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 controls the pancreatic exocrine function in Japanese flounder, Paralichthys olivaceus, the expression patterns of cholecystokinin (CCK) and pancreatic polypeptide (PP) during the larval stage were analyzed by immunohistochemistry and in situ hybridization. Expression of CCK in the intestinal epithelia started at 2 days posthatching (dph), 1 day prior to the first feeding. Endocrine pancreatic cells containing insulin were present in the pancreas primodium at hatching, and these endocrine cells formed an islet at 3 dph and developed into the principle islet neighboring gallbladder at 20 dph. However, PP cells were not contained in the principle islet even after metamorphosis. 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 enzyme secretion by CCK starts to function at the first feeding, whereas the restrictive regulation by PP develops about 1 month later in flounder larvae. In addition, we observed that CCK immunoreactive cells appeared in the accessory islets at 30 dph, similar to PP cells, even though CCK mRNA expression could not be detected in cells from the islets. This indicates the possibility that a peptide that is cross-reacted with the CCK antibody, i.e., gastrin, is synthesized in the flounder islets.