Classification of endocrinocytes
In relation to the lumen of the gastric gland, endocrine cells are divided into two main types:
- “Open” endocrinocytes. They have a narrowed apical part that opens directly into the gland lumen. These cells act as receptors: they “assess” the chemical composition of the contents (e.g., acidity or food components) and release hormones in response.
- “Closed” endocrinocytes. They have no contact with the lumen. Their activity is regulated not by food, but by nerve impulses or humoral factors from the blood and tissue fluid.
Regarding the delivery method of hormones, there is an endocrine mechanism (hormone enters the bloodstream to reach distant organs) and a paracrine mechanism (the substance diffuses to neighboring cells).
Key cells of proper (fundic) glands
Three types of endocrine cells predominate in the fundic region of the stomach, each performing a specific task:
- EC cells (enterochromaffin cells). These are open cells with a paracrine action. Upon contact with food, they release serotonin, which stimulates the production of enzymes and acid by exocrinocytes and enhances motility. Under the influence of the sympathetic nervous system (associated with visual pathways and the pineal gland), they secrete melatonin, which sets the circadian rhythms of the GI tract.
- ECL cells (enterochromaffin-like cells). Closed cells with a paracrine type of secretion. They produce histamine in response to nervous or humoral stimuli. Histamine powerfully stimulates parietal cells to produce hydrochloric acid.
- A-like cells. Closed cells operating via a true endocrine mechanism. They release enteroglucagon (gastric glucagon) directly into the blood. Its task is to break down stored carbohydrates and fats between meals, similarly to a pancreatic hormone.
Chemical nature of hormones and other regulators
The synthesized hormones have different biochemical origins. For example, serotonin and melatonin (hormones of EC cells) are derived from the amino acid tryptophan. Histamine (ECL cells) is a derivative of histidine. Enteroglucagon is a polypeptide.
In addition to the main cells of the stomach, other GI hormones participate in the regulation of digestion:
- I cells: produce cholecystokinin, which stimulates the release of bile and pancreatic juice.
- S cells: secrete secretin, which inhibits gastrin production while activating the secretion of pancreatic juice.
- D cells: secrete somatostatin, a universal inhibitor of GI functions.
- D1 cells: produce VIP (vasoactive intestinal peptide), which dilates blood vessels, lowers blood pressure, and acts as an antagonist to somatostatin.
Briefly: answers to topical questions
Which gastric endocrine cells secrete gastrin?
Gastrin is secreted by gastric G cells. They belong to the “open” type of endocrinocytes and are located predominantly in the pyloric and cardiac glands.
Functional features of G cells:
- Secrete two hormones: gastrin and enkephalin.
- Are activated when food enters the pyloric region.
- The released gastrin enters the bloodstream and stimulates the secretion of hydrochloric acid by parietal cells, pepsinogen by chief cells, and also enhances gastric motility.
Do gastric endocrine cells belong to the APUD system?
Yes, endocrine cells of the gastrointestinal tract belong to the APUD system. They form the diffuse endocrine system of the digestive tract, which is also called the enteric system.
Within the APUD system of the gastrointestinal tract, these cells are responsible for the secretion of polypeptide gastrointestinal hormones (such as gastrin, secretin, somatostatin, cholecystokinin, and vasoactive intestinal peptide). The main function of these substances is to regulate digestion, secretion, motility, and absorption processes.
Class and exam questions
How do open enteroendocrinocytes differ from closed ones?
Open enteroendocrinocytes contact the lumen of the gland with their apical pole and respond to the chemical composition of food. Closed ones do not reach the lumen and are regulated solely via neural or humoral signals.
What hormone do ECL cells produce and what is its purpose?
ECL cells secrete histamine. It acts in a paracrine manner and is a potent stimulator of hydrochloric acid secretion by parietal cells.
Why is the histamine test used in clinical practice?
The histamine stimulation test is used to assess gastric glands in cases of reduced acidity. If acidity rises after the injection, the problem is functional (nervous regulation is disrupted); if it does not, it is organic (the glands are atrophied).
What else to study on this topic
- Mechanisms of paracrine interaction in the mucosa
- Synthesis of serotonin and melatonin from tryptophan
- Influence of the sympathetic nervous system on GI circadian rhythms
- Interaction of histamine and parietal cells
- Clinical interpretation of functional gastric tests
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