Oocyte maturation in the mummichog (Fundulus heteroclitus): effects of steroids on germinal vesicle breakdown of intact follicles in vitro.

The effects of several steroids on the maturation of follicle-enclosed oocytes of the mummichog Fundulus heteroclitus in vitro were examined. At a relatively high concentration (1.0 microgram/ml), a number of different steroids, including pregnenolone, 17 alpha-hydroxypregnenolone, corticosterone, cortisol, 11-deoxycorticosterone, 11-deoxycortisol, androstenedione, testosterone, progesterone, 17 alpha-hydroxyprogesterone, 20 beta-dihydroprogesterone, and 17 alpha-hydroxy-20 beta-dihydroprogesterone, were able to induce germinal vesicle breakdown (GVBD) in prematuration oocytes. Cholesterol, 17 beta-estradiol, 11-ketotestosterone, and 11 beta-hydroxytestosterone were totally ineffective. In general, 11-oxysteroids tended to be less effective than their 11-deoxysteroid counterparts. Two 20 beta-dihydroprogestins--17 alpha-hydroxy-20 beta-dihydroprogesterone and 20 beta-dihydroprogesterone--were the most potent maturation-inducing steroids, initiating 50% GVBD at 1 ng/ml in follicles obtained from ovaries containing mature or maturing follicles in vivo, or at 2.5-4.0 ng/ml in follicles from ovaries lacking mature or maturing oocytes in vivo. These results are consistent with several previous studies involving salmonids and various other teleosts, and suggest a possible physiological role for a 20 beta-dihydroprogestin in the resumption of meiotic maturation in F. heteroclitus.