Effect of physiological levels of phytoestrogens on mouse oocyte maturation in vitro.

Phytoestrogens are a group of naturally occurring compounds that have weak estrogenic activity. Genistein and daidzein are major phytoestrogens produced by soybeans. It has been reported previously that at high concentration, some phytoestrogens inhibit cell cycle progression of mouse germinal vesicle (GV) oocytes, but the environmentally relevant level is much lower. Here we show the effects of low concentrations of the isoflavones genistein, daidzein and the daidzein metabolite, equol, on mouse oocyte maturation. GV oocytes denuded of cumulus cells were cultured in TaM medium containing low levels (5 μM) of genistein, daidzein. or equol. In all cases, the oocytes underwent normal GV break down, first polar body extrusion and became arrested at metaphase II (mII). As judged by fluorescence microscopy, the treated mII oocytes exhibited normal distributions of actin microfilaments, cortical granules and metaphase spindle formation with condensed metaphase chromatin. Moreover, mRNA expression levels of the cytostatic factors Emi2 and Mos were similar to those of their respective controls. These data suggest that exposure of maturing GV oocytes to environmental levels of genistein, daidzein or equol in vitro do not cause negative effects on maturation to produce mII oocytes.