Ultrastructure of pancreatic B-cells of Atlantic salmon ( Salmo salar) during smoltification and seawater adaptation

Abstract

Pancreatic B-cells of Atlantic salmon ( Salmo salar) were identified by immunostaining and the ultrastructure and morphology of these cells were investigated during smoltification and acclimation to seawater. Fish were classified as smolts based on length, condition factor, silvering, branchial Na + K + - ATPase activity and salinity tolerance. Individuals measuring less than 15 cm long were used as age-matched, non-smolting controls. Tissue samples were collected from juvenile salmon in February, April and May and, also, in June from post-smolts retained in fresh water or transferred to seawater for 1 month. Pancreas islet size was measured and B-cells were identified by immunohistochemistry and immunocytochemistry. The surface areas of B-cells, nuclei and granules within the cells were quantified, allowing comparison of nucleus: cell (N:C) and total granule: cytoplasm (G:Cyt) ratios. Additionally, the ratio of the number of homogeneous granules: total granules (Hom:G) was determined. No significant changes in size of islets were detected in parr and smolts. In fish undergoing smoltification a slight increase in G:Cyt and a significant decrease in Hom:G ratios ( P<0.05) were observed, whereas in non-smolting parr the G:Cyt ratio increased significantly ( P<0.05) and the Hom:G ratio remained significantly higher than in smolts. A decrease in B-cell area ( P<0.05) was observed in post-smolts retained in fresh water or transferred to seawater. Results are consistent with decreased insulin synthesis or secretion during parr-smolt transformation. In parr during this same period, enhanced B-cell activity was indicated by increases in G:Cyt and elevated Hom:G ratios compared to smolts.