The "normal" endocrine cell of the gut: changing concepts and new evidences.

The endocrine cells of the gut are a highly specialized mucosal cell subpopulation. Within the gastrointestinal tract at least 14 different cell types produce a wide range of hormones with a specific regional distribution. The gut endocrine cells belong to the diffuse endocrine system. These cells present two regulated pathways of secretion characterized by large dense core vesicles (LDCV) and synaptic-like microvesicles (SLMV). Gut endocrine cells are recognized by the expression of several "general" markers, including the LDCV marker chromogranin A and the SLMV marker synaptophysin, in addition to the cytosolic markers neuron-specific enolase and protein gene product 9.5. The expression of different hormones identifies specific cell types. The gut endocrine cells are reputed to be terminally differentiated and incapable of proliferation. However, some data suggest that the number of gut endocrine cells may adapt in response to tissue-specific physiological stimuli. Gut endocrine cell differentiation appears to follow a "constitutive" tissue-specific pathway, which may be disrupted and investigated by genetic manipulation in mice. It is suggested that endocrine cell homeostasis is maintained by the entry of new endocrine-committed cells along the differentiation pathway and that such intermediate cells may be sensitive to physiological stimuli as well as transforming agents.