The endocrine cells of the digestive tract. General concepts and historic perspective.

The chromaffin cells of the digestive mucosa were discovered by Heidenhain in 1870, long before the hormone concept was formulated. Following the discovery of the first hormone, secretin, by Bayliss and Starling in 1902, the study of the cells producing such messengers acquired new impetus. After Masson, in 1914, showed that the digestive chromaffin cells were argentaffin and suggested their endocrine nature, a series of technological advances led to the gradual characterization of diverse cell types, their classification, and the discovery of their function. Histochemistry, including silver impregnations, fluorescence microscopy, and aniline stains, exposed the complexity of the digestive endocrine cells. Electron microscopy provided structural markers for their characterization and the basis for the formulation of the first universally accepted classifications. Light- and electron-microscopic immunocytochemistry contributed to the understanding of the function of many of these cells, thus opening the way for modern classifications. Many unsolved problems still remain. These include the existence of cells without defined function, of chemical messengers without known cell of origin, and the presence of multiple messengers in some cell types. Answers are expected to emerge from further application of immunocytochemistry and from the introduction of modern approaches, such as in-situ hybridization histochemistry.