Development of cholecystokinin and pancreatic polypeptide endocrine systems during the larval stage of Japanese flounder, Paralichthys olivaceus

To understand the developmental process of the endocrine system, which cont rols the pancreatic exocrine function in Japanese flounder, Paralichthys ol ivaceus, the expression patterns of cholecystokinin (CCK) and pancreatic po lypeptide (PP) during the larval stage were analyzed by immunohistochemistr y and in situ hybridization. Expression of CCK in the intestinal epithelia started at 2 days posthatching (dph), 1 day prior to the first feeding Endo crine pancreatic cells containing insulin were present in the pancreas prim odium at hatching, and these endocrine cells formed an islet at 3 dph and d eveloped into the principle islet neighboring gallbladder at 20:dph. Howeve r, PP cells were not contained in the principle islet even after metamorpho sis. PP cells were contained in the accessory islets differentiated around the proximal part of the pyloric appendages at 30 dph (early metamorphosis stage). Therefore, we infer that the stimulative regulation of pancreatic e nzyme secretion by CCK starts to function at the first feeding, whereas the restrictive regulation by PP develops about 1 month later in flounder larv ae. In addition, we observed that CCK immunoreactive cells appeared in the accessory islets at 30 dph, similar to PP cells, even though CCK mRNA expre ssion could not be detected in cells from the islets. This indicates the po ssibility that a peptide that is crossreacted with the CCK antibody, i.e., gastrin, is synthesized in the flounder islets. (C) 2000 Academic Press.