Ultrastructural studies on experimentally induced vitellogenesis in juvenile rainbow trout (Salmo gairdneri R.)

Juvenile rainbow trout (weighing 10 or 20 g) were treated thrice weekly for 10 weeks with salmon-gonadotropin (S-GTH), salmon pituitary extract (S-PE), S-GTH plus estradiol-17!, and estradiol-17p alone.The effects of these treatments on the oocytes were studied at ultrastructural level.Saline-injected control fish contained oocytes at previtellogenic stage of development.S-GTH (0.1 or 0.5 pg/g) induced a synthesis of endogenous yolk in the oocyte cytoplasm but failed to initiate incorporation of exogenous yolk or vitellogenin into the oocytes.S-GTH (0.1 !g/g) in combination with estradiol-17!(1 gg)g) produced similar results to those with S-GTH alone.Estradiol-17fi alone failed to induce vitellogenesis.In contrast, S-PE (10 !kg/g) induced the incorporation of exogenous yolk into the oocyte indicated by an extensive pinocytotic activity at the oocyte membrane.It is suggested that gonadotropin is necessary for the induction of endogenous yolk synthesis and that some other pituitary hormone(s) is involved in the process of incorpora- tion of exogenous yolk or vitellogenin into the oocyte.