Classification of Endocrinocytes
Depending on their relationship to the lumen of the gastric gland, endocrine cells are divided into two main types:
- "Open" endocrinocytes. These have a narrowed apical part that opens directly into the gland lumen. These cells act as receptors: they "sample" the chemical composition of the luminal contents (such as acidity or food components) and release hormones in response.
- "Closed" endocrinocytes. These lack contact with the lumen. Their activity is regulated not by food, but by neural impulses or humoral factors from the blood and tissue fluid.
Regarding the mode of hormone delivery, the endocrine mechanism involves hormones entering the bloodstream to reach distant organs, whereas the paracrine mechanism involves substances diffusing to neighboring cells.
Key Cells of Proper (Fundic) Glands
In the fundic region of the stomach, three types of endocrine cells predominate, each performing a specific task:
- EC cells (enterochromaffin cells). These are open paracrine cells. Upon contact with food, they release serotonin, which stimulates enzyme and acid production by exocrinocytes and enhances motility. Under the influence of the sympathetic nervous system (linked to visual pathways and the pineal gland), they secrete melatonin, which establishes the circadian rhythms of GI tract function.
- ECL cells (enterochromaffin-like cells). These are closed cells with a paracrine mode of secretion. They produce histamine in response to neural or humoral stimuli. Histamine powerfully stimulates parietal cells to produce hydrochloric acid.
- A-like cells. These are 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 pancreatic hormones.
Chemical Nature of Hormones and Other Regulators
The synthesized hormones have diverse biochemical origins. For example, serotonin and melatonin (hormones of EC cells) are derived from the amino acid tryptophan. Histamine (ECL cells) is derived from histidine. Enteroglucagon is a polypeptide.
In addition to the primary gastric cells, other GI hormones participate in digestive regulation:
- I cells: produce cholecystokinin, which stimulates bile release and pancreatic juice secretion.
- S cells: secrete secretin, which inhibits gastrin formation while activating pancreatic juice secretion.
- D cells: release somatostatin, a universal inhibitor of GI functions.
- D1 cells: produce VIP (vasoactive intestinal peptide), which dilates blood vessels, lowers blood pressure, and acts as a somatostatin antagonist.