Main cells are enzyme secreting cells in the stomach. They are found in the gastric glands and produce pepsinogen, the inactive precursor to pepsin, and gastric lipase. The enzymes produced by the main cells mix with hydrochloric acid and other secretions to produce gastric juice, which is an important part of protein and fat digestion in the stomach.
What is a main cell?
A main gastric cell (AKA a zymogenic cell) is a type of epithelial cell that is found in the lining of the stomach. Its main function is to synthesize and secrete pepsinogen; the inactive precursor of the enzyme pepsin, which is activated on contact with hydrochloric acid. Main cells of the stomach also produce the enzyme gastric lipase. Therefore, main gastric cells play a central role in the digestion of dietary proteins and lipids in the stomach.
Location of the main cells
The lining of the stomach lining contains numerous gastric glands, each of which contains a large number of specialized cells. Each cell in the gastric gland secretes part of the gastric fluid, which is then released into the stomach.
The specialized cells of the gastric glands include main cells (which secrete pepsinogen and gastric lipase) and parietal cells (which produce hydrochloric acid) as well as mucus and hormone secreting cells. These secretions combine to form gastric juice, which initiates protein and fat digestion in the stomach.
Structure and function of the main cells
The main function of a main gastric cell is to produce pepsinogen (the inactive precursor to pepsin) and gastric lipase. Major cells are highly adapted to keep up with the high rate of protein synthesis required for enzyme secretion.
Pepsinogen is the inactive precursor to pepsin, a protease. It is produced and secreted by the main stomach cells and activated in the gastric juice upon contact with hydrochloric acid. Once activated, pepsin begins to digest proteins into smaller peptides and amino acids in the stomach.
Main cells are the main producers of pepsinogen in the body and contain abundant rough endoplasmic reticulum (RER) for protein synthesis. They also host many secretory granules (called Zymogen granules), which store pepsinogen and other digestive enzymes until needed.
Gastric lipase production
Major cells also produce gastric lipase, which helps digest triglycerides into simple fatty acids, diglycerides, and monoglycerides. Like pepsinogen, gastric lipase is synthesized by the RER and stored in the secretory granules of the main cells.
Stimulation of the main cells
Digestive enzymes are stored in the secretory vesicles of the main cells and are released in response to various stimuli. Low pH, the neurotransmitter Acetylcholine, and the hormone Gastrin are all known to stimulate main gastric cells.
The first stage of digestion is that Head phase, where the sight, thought or smell of food is the Vagus nerve. The vagus nerve releases a type of neurotransmitter called acetylcholine, which triggers the release of pepsinogen from the main stomach cells. Acetylcholine from the vagus nerve also stimulates the parietal cells to secrete hydrochloric acid, which converts pepsinogen into the active enzyme pepsin.
Gastrin is a gastrointestinal hormone released by G cells in the stomach and duodenum of the small intestine. G cells produce gastrin in response to various stimuli, including gastric distension, high levels of amino acids and peptides, low pH, and vagus nerve stimulation. Gastrin stimulates the main cells to secrete pepsinogen and the parietal cells to release hydrochloric acid.
The low pH of the stomach also stimulates the main stomach cells. When the parietal cells secrete hydrochloric acid, the stomach contents become more acidic and the main cells are made to release pepsinogen. Upon contact with the hydrochloric acid, the pepsinogen is activated and protein digestion can begin.