Ovarian steroidogenesis and the role of sex steroid hormones on ovarian growth and maturation of the Japanese eel

Three sex steroid hormones, estradiol-17β (E2), 11-ketotestosterone (11-KT), and 17α,20β-dihydroxy-4-pregnen-3-one (DHP), are well established as primary estrogen, androgen, and progestin, respectively, in teleost fish. Japanese eel, Anguilla japonica, would be a suitable candidate to study ovarian steroid physiology of fish because the ovarian growth and steroidogenesis is dormant under laboratory condition but can be induced by administration of exogenous gonadotropic reagents. In this review, we summarized our work on the function and production of sex steroid hormones in the ovary of the Japanese eel during ovarian growth and oocyte maturation artificially induced by treatment with extract of salmon pituitary. In vitro and in vivo assays suggest that 11-KT and E2 play primary roles in previtellogenic and vitellogenic growth of oocytes, respectively, whereas DHP is essential for induction of final oocyte maturation. We also reviewed the correlation between ovarian steroidogenesis to produce these sex steroid hormones, serum titers and gene expression.