Effects of steroids on mouse oocyte maturation in vitro.

Oestrone and dihydrotestosterone had no significant action. Other steroids inhibited maturation. The stage of maturation most affected and the median effective concentration (MEC) at this stage varied with different steroids. The predominant effect of pregnenolone (MEC = 6.4 microM), progesterone (MEC = 5.3 microM), androstenedione (MEC = 28.0 microM) and oestradiol-17 beta (MEC = 23.0 microM) was to block maturation after the resumption of meiosis but before completion of the first meiotic division. Testosterone was also effective at this stage (MEC = 23.5 microM) but at higher concentrations it prevented germinal vesicle breakdown (MEC = 40 microM) without causing oocyte degeneration. The inhibitory actions of all steroids were reversible in oocytes exposed for 4 or 18 h.