Abstract
A group of apparently unrelated endocrine cells, some in endocrine glands, others in nonendocrine tissues, share a number of cytochemical and ultrastructural characteristics. These characteristics, from the initial letters of four of which the term APUD is derived, indicate the possession of a common metabolic pattern and common synthetic, storage and secretion mechanisms. It is postulated that the various characteristics reflect the production and storage of hormone precursor protein which has a predominantly random coil conformation. Several explanations are possible to account for the APUD characteristics but if the cells, in fact, share a common ancestor the only possible candidate is the neural crest cell.