Ultrastructure of the liver of Lake Erie coho salmon from post-hatching until spawning.

Hepatocytes were examined in coho salmon, Oncorhynchus kisutch, at several stages in their life cycles, using feral and laboratory-reared fish. Yolksac fry sampled before their first exogenous feed had low glycogen and lipid reserves in the hepatocytes and although 'light' cells were predominant, there was already evidence of a dimorphism in the electron density of hepatocytic cytoplasm. Within 1 day of the first exogenous feeding (a commercial salmonid starter diet) the hepatocytes in the fry contained extensive glycogen pools (up to 50% of the cell volume) and lipid droplets, often contained within the glycogen pools. This high glycogen and lipid condition was also evident in laboratory-reared male and female fish which had undergone smoltification. In sexually immature feral fish collected prior to the growth of their gonads, the hepatocytes were predominantly large 'light' cells. There was little glycogen in the hepatocytes of either males or females, lipid was abundant (particularly in females) as was ribosomal endoplasmic reticulum and smooth endoplasmic reticulum. In feral prespawning and spawning adults there was some loss of lipid and an apparent increase in the number of 'dark' hepatocytes. Treatment of fingerling fish with methyl testosterone resulted in a decrease of hepatocytic lipid content, whereas 17 beta-oestradiol stimulated an increase in hepatocytic lipid and ribosomal endoplasmic reticulum and a decrease in glycogen.