parietal cell
Summary
Parietal cells (oxyntic cells) are located in the gastric glands of the fundus and body of the stomach. They secrete hydrochloric acid (HCl) and intrinsic factor, both essential for digestion and vitamin B12 absorption, respectively. They are stimulated by gastrin, histamine, and acetylcholine, and are the target of proton pump inhibitors and H2 blockers.
Detail
Parietal cells are found in the gastric glands, predominantly in the fundus and body of the stomach, and are essential for both digestion and micronutrient absorption. They perform two primary functions: (1) secretion of hydrochloric acid via the H+/K+ ATPase (proton pump) on their apical membrane, which acidifies the stomach lumen to activate pepsinogen to pepsin and kill ingested pathogens; and (2) secretion of intrinsic factor, a glycoprotein required for binding and absorption of vitamin B12 (cobalamin) in the terminal ileum.
Acid secretion is regulated by three main stimulatory pathways that converge on the parietal cell: (1) gastrin, released by G cells in response to gastric distension and peptides, acts on CCK-B receptors; (2) histamine, released by enterochromaffin-like (ECL) cells in response to gastrin and vagal stimulation, acts on H2 receptors (this is the main target of H2 blockers like ranitidine); and (3) acetylcholine, released by vagal efferents, acts on M3 muscarinic receptors. All three pathways ultimately increase intracellular cAMP or calcium, leading to activation and insertion of H+/K+ ATPase pumps into the apical membrane, driving proton secretion into the stomach lumen in exchange for potassium.
Clinically, parietal cells are central to several important pathologies. Autoimmune destruction of parietal cells causes autoimmune (Type A) gastritis, leading to achlorhydria, loss of intrinsic factor, and pernicious anemia (vitamin B12 deficiency) due to impaired ileal absorption. This is characterized by anti-parietal cell and anti-intrinsic factor antibodies, and it also carries an increased risk of gastric carcinoid tumors due to chronic hypergastrinemia (loss of negative feedback from acid). Zollinger-Ellison syndrome involves a gastrin-secreting tumor that overstimulates parietal cells, causing excessive acid production and refractory peptic ulcers. Pharmacologically, parietal cells are targeted by proton pump inhibitors (e.g., omeprazole), which irreversibly inhibit the H+/K+ ATPase, and H2 receptor antagonists (e.g., ranitidine, famotidine), both used to treat peptic ulcer disease, GERD, and Zollinger-Ellison syndrome.
Sources
- First Aid for the USMLE Step 1
- Guyton and Hall Textbook of Medical Physiology
- Robbins and Cotran Pathologic Basis of Disease
- Katzung's Basic and Clinical Pharmacology
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