkaline fluid provide protection to the epithelium from mechanical injury and gastric acid. The surface of the gastric mucosa also contains numerous duct openings called gastric pits, in which one or more gastric glands empty.
The gastric mucosa is divided into three regions including the cardiac glandular region located just below the lower esophageal sphincter, the oxyntic glandular region located in the body and fundus, and the pyloric glandular region located in the antrum. Both the cardiac and pyloric glandular regions contain primarily mucus-secreting gland cells, but the latter also contains gastrin cells (G cells), which secrete the hormone gastrin. The oxyntic (acid-forming) glandular region contains three types of cells including mucus secreting mucous neck cells, parietal or oxyntic cells which secrete hydrochloric acid and intrinsic factor, and pepsinogen secreting peptic or chief cells.
Gastrin promotes the secretion of hydrochloric acid and pepsinogens by stimulating parietal cells and peptic cells. Hydrochloric acid breaks down food material, kills most ingested microorganisms, and catalyzes the conversion of inactive pepsinogen to active pepsin, which is responsible for protein digestion. Pepsinogens are contained in zymogen granules in the peptic or chief cells, and are released by exocytosis. The presence of hydrochloric acid provides the necessary acidic conditions for the conversion of pepsinogen to pepsin. Intrinsic factor binds vitamin B12 and allows it to be absorbed in the ileum.
The basic factors that stimulate gastric secretions are acetylcholine, gastrin, and histamine. Acetylcholine stimulates the secretion of mucus by mucus cells, hydrochloric acid by parietal or oxyntic cells, and pepsinogen by peptic or chief cells. Gastrin and histamine exert their effects only on parietal or oxyntic cells.
During normal gastric function, surface epithelial cells are normally exfoliated into the lumen. These cells are replaced by regenerative cells, which have the ability to differentiate into columnar epithelial cells and to migrate to their new location. In addition, these cells are columnar stem cells interspersed among the mucous neck cells. The stomach uses these cells to repair epithelial damage to the surface.
There are also diffuse neuroendocrine system (DNES) cells dispersed among the other epithelial cells of the gastric mucosa. DNES cells are endocrine cell types in diverse locations throughout the body. These cells secrete hormone-like substances.