Development of the steroidogenic capacity of medaka (Oryzias latipes) ovarian follicles during vitellogenesis and oocyte maturation.

Developmental changes in the steroidogenic capacity of medaka, Oryzias latipes, ovarian follicles at 12 different stages during vitellogenesis and oocyte maturation were examined using 18-hr incubations. Medaka were acclimated to conditions of 26 degrees on a lighting regime of 14 hr light and 10 hr dark. Under these conditions, females usually spawn daily within 1 hr of the onset of light. The process of vitellogenesis and oocyte maturation occurs within 72 hr, the breakdown of the germinal vesicle (GVBD) and ovulation being completed at 6 and 1 hr, respectively, before the expected time of spawning. Vitellogenic follicles between 32 and 16 hr before spawning produced large amounts of estradiol-17 beta spontaneously and in response to partially purified chum salmon gonadotropin (SGA) or pregnant mare's serum gonadotropin (PMSG). However, postvitellogenic follicles between 12 and 4 hr before spawning showed very little evidence of estradiol-17 beta production. By contrast, basal concentrations of 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOHprog) remained very low in follicles during vitellogenesis and were elevated in those collected during oocyte maturation; there was a close relationship between the medium concentration of 17 alpha,20 beta-diOH-prog and the percentage GVBD in the oocytes. 17 alpha,20 beta-DiOHprog production in response to PMSG was very low in follicles during early and mid-vitellogenesis and increased in those collected at 28 hr before spawning, a time which coincided with the first acquisition of the ability of the follicles to undergo maturation in response to gonadotropin. These results clearly demonstrate that a distinct shift from the secretion of predominantly estradiol-17 beta to the secretion of 17 alpha,20 beta-diOHprog occurs in the medaka ovarian follicle immediately prior to oocyte maturation. Considering the potency of 17 alpha,20 beta-diOHprog for the induction of oocyte maturation in vitro, these results further suggest that 17 alpha,20 beta-diOHprog is a naturally occurring steroidal mediator of oocyte maturation in the medaka.