Estradiol and its membrane-impermeable conjugate (estradiol-bovine serum albumin) during in vitro maturation of bovine oocytes: Effects on nuclear and cytoplasmic maturation, cytoskeleton, and embryo quality

in various cell types, there is increasing evidence for non-genomic 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 (E,) on mammalian oocyte in vitro maturation (IVM). The aim of this study was to investigate whether the effects of E, on bovine oocyte lVM are mediated via a plasma membrane receptor (nongenomic). First, we investigated the expression of estradiol (classical) receptor alpha (ERalpha) and beta (ERbeta) mRNA in oocytes and cumulus cells (CC). We also studied the effects of different exposure times to E, (before and after germinal vesicle breakdown, GVBD) on nuclear maturation. To study the possible involvement of the putative estradiol plasma membrane receptor on the lVM of oocytes, we used E, conjugated with bovine serum albumin (E-2-BSA), which cannot cross the plasma membranes. Our results demonstrate that oocytes expressed ERbeta mRNA, while CC expressed both ERalpha and ERP mRNA. Exposure to E, during the first 8 h of culture (before GVBD) induced a block at the metaphase I stage (MI). However, the presence of E, after GVBD induced an increase of oocytes with nuclear aberrations. Meiotic spindle organization was severely affected by E, during lVM and multipolar spindle was the most frequently observed aberration. Exposure of oocytes to E-2-BSA did not affect nuclear maturation, blastocyst formation rate, nor embryo quality. Our results suggest that the detrimental effects of E-2 on in vitro nuclear maturation of bovine oocyte are not exerted via a plasma membrane receptor.