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Functional Considerations: Enteroendocrine Cells, APUD Cells, and Gastrointestinal Hormones

Enteroendocrine cells are present in most of the digestive tract, including the ducts of the pancreas and liver, and in the respiratory system, another endodermal derivative that originates by invagination of the epithelium of the embryonic foregut. The endocrine islets (of Langerhans) of the pancreas can be considered specialized accumulations of enteroendocrine cells derived from pancreatic buds that also arise from the embryonic foregut. It has been estimated that the enteroendocrine cells collectively would constitute the largest endocrine "organ" in the body. These cells have also been called gastroenteropancreatic (GEP) endocrine cells and closely resemble neurosecretory cells of the central nervous system that secrete many of the same hormones and regulatory agents. For that reason, they are also described as constituting part of a diffuse neuroendocrine system. These endocrine cells are not grouped as clusters in any specific part of the gastrointestinal tract. Rather, they are distributed singly throughout the gastrointestinal epithelium. Figure 16.13 shows the parts of the gastrointestinal tract from which the hormones are released into the blood.

Some enteroendocrine cells may be classifiable functionally as amine precursor uptake and decarboxylation (APUD) cells. They should not, however, be confused with the APUD cells that are derived from the embryonic neural crest and migrate to other sites in the body. Enteroendocrine cells, as discussed above, differentiate from the progeny of the same stem cells as all of the other epithelial cells of the digestive tract. The fact that two different cells may produce similar products should not imply that they have the same origin.

Enteroendocrine cells produce not only gastrointestinal hormones such as secretin, gastrin, cholecystokinin (CCK), gastric inhibitory peptide (GIP), and motilin but also paracrine substances (paracrines). A paracrine substance differs from a hormone in that it diffuses locally to its target cell instead of being carried by the bloodstream to a target cell. A well-known substance that appears to act as a paracrine substance within the gastrointestinal tract and pancreas is somatostatin, which inhibits other gastrointestinal and pancreatic islet endocrine cells. APUD cells secrete a variety of regulator substances in tissues and organs including the respiratory epithelium, adrenal medulla, islets of Langerhans, thyroid gland (parafollicular cells), and pituitary gland.

In addition to the established gastrointestinal hormones, several gastrointestinal peptides have not been definitely classified as hormones or paracrines. These peptides are designated candidate or putative hormones.

Other locally active agents isolated from the gastrointestinal mucosa are neurotransmitters. These agents are released from nerve endings close to the target cell, usually the smooth muscle of the muscularis mucosae, the muscularis externa, or the tunica media of a blood vessel. In addition to acetylcholine (not a peptide), peptides found in nerve fibers of the gastrointestinal tract are vasoactive intestinal peptide (VIP), bombesin, and enkephalins. Thus, a particular peptide may be produced by endocrine and paracrine cells and also be localized in nerve fibers.

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