Parietal cell

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

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

Synthesis

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

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

• Histamine, stimulates H₂ histamine receptors (most significant contribution).
• Acetylcholine, from parasympathetic activity via the vagus nerve and enteric nervous system, stimulating M₃ receptors.^[2]
• Gastrin, stimulating CCK2 receptors (least significant contribution, but also causes histamine secretion by local ECL cells)

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

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 anemia 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

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 membrane of parietal cells 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

Immunofluorescence staining pattern of gastric parietal antibodies on a stomach section

• Peptic ulcers can result from over-acidity in the stomach. Antacids can be used to enhance the natural tolerance of the gastric lining. Antimuscarinic drugs such as pirenzepine or H₂ antihistamines can reduce acid secretion. Proton pump inhibitors are more potent at reducing gastric acid production since that is the final common pathway of all stimulation of acid production.
• In pernicious anemia, autoantibodies directed against parietal cells or intrinsic factor cause a reduction in vitamin B₁₂ absorption. It can be treated with injections of replacement vitamin B₁₂ (methylcobalamin, hydroxocobalamin or cyanocobalamin).
• Achlorhydria is another autoimmune disease of the parietal cells. The damaged parietal cells are unable to produce the required amount of gastric acid. This leads to an increase in gastric pH, impaired digestion of food and increased risk of gastroenteritis.

See also

• Chief cell
• Fundic glands
• Stomach
• Digestion
• Gastroesophageal reflux disease
• Discovery and Development of Proton Pump Inhibitors

References

External links

• Illustration of Chief cells and Parietal cells at anatomyatlases.org
• The Parietal Cell: Mechanism of Acid Secretion at vivo.colostate.edu
• BU Histology Learning System: 11303loa - Digestive System: Alimentary Canal: fundic stomach, gastric glands, lumen"
• Physiology at MCG 6/6ch4/s6ch4_8
• Physiology at MCG 6/6ch4/s6ch4_14
• Parietal cell antibody
• Antibody to GPC