The APUD system and its apudomas.

As sensitive radioimmunoassays for the detection of polypeptide hormones are developed, the exciting discovery of a diffusely distributed system of interrelated endocrine cells has begun a new era of endocrinology. This system, although anatomically disassociated, is bound together by a number of common features such as its biosynthetic mechanism, histochemical and ultrastructural features, and embryologic origin (Table I). The most prominent feature, however, is their biosynthetic pathways for hormone production, from which the acronym APUD has been derived. These are the capacity for Amine Precursor Uptake such as DOPA and then subsequent Decarboxylation, resulting in the synthesis of bioactive amines or polypeptide hormones. Hyperplasias or neoplasms of these cells are defined as apudomas. In the last ten years a great deal of research has rapidly altered the original concepts of this system, especially in terms of its embryologic origin, physiologic interrelationships, classification, as well as the addition of many new APUD cell members. These will be reviewed, and the origin, diagnosis, and treatment of each recognized apudoma will be synthesized in light of its membership within the APUD system.