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Parietal cells (also known as oxyntic cells) are
epithelial cells Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...
in the
stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital digestive organ. In the digestive system ...
that secrete
hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the diges ...
(HCl) and intrinsic factor. These cells are located in the gastric glands found in the lining of the fundus and
body Body may refer to: In science * Physical body, an object in physics that represents a large amount, has mass or takes up space * Body (biology), the physical material of an organism * Body plan, the physical features shared by a group of animals ...
regions of the stomach. They contain an extensive secretory network of canaliculi from which the HCl is secreted by
active transport In cellular biology, active transport is the movement of molecules across a cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the pla ...
into the stomach. The enzyme
hydrogen potassium ATPase
hydrogen potassium ATPase
(H+/K+ ATPase) is unique to the parietal cells and transports the H+ against a
concentration In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they unde ...

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 Histamine is an organic nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele a ...

histamine
,
acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter—a chemical message released by nerve cells to send signals to other cells, such as neurons, muscl ...

acetylcholine
and
gastrin Gastrin is a peptide hormonePeptide hormones or protein hormones are hormones whose molecules are peptides or proteins, respectively. The latter have longer amino acid chain lengths than the former. These hormones have an effect on the endocrine s ...
signalling from both central and local modulators.


Structure


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 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.


Function


Hydrochloric acid secretion

Hydrochloric acid is formed in the following manner: * Hydrogen ions are formed from the dissociation of carbonic acid. Water is a very minor source of hydrogen ions in comparison to carbonic acid. Carbonic acid is formed from
carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occurs naturally in At ...

carbon dioxide
and water by
carbonic anhydrase
carbonic anhydrase
. * 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 Parietal cell#Canaliculus, canaliculi. * Hydrogen ions are pumped out of the cell into the canaliculi in exchange for potassium ions, via the hydrogen potassium ATPase, H+/K+ ATPase. These receptors are increased in number on lumenal side by fusion of tubulovesicles during activation of parietal cells and removed during deactivation. This receptor maintains a million-fold difference in proton concentration. ATP is provided by the numerous mitochondria. 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 Denaturation (biochemistry), denaturing) of ingested proteins. As proteins unfold, the peptide bonds linking component amino acids are exposed. Gastric HCl simultaneously cleaves pepsinogen, a zymogen, into active pepsin, an endopeptidase that advances the digestive process by breaking the now-exposed peptide bonds, a process known as proteolysis.


Regulation

Parietal cells secretion, secrete acid in response to three types of stimulus (physiology), stimuli: * Histamine, stimulates Histamine H2 receptor, H2 histamine receptors (most significant contribution). * Acetylcholine, from parasympathetic activity via the vagus nerve and enteric nervous system, stimulating Muscarinic acetylcholine receptor M3, M3 receptors. * Gastrin, stimulating Cholecystokinin B receptor, CCK2 receptors (least significant contribution, but also causes histamine secretion by local Enterochromaffin-like cell, ECL cells) Activation of histamine through H2 receptor causes increases intracellular Cyclic_adenosine_monophosphate, cAMP level while Ach through M3 receptor and gastrin through CCK2 receptor increases intracellular calcium level. These receptors are present on basolateral side of membrane. Increased cAMP level results in increased protein kinase A. Protein kinase A phosphorylates proteins involved in the transport of Hydrogen potassium ATPase, 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 Enterochromaffin-like cell, ECL cells,which in turn signal parietal cells via histamine release/H2 stimulation. Gastrin itself has no effect on the maximum histamine-stimulated gastric acid secretion. The effect of histamine, acetylcholine and gastrin is synergistic, that is, effect of two simultaneously is more than additive of effect of the two individually. It helps in non-linear increase of secretion with stimuli physiologically.


Intrinsic factor secretion

Parietal cells also produce a glycoprotein known as intrinsic factor. Intrinsic factor is required for the absorption of Vitamin B12, vitamin B in the diet. A long-term Vitamin B12 deficiency, deficiency in vitamin B12 can lead to megaloblastic anemia, characterized by large fragile red blood cells. Pernicious anaemia results from autoimmune destruction of gastric parietal cells, precluding the synthesis of intrinsic factor and, by extension, absorption of Vitamin B12. Pernicious anemia also leads to megaloblastic 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.


Clinical significance

* ''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'', autoantibody, 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

* Gastric chief cell * Digestion * Gastroesophageal reflux disease * Discovery and Development of Proton Pump Inhibitors *List of human cell types derived from the germ layers


References


External links


Illustration of Chief cells and Parietal cells at anatomyatlases.org


* - Digestive System: Alimentary Canal: fundic stomach, gastric glands, lumen" * *
Parietal cell antibody
{{Authority control Epithelial cells Animal cells Human cells Acid secreting cells Stomach