Entero-endocrine cells are scattered within the epithelia throughout the whole gastrointestinal tract, showing remarkable accumulations in the pyloric region and the duodenum. By light microscopical means these cells and their specific secretion granules respectively can be demonstrated by numerous staining methods (about 100), in which the argyrophilic reaction (according to Grimelius) is mainly applied today. Histochemical methods for the demonstration of entero-endocrine cells are based on the histochemical fluorescent demonstration of biogenic monoamines (endogenous occurrence or following exogenous application of amine precursor substances) and on the immunocytochemical demonstration of gastrointestinal polypeptide hormones. Most of the endocrine cell types (about 16) are responsible for the synthesis of certain enteric hormones. Furthermore, some entero-endocrine cells may synthesize more than one peptide hormone. Moreover, it is remarkable that some polypeptides (e.g. Gastrin, Cholecystokinin-Pancreozymin, Neurotensin, Endorphin, Enkephalin, ACTH) may occur simultaneously in the central nervous system and in the gastrointestinal tract. Ultrastructurally, entero-endocrine cells in their entirety mainly exhibit a pale cytoplasm, well developed microvilli and specific secretion granules varying in shape, size and electron density within different cell types. With respect to their mode of function, entero-endocrine cells have also been tentatively described as modified 'sensory cells'. The hormones of these cells may accordingly be interpreted as equivalents of neurotransmitter substances. Morphological investigations suggest that entero-endocrine cells are controlled by chemical and physical stimuli including those due to the composition of the chyme in the gut lumen. The enteric hormones can either directly influence the environment by a 'paracrinous' route or can also effect 'target organs' by an 'endocrinous' pathway. The entero-endocrine cells are summed up, together with similar cells of the pancreas and bile duct system as gastro-entero-pancreatic endocrine cells (GEP-endocrine system). Furthermore, cytochemical characteristics of many cells in different organs have lead to the definition of the 'APUD cell series' (Pearse 1969). According also to functional aspects, the entero-endocrine cells are members of the so-called 'paraneurons' (Fujita 1977). The progenitor cells of entero-endocrine cells have been claimed to originate from the neural crest, the neural ridge or the neurectoderm respectively. The final proof of this hypothesis is, however, still lacking. Concerning the differentiation and cell cycle of entero-endocrine cells, little knowledge dealing with these problems is actually available.
