Steroid hormones and their synthetic pathways in the vitellogenic ovarian follicles of yellowtail, Seriola quinqueradiata

In the present study, we clarified the steroid hormones produced and their synthetic pathways, particularly focusing the estradiol-17β (E2) synthesis, in the vitellogenic ovarian follicles to provide information on the endocrine control of vitellogenesis in yellowtail, Seriola quinqueradiata. Intact vitellogenic ovarian follicles were isolated and incubated with radioactive [³H]pregnenolone and [¹⁴C]androstenedione and steroid metabolites were identified by thin layer chromatography (TLC) followed by recrystallization to constant specific activity. Results obtained clearly indicated that testosterone (T) is the substrate precursor of E2 synthesis. In the vitellogenic ovarian follicles, the steroid metabolites produced and major pathway followed were pregnenolone, 17-hydroxypregnenolone, dehydroepiandrosterone, androstenedione (AD), T and E2. AD, T and E2 were also present in the serum of fish during vitellogenesis, and serum level of T was the highest (3.7ng/ml), followed by E2 (2.3ng/ml). Thus, this study demonstrated the complete steroidogenic pathway of E2 synthesis in the ovarian follicles of yellowtail, and revealed that T is the major precursor of E2.