Nongenomic effects of 17 beta-estradiol on maturing human oocytes: relationship to oocyte developmental potential

There is increasing evidence for nongenomic steroid effects in various cell types. This study is the first to demonstrate that 17 beta-estradiol (E2) exerts a direct nongenomic effect on maturing human oocytes by inducing a series of transient increases in the intracellular free calcium concentration ([Ca2+]i). This effect of E2 appeared to be specific and was mediated by steroid action on the cell surface. The first of the E2-induced [Ca2+]i increases was induced by an influx of extracellular Ca2+ ions. However, the release of Ca2+ from intracellular stores contributed more significantly to [Ca2+]i waves characterizing the subsequent series of secondary [Ca2+]i increases, although the presence of extracellular Ca2+ still was an absolute requirement for these [Ca2+]i oscillations. The addition of E2 to oocyte maturation medium did not produce any apparent effects on either germinal vesicle breakdown or further progression of meiosis, but it did increase the fertilization and cleavage rates of the in vitro matured oocytes. These findings show that E2 can directly influence the quality of maturing oocytes. This effect is due to steroid action on the cell surface, implies Ca2+ as a second messenger, and contributes to oocyte capacitation for fertilization and early postfertilization development.