Insulin during the early postembryonic development of the gilt-head sea bream, Sparus aurata: ultrastructural, immunohistochemical, and biochemical studies.

Insulin was sought during the early postembryonic development of gilt-head sea bream, Sparus aurata, using ultrastructural, immunohistochemical and biochemical techniques. The endocrine pancreas appeared at hatching (Day 0) as a single cluster of morphologically similar cells. Secretory granules formed from Day 1 onwards but the cells could only be identified as insulin-producing B cells at the end of the endotrophic period (Day 3-Day 4). Insulin-immunoreactive cells were detected in the pancreatic primordium from hatching onwards and their number increased after the end of the endo-exotrophic period. Early insulin production was also found using an ELISA method on homogenates of prelarvae and larvae. Insulin levels were fairly high during the endotrophic period, decreased strongly at mouth opening, and then increased at the end of the endo-exocrine period. The origin and role of the large amount of hormone detected during the strictly endotrophic phase of ontogenesis are discussed in light of data on other vertebrates.