What Is a Parietal Cell Structure Function and Secretions
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.
FAQs on Parietal Cell in the Stomach Structure and Role in Digestion
1. What is a parietal cell?
A parietal cell is a specialized epithelial cell in the stomach that secretes hydrochloric acid (HCl) and intrinsic factor. It is located in the gastric glands of the fundus and body of the stomach. These cells play a key role in digestion and vitamin absorption by:
- Producing HCl to maintain an acidic gastric pH
- Activating digestive enzymes like pepsin
- Secreting intrinsic factor needed for vitamin B12 absorption
2. What is the function of parietal cells?
The main function of parietal cells is to secrete hydrochloric acid and intrinsic factor in the stomach. Their functions include:
- Lowering gastric pH to around 1–3
- Converting pepsinogen into active pepsin
- Destroying ingested microbes
- Enabling vitamin B12 absorption in the ileum via intrinsic factor
3. Where are parietal cells located?
Parietal cells are located in the gastric glands of the fundus and body (corpus) of the stomach. They are found mainly in the middle region of the gastric glands. These cells are large, eosinophilic, and contain numerous mitochondria to support acid secretion.
4. How do parietal cells secrete hydrochloric acid?
Parietal cells secrete hydrochloric acid using the H+/K+ ATPase proton pump located in their apical membrane. The process occurs in steps:
- Carbon dioxide combines with water via carbonic anhydrase to form carbonic acid
- Carbonic acid dissociates into H+ and HCO3−
- H+ is pumped into the stomach lumen via the proton pump
- Cl− diffuses into the lumen and combines with H+ to form HCl
5. What is intrinsic factor and why is it important?
Intrinsic factor is a glycoprotein secreted by parietal cells that is essential for vitamin B12 absorption. It binds to vitamin B12 in the stomach and protects it during digestion. The intrinsic factor–B12 complex is then absorbed in the ileum of the small intestine. Without intrinsic factor, vitamin B12 deficiency and pernicious anemia can occur.
6. What stimulates parietal cells to secrete acid?
Parietal cells are stimulated to secrete acid by gastrin, histamine, and acetylcholine. These stimulators act as follows:
- Gastrin: Released from G cells in response to food
- Histamine: Released from enterochromaffin-like (ECL) cells
- Acetylcholine: Released from vagus nerve endings
7. What happens if parietal cells are damaged?
Damage to parietal cells leads to reduced acid secretion and lack of intrinsic factor. This can cause:
- Hypochlorhydria or achlorhydria (low or absent stomach acid)
- Impaired protein digestion
- Vitamin B12 deficiency
- Pernicious anemia due to intrinsic factor deficiency
8. What is the difference between parietal cells and chief cells?
The main difference is that parietal cells secrete hydrochloric acid and intrinsic factor, while chief cells secrete pepsinogen. Key distinctions include:
- Parietal cells: Acid secretion, intrinsic factor production, abundant mitochondria
- Chief cells: Secrete pepsinogen, located deeper in gastric glands
9. Why do parietal cells have many mitochondria?
Parietal cells contain numerous mitochondria because acid secretion requires large amounts of ATP. The H+/K+ ATPase proton pump actively transports hydrogen ions against a concentration gradient, which is an energy-intensive process. The high mitochondrial density supports continuous and efficient gastric acid production.
10. How is gastric acid secretion regulated?
Gastric acid secretion by parietal cells is regulated through neural, hormonal, and paracrine mechanisms. Regulation occurs in phases:
- Cephalic phase: Stimulated by sight, smell, or thought of food via the vagus nerve
- Gastric phase: Triggered by stomach distension and gastrin release
- Intestinal phase: Modulated by signals from the small intestine
