Comparative Morphology of Pancreatic Islets in Animals

About 5 years ago the comparative endocrinology of the pancreatic islets was analyzed in two reviews from our laboratory,1,2 when we tried to cover most of the relevant literature in that field published before 1971. Since then, remarkable advances have been made in the understanding of pancreatic islet hormone formation, secretion, molecular structure, and membrane receptor sites.3,4 The intimate relationship that was found both phylogenetically and ontogenetically between the islet parenchyma and the endocrine cells of the gastrointestinal (GI) tract1,2 has been further substantiated structurally,5 as well as biochemically, physiologically, and clinically.6–10 Moreover, the APUD (amine precursor uptake and decarboxylation) concept has become so well established that it is now obvious that the islet and GI endocrine cells belong to one and the same system11–14 As a matter of fact, the whole field of peptide endocrinology seems to become neuroendocrinology,13 and a completely new approach has been offered to solve some crucial problems in the pathology of endocrine neoplasms in general and of so-called islet cell tumors in particular.11,12 Recently, new aspects have also come to light on the production sites in the pancreas and the GI tract of somatostatin,15,16 gastrin,17,18 the newly discovered pancreatic polypeptide (PP),19–21 and other hormones of the gastro-entero-pancreatic (GEP) endocrine system.5,8–14 The methods for the structural analysis of GEP cells have been critically scrutinized,22 improved, and standardized,11,12 and, as a result.