Visualisation of the diffuse endocrine system.

Abstract

Bacon declared that 'the informed mind is more apt to see'; in the matter of detecting endocrine cells within the gastrointestinal tract, though some 400 years in advance of his time, he was correct. Endocrine cells are distinguishable from other gastrointestinal epithelial cells by the presence within their cytoplasm of secretory granules, and with due care and expectation of their presence these may often be recognised in conventionally stained paraffin embedded material, as many 19th and early 20th century histologists appreciated (see Clara, 1957). Not all endocrine cells, however, have granules which are recognisable, nor is differentiation between the shape and content of one granule and another possible on conventional haematoxylin and eosin staining. The purpose of this presentation is briefly to review the different types of endocrine cell found in the gut and the techniques by which one may be separated from another (Dawson, 1970, 1976). Endocrine cells in the gut are constructed as variants on a common pattern. They tend to be pear shaped or triangular, with a broad base abutting onto the basement membrane and a narrow apex which usually reaches the intestinal lumen and often bears microvilli. There is a conspicuous rough endoplasmic reticulum and Golgi, and mitochondria rich in decarboxylases and esterases. The most significant feature, however, is the presence within the cytoplasm of storage granules surrounded by a lipoprotein envelope, which tend to be especially numerous at the base rather than at the apex of each cell. Endocrine cells are classified by the content of these granules which can potentially be determined either by histochemical or by immunocytological techniques. Since granule size, shape, and density also seem to be related to granular content, ultrastructural studies may afford a separate means of identifying cell function. The granules in most endocrine cells in the gut contain a polypeptide hormone. Some contain an identifiable biological amine of which 5-hydroxytryptamine (SHT) is the most important. It is the supposed ability of all endocrine cells to be able to decarboxylate amine precursors which has led to their being referred to as APUD (Amine Precursor Uptake and Decarboxylation) cells, a term which 7 Pearse and his co-workers have enthusiastically embraced in their many detailed studies of an evergrowing family (Pearse, 1968, 1969). Since not all cells which secrete polypeptide hormones can be shown to decarboxylate and store amine precursors, I prefer the non-committal term endocrine cells; certainly endocrine cell tumour, when the precise secretion (for example, insulinoma) cannot be named, is preferable to the ugly word Apudoma.