The presence of endocrine cells in the various portions of the
digestive tract is very well documented. The stomach and intestines contain cells that produce true hormones: chemical materials released into the blood and targeting distant cells. You can consider these as endocrine organs in the classic sense. Another way to look at this enteroendocrine system is as a diffuse endocrine organ, producing a number of different hormones, and localized in the mucosa of the digestive tract.
The functions of many of these endocrine cells have been established. Actually, their presence has long been known, but it's only in the past couple of decades that techniques have been made available to establish their variety and their roles in gastrointestinal physiology. In the image above an enteroendocrine cell has been revealed by a classic silver deposition method. Because these cells stain strongly with this sort of method, they have long been called "argetaffin cells" ("silver affinity"). The reaction precipitates metallic silver as dark granules.
Enteroendocrine cells of different types (there are at least 20) may be found in all parts of the stomach and small intestine. These cells do not discharge their product into the lumen of the gut. Hormones, by definition, are carried in the blood. The cell in this image is polarized away from the lumen and towards the blood vessels of the underlying lamina propria. The gland product will be discharged into the extracellular space on the basal side of the cell. It diffuses a short distance and passes into the capillary bed underneath, to be carried to the target organ in the blood like any other hormone would be.
This electron microscope image demonstrates the nature and polarization of an enteroendocrine cell nicely. The round cell is located right on the basement membrane (BM) and its granules are located on the side below the nucleus and against the membrane. The nucleus of this cell is vesicular, and there's a prominent nucleolus. That plus the numerous secretory vesicles in the cytoplasm are evidence of its function.
There's no way to tell from an image like this one (or the light micrograph above) what the cell is producing; but such information can be obtained with immunolabeling studies. This method has defined at least 20 different types of enteroendocrine cells, most of which have target organs in distant regions of the digestive system. One is known to stimulate secretion in the pancreas, another to cause contraction of the smooth muscle in the wall of the gall bladder; and others target specific cells of the endocrine system itself.
There's considerable evidence that many enteroendocrine cells can discharge in response to nervous stimulation, and that in some instances nerve fibers innervate them directly. Thus we have one more example of how the two systems for response to stimulus are integrated.
Enteroendocrine cell, turkey gut; TEM preparation, 20,000x
Image Credit: I am indebted to Dr. Ihab Mahmoud El-Zoghby of the Faculty of Veterinary Medicine, Zagazig University, Egypt, for this picture, which is from his PhD thesis.
