Parietal cells are one type of special cell that play a very crucial part in the digestive system of humans. Parietal cells are also called oxyntic cells. Parietal cells are one type of secretory epithelium cells that secrete hydrochloric acid. This hydrochloric acid makes the gastric juice acidic (pH 2.0 - 3.7). These oxyntic cells are found in the inner linings of the stomach. Besides hydrochloric acid, parietal cells secrete an intrinsic factor that helps in the absorption of vitamin B12. Such secretion of oxyntic cells is controlled by central and local modulators by some regulating factors such as histamine, acetylcholine, gastrin, etc.
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Structure and Location of Parietal Cells
Parietal cells are part of the human stomach. The stomach is the widest and distensible C-shaped part of the alimentary canal which is obliquely behind the diaphragm on the left side. The stomach wall is made of many folds that are known as gastric rogues. The stomach has mainly three parts - fundus, body and pyloric. Gastric parietal cells are the glandular part of the fundus and body regions of the stomach.
To explain the question of what are parietal cells, it is necessary to mention that it is a kind of epithelium cell that secrete hydrochloric acid and intrinsic factors. Oxyntic cells are made of canaliculi. Canaliculi is an extensive secretory network. To increase the surface area such canaliculi has a deep infolding. An interesting mechanism is followed here. Interestingly the number of canaliculi largely depends on the secretory needs. This canaliculi network helps in the secretion of hydrochloric acid. In the stomach, the secretion of hydrochloric acid needs active transport as it is transported against the H+concentration. That is why the enzyme hydrogen potassium ATPase is present in parietal cells to conduct active transport of hydrochloric acid.
Function of Oxyntic Cells
To discuss parietal cells function, it is needed to mention that to work properly gastric digestive enzymes require an acidic medium. Hydrochloric acid secreted by parietal cells maintains the acidic medium within the stomach.
The source of the hydrogen ion of hydrochloric acid is carbonic acid which is formed by carbon dioxide and water with the help of an enzyme called carbonic anhydrase. There is an exchange of bicarbonate ions with chloride ions through the membrane of parietal cells. Such ion exchange results in an alkaline tide phenomenon. Huge amounts of potassium and chloride ions are absorbed by simple diffusion. With the help of a potassium hydrogen pump and ATP, the hydrogen ion is absorbed into the parietal cells by the active ion exchange method. Interestingly the efficiency of this hydrogen potassium pump increases during the activation of parietal cells stomach.
Hydrochloric acid plays a vital role in the digestion of food within the stomach. It helps in the digestion of proteins by unfolding the peptide bonds of protein and thus helps the gastric enzymes to work properly on it. Hydrochloric acid transforms the inactive pepsinogen and prorenin into active enzymes pepsin and renin and thus helps in proteolysis. Parietal cells secrete an intrinsic factor which is one type of glycoprotein. This intrinsic factor plays an important role in the absorption of vitamin B12.
Regulation of Parietal Cells
Secretion of oxyntic cells is controlled by central and local modulators by some regulating factors such as histamine, acetylcholine, gastrin, etc. Histamine plays a major role in such stimulation by activating H₂ histamine receptors. On the other hand, the vagus nerve and enteric nervous system control the secretion by activating the C3 receptors with the help of acetylcholine. Gastrin activates the CCK2 receptors and also contributes to the secretion of histamine.
Did You Know?
Parietal cells secrete an intrinsic factor which is one type of glycoprotein. This intrinsic factor plays an important role in the absorption of vitamin B12. The deficiency of vitamin B12 can cause megaloblastic anaemia which can affect the transport of oxygen in the blood. Autoimmune destruction of parietal cells causes pernicious anaemia. Thus, the intrinsic factor secreted by oxyntic cells plays an important role in the formation of haemoglobin.
Intracellular cAMP is stimulated by H2 receptors of parietal cells and the calcium level is increased by M3 and CCK2 receptors. That results in the increased activity of kinase A. Kinase A induces the cell membrane and increases the hydrogen ion secretion and the potassium ion reabsorption and that makes the secretion of hydrochloric acid within the stomach cavity.