29.2 ). The gastric pits account for a significant fraction of the total surface area of the gastric mucosa. The gastric mucosa is divided into three distinct regions based on the structure of the glands. The small cardiac glandular region, located just below the lower esophageal sphincter (LES), primarily contains mucus-secreting gland cells. The remainder of the gastric mucosa is divided into the oxyntic or parietal (acid-secreting) gland region, located above the gastric notch (equivalent to the proximal part of the stomach), and the pyloric gland region, located below the notch (equivalent to the distal part of the stomach).
The structure of a gastric gland from the oxyntic glandular region is illustrated in
Fig. 29.2 . Surface epithelial cells extend slightly into the duct opening. The opening of the gland is called the isthmus and is lined with surface mucous cells and a few parietal cells. Mucous neck cells are located in the narrow neck of the gland. Parietal or oxyntic cells, which secrete HCl and intrinsic factor (involved in absorption of vitamin B12), and chief or peptic cells, which secrete pepsinogens, are located deeper in the gland. Oxyntic glands also contain enterochromaffin-like (ECL) cells that secrete histamine, and D cells that secrete somatostatin. Parietal cells are particularly numerous in glands in the fundus, whereas mucus-secreting cells are more numerous in glands •BOX 29.1FunctionsoftheStomach
Storage—acts as temporary reservoir for the meal
Secretion of H+ to kill microorganisms and convert pepsinogen to its active form
Secretion of intrinsic factor to absorb vitamin B12 (cobalamin)
Secretion of mucus and HCO3 − to protect the gastric mucosa
Secretion of water for lubrication and to provide aqueous suspension of nutrients
Motor activity for mixing secretions (H+ and pepsin) with ingested food
Coordinated motor activity to regulate the emptying of contents into the duodenum of the pyloric (antral) glandular region. In addition the pyloric glands contain G cells that secrete the hormone gastrin. The parietal glands are also divided into regions: the neck (mucous neck cells and parietal cells) and the base (peptic/chief and parietal cells). Endocrine cells are scattered throughout the glands.
The fluid secreted into the stomach is called gastric juice. Gastric juice is a mixture of the secretions of the surface epithelial cells and the secretions of gastric glands. One of the most important components of gastric juice is H+, which is secreted against a very large concentration gradient. Thus H+ secretion by the parietal mucosa is an energy-intensive process. The cytoplasm of the parietal cell is densely packed with mitochondria, which have been estimated to fill 30% to 40% of the cell’s volume. One major function of H+ is conversion of inactive pepsinogen (the major enzyme product of the stomach) to pepsins, which initiate protein digestion in the stomach. Additionally, H+ ions are important for preventing invasion and colonization of the gut by bacteria and other pathogens that may be ingested with food. The stomach also secretes significant amounts of HCO3 − and mucus, which are important for protection of the gastric mucosa against the acidic and peptic luminal environment. The gastric epithelium also secretes intrinsic factor, which is necessary for absorption of vitamin B12 (cobalamin). The functions of other components of gastric juice are redundant with secretions provided more distally in the GI tract.
Composition of Gastric Secretions