1996 Volume 44 Issue 2 Pages 189-195
The changes in ovarian development and serum steroid hormone levels of the yellowtail, Seriola quinqueradiata, during final oocyte maturation induced by a human chorionic gonadotropin (HCG) were monitored at six different times (0, 6, 12, 24, 36, and 48 hours) after HCG injection to understand the mode of ovarian development and endocrine control of oocyte maturation of this species.
The ovarian stage of fish sampled at any one time showed a tendency to be similar to one another. Vitellogenesis was completed and germinal vesicle migration began in fish sampled at 24-36 hours. Fish sampled at 48 hours had fully mature oocytes. Serum testosterone (T) and estradiol-17β (E2) were maintained at a relatively high level until 24 hours after HCG injection (T, 3.0-4.7 ng/ml; E2, 16.7-20.0 ng/ml), then the levels decreased and showed the lowest values (T, 0.8 ng/ml; E2, 5.9ng/ml) at 48 hours. The putative maturation-inducing hormone (MIH) of many teleost species, 17α, 20β-dihydroxy-4-pregnen-3-one, showed a rapid increase (0.42 ng/ml) in serum concentration at 6 hours after HCG injection, but maintained constant low levels (0.12-0.21 ng/ml) during final oocyte maturation from 24 to 48 hours. These results are discussed in relation to other studies on changes in serum steroid levels during final oocyte maturation.
