The Correct Answer: Parietal Cells Secrete Intrinsic Factor
In humans, parietal cells, also known as oxyntic cells, are responsible for secreting intrinsic factor. These cells are found in the gastric glands, primarily in the fundus and body of the stomach. Parietal cells also secrete hydrochloric acid (HCl), which creates the stomach's acidic environment and activates pepsinogen. This acidic environment is crucial for releasing vitamin B12 from food proteins.
The Critical Role of Intrinsic Factor
Intrinsic factor is a glycoprotein essential for vitamin B12 absorption. It binds to vitamin B12 in the digestive tract, and this complex is absorbed in the terminal ileum of the small intestine. Without sufficient intrinsic factor, vitamin B12 cannot be absorbed effectively.
The True Function of Chief Cells
Chief cells, also located in the gastric glands, primarily secrete the inactive enzyme precursor pepsinogen and gastric lipase. Hydrochloric acid from parietal cells converts pepsinogen into active pepsin, which begins protein digestion. This separation of function helps protect the stomach lining from self-digestion.
Where the Misconception Comes From
The confusion often arises from the close proximity of parietal and chief cells within the gastric glands and the fact that chief cells secrete intrinsic factor in some other species, like rodents. While minor intrinsic factor expression might occur in a small number of human chief cells, it's not their primary function and is not a significant source compared to parietal cells.
Comparison of Gastric Cells
To clarify the roles of these specialized stomach cells, here is a comparison:
| Feature | Parietal Cells (Oxyntic Cells) | Chief Cells (Zymogenic Cells) |
|---|---|---|
| Primary Secretions | Hydrochloric Acid (HCl) and Intrinsic Factor | Pepsinogen and Gastric Lipase |
| Location in Gland | Primarily in the middle region | Primarily in the base or deeper regions |
| Role in Digestion | Activates pepsinogen; sterilizes food; facilitates B12 absorption | Initiates protein digestion via pepsinogen |
| Key Function | Acid and intrinsic factor production | Enzyme precursor production |
The Impact of Intrinsic Factor Deficiency
Insufficient intrinsic factor leads to impaired vitamin B12 absorption and can cause pernicious anemia. This autoimmune disorder involves the destruction of parietal cells. Other causes include atrophic gastritis or gastric surgery. Vitamin B12 injections are a common treatment to bypass the need for intrinsic factor-dependent absorption. You can find additional information on this topic from resources like the National Institutes of Health (NIH) website(https://www.ncbi.nlm.nih.gov/books/NBK546655/).
Conclusion
In conclusion, it is important to understand that in humans, parietal cells, not chief cells, are responsible for secreting intrinsic factor. This glycoprotein is vital for the absorption of vitamin B12. Chief cells play a different crucial role in digestion by secreting pepsinogen. Both cell types are essential for proper stomach function.
What is the primary function of chief cells in the stomach?
Pepsinogen secretion: Chief cells primarily secrete pepsinogen, an inactive enzyme precursor that is converted into the active protein-digesting enzyme pepsin by the stomach's acid.
What are parietal cells also known as?
Oxyntic cells: Parietal cells are also known as oxyntic cells, derived from the Greek word for acid, 'oxyntos', reflecting their function in secreting hydrochloric acid.
What condition results from a lack of intrinsic factor?
Pernicious anemia: A deficiency of intrinsic factor prevents proper vitamin B12 absorption, which leads to a type of megaloblastic anemia called pernicious anemia.
How is vitamin B12 absorbed in the body?
Via intrinsic factor: After being released from food, vitamin B12 binds to intrinsic factor in the stomach and is subsequently absorbed through receptors in the terminal ileum.
Why don't chief cells secrete intrinsic factor in humans?
Specialized function: Chief cells are specialized to produce digestive enzymes like pepsinogen, while the role of producing intrinsic factor evolved to be a specific function of parietal cells in humans.
How does the stomach protect itself from its own acid and enzymes?
Mucosal barrier: The stomach is protected by a thick, bicarbonate-rich layer of mucus secreted by surface mucous cells, and tight junctions between epithelial cells prevent gastric juice from penetrating deeper.
Can vitamin B12 be absorbed without intrinsic factor?
In small amounts: While intrinsic factor is necessary for efficient absorption, a very small percentage of vitamin B12 can be absorbed passively in its absence, but not enough to prevent deficiency in most cases.
