In various cell types, there is increasing evidence for nongenomic steroid effects, i.e., effects that are not mediated via the classical steroid receptors. However, little is known about the involvement of the nongenomic pathway of estradiol (E2) on mammalian oocyte in vitro maturation (IVM). The aim of this study was to investigate whether the effects of E2 on bovine oocyte IVM are mediated via a plasma membrane receptor (nongenomic). First, we investigated the expression of estradiol (classical) receptor alpha (ERα) and beta (ERβ) mRNA in oocytes and cumulus cells (CC). We also studied the effects of different exposure times to E2 (before and after germinal vesicle breakdown, GVBD) on nuclear maturation. To study the possible involvement of the putative estradiol plasma membrane receptor on the IVM of oocytes, we used E2 conjugated with bovine serum albumin (E2-BSA), which cannot cross the plasma membranes. Our results demonstrate that oocytes expressed ERβ mRNA, while CC expressed both ERα and ERβ mRNA. Exposure to E2 during the first 8 h of culture (before GVBD) induced a block at the metaphase I stage (MI). However, the presence of E2 after GVBD induced an increase of oocytes with nuclear aberrations. Meiotic spindle organization was severely affected by E2 during IVM and multipolar spindle was the most frequently observed aberration. Exposure of oocytes to E2-BSA did not affect nuclear maturation, blastocyst formation rate, nor embryo quality. Our results suggest that the detrimental effects of E2 on in vitro nuclear maturation of bovine oocyte are not exerted via a plasma membrane receptor.
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