Parietal cell

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Parietal cell
Parietal cells.jpg
Human parietal cells (pink staining) - stomach
Control-of-stomach-acid-sec.png
Control of stomach acid
Latin exocrinocytus parietalis
MeSH Gastric+Parietal+Cells
Code TH H3.04.02.1.00033
Anatomical terminology

Parietal cells (also known as oxyntic or delomorphous cells), are the stomach epithelial cells that secrete gastric acid (HCl) and intrinsic factor. These cells are only located in glands found in a particular area of the stomach, known as the fundus. They contain an extensive secretory network (called canaliculi) from which the HCl is secreted by active transport into the stomach. The enzyme hydrogen potassium ATPase (H+/K+ ATPase) is unique to the parietal cells and transports the H+ against a concentration gradient of about 3 million to 1, which is the steepest ion gradient formed in the human body. Parietal cells are primarily regulated via histamine, acetylcholine and gastrin signaling from both central and local modulators (see 'Regulation').

Secretion of Hydrochloric Acid[edit]

Synthesis[edit]

Hydrochloric acid is formed in the following manner:

  • Hydrogen ions are formed from the dissociation of water molecules. The enzyme carbonic anhydrase converts one molecule of carbon dioxide and one molecule of water indirectly into a bicarbonate ion (HCO3-) and a hydrogen ion (H+).
  • The bicarbonate ion (HCO3-) is exchanged for a chloride ion (Cl-) on the basal side of the cell and the bicarbonate diffuses into the venous blood, leading to an alkaline tide phenomenon.
  • Potassium (K+) and chloride (Cl-) ions diffuse into the canaliculi.
  • Hydrogen ions are pumped out of the cell into the canaliculi in exchange for potassium ions, via the H+/K+ ATPase.

As a result of the cellular export of hydrogen ions, the gastric lumen is maintained as a highly-acidic environment. The acidity aids in digestion of food by promoting the unfolding (or denaturing) of ingested proteins. As proteins unfold, the peptide bonds linking component amino acids are exposed. Gastric HCl simultaneously activates pepsinogen, an endopeptidase that advances the digestive process by breaking the now-exposed peptide bonds, a process known as proteolysis.

Regulation[edit]

Parietal cells secrete acid in response to three types of stimuli:[1]

Input through these receptors regulates acid secretion by increasing intracellular levels cAMP and Ca2+ by independent signaling pathways. The histamine receptors act by causing activation of increasing adenylate cyclase in the parietal cell. This increases intracellular cyclic AMP, which leads to activation of protein kinase A. By contrast, the M3 and gastrin receptor pathways lead to protein kinase activation by a different pathway that involves increasing intracellular Ca2+ levels. Protein kinase A then phosphorylates proteins involved in the transport of H+/K+ ATPase from the cytoplasm to the cell membrane. This causes resorption of K+ ions and secretion of H+ ions. The pH of the secreted fluid can fall by 0.8.

Gastrin primarily induces acid-secretion indirectly, increasing histamine synthesis in ECL cells,which in turn signal parietal cells via histamine release/H2 stimulation.[3] Gastrin itself has no effect on the maximum histamine-stimulated gastric acid secretion.[4]

Intrinsic factor[edit]

Parietal cells also produce intrinsic factor. Intrinsic factor is required for the absorption of Vitamin B12 in the diet. A long-term deficiency in vitamin B12 can lead to megaloblastic anemia, characterized by large fragile erythrocytes.Pernicious anaemia is a condition where intrinsic factor is not produced and leads to the same type of anemia. Atrophic gastritis, particularly in the elderly, will cause an inability to absorb B12 and can lead to deficiencies such as decreased DNA synthesis and nucleotide metabolism in the bone marrow.

Canaliculus[edit]

A canaliculus is an adaptation found on gastric parietal cells. It is a deep infolding, or little channel, which serves to increase the surface area, e.g. for secretion. The parietal cell membrane is dynamic; the numbers of canaliculi rise and fall according to secretory need. This is accomplished by the fusion of canalicular precursors, or "tubulovesicles", with the membrane to increase surface area, and the reciprocal endocytosis of the canaliculi (reforming the tubulovesicles) to decrease it.

Diseases of parietal cells[edit]

Immunofluorescence staining pattern of gastric parietal antibodies on a stomach section

See also[edit]

References[edit]

  1. ^ Boulpaep, Walter (2009). Medical Physiology. Philadelphia: Saunders. pp. 898–899. ISBN 978-1-4160-3115-4. 
  2. ^ "Gastric acid secretion - Homo sapiens". KEGG. Retrieved June 1, 2011. 
  3. ^ Waldum, Helge L., Kleveland, Per M., et al. (2009)'Interactions between gastric acid secretagogues and the localization of the gastrin receptor',Scandinavian Journal of Gastroenterology,44:4,390 — 393
  4. ^ Kleveland PM, Waldum HL, Larsson M. Gastric acid secretion in the totally isolated, vascularly perfused rat stomach. A selective muscarinic-1 agent does, whereas gastrin does not, augment maximal histamine-stimulated acid secretion. Scand J Gastroenterol 1987;/22:/705�13.

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