Steroids involved with final oocyte maturation in the winter flounder

A number of androgens and progestogens including 17α,20β-dihydroxy-4-pregnen-3-one (17,20-P) were examined in female winter flounder as possible maturation inducing steroids (MIS). During final oocyte maturation serum levels of testosterone (T) and 17β-hydroxy-5β-androsten-3-one (5β-T) peaking at over 200 ng/ml and pregnenolone (PE) at 40 ng/ml were the predominant steroids found from each major group. High levels of T and 5β-T were correlated with oocyte stages characterized by germinal vesicle migration. Of the PEs measured, maximum serum levels of PE, 3β,17α-hydroxy-5-pregnen-20-one (17-PE) and 3β,17α,20β-dihydroxy-5-pregnene (17,20-PE) were found during later oocyte stages associated with germinal vesicle breakdown. Levels of 17,20-P, an established MIS in most fish, were almost non-detectable (<0.1 ng/ml serum) in females throughout all stages of final oocyte maturation. Incubations of ovarian follicles in vitro with physiological concentrations of T and 5β-T indicated that these steroids could induce all stages of final oocyte maturation. Similar in vitro incubations showed that 17-PE and 17,20-PE were only effective on germinal vesicle breakdown. The principal conclusions are that T, 5β-T and the PEs can be considered as MISs in winter flounder and the PE pathway predominates during the final stages of oocyte maturation in winter flounder in contrast to progesterones which predominate in other fish species, mostly salmonids, studied to date.