ONTOGENY OF IGF-1 AND THE CLASSICAL ISLET HORMONES IN THE TURBOT, SCOPHTHALMUS-MAXIMUS

The ontogeny of the endocrine pancreas of a teleost, the turbot (Scophthalmus maximus), was investigated by the use of double immunofluorescence. Clustered insulin (INS)-immunoreactive (IR) cells were observed on the first day after hatching. During the following days, the islet largely increased in size and some smaller islets appeared. All islets consisted only of INS-IR cells. Between day 5 (onset of exogenous feeding) and 7, somatostatin (SOM) and glucagon (GLUC) cells appeared. In the large (principal) islet, the SOM-IR cells intermingled with the INS-IR cells. In the secondary islets, they occurred at the periphery. The GLUC-IR cells were located at the periphery in all islets. Subsequently, two-four additional small principal islets appeared. At day 11,pancreatic polypeptide (PP)-IR cells were present in principal islets and secondary islets. Starting with day 11, in all islets, insulin-like growth factor 1 (IGF-1) immunoreactivity was localized in numerous PP-IR cells and GLUC-IR and some SOM-IR cells. It also occurred in enteroendocrine cells that seemed to contain none of the classical islet hormones. The early appearence of INS correlates with its key role in the regulation of fish protein and lipid metabolism. Islet-derived IGF-1 might inhibit the regulation of INS secretion in a paracrine manner and may be highly involved in growth-promoting processes.