Variations in bovine embryo production between individual donors for OPU-IVF are closely related to glutathione concentrations in oocytes during in vitro maturation

Variations in embryo production between individual oocyte donors represent a serious problem in cattle production, when implementing ovum pick-up (OPU) and in vitro maturation (IVM) of oocytes. However, the precise cause of this problem is unknown. Here, we aimed to investigate the relationship between the glutathione (GSH) concentration in IVM oocytes and embryo development to explore a potential cause of the variations between individual donors. First, we found a high positive correlation (r = 0.80) between the GSH concentration in IVM oocytes and the blastocyst development rate of oocytes collected in the same OPU session for each donor (N = 11). Second, we selected two donors with significantly different blastocyst development rates. In samples from these donors, we examined the dynamics of oocytes GSH concentrations, and the gene expression of the glutathione synthetase (GSS) and glutathione peroxidase (GPX) genes in cumulus-oocyte complexes (COCs) during IVM. At 0 and 24 h after IVM, oocytes from the donor with the highest blastocyst development rate (high donor) exhibited significantly higher oocyte GSH concentrations than oocytes from the donor with the lowest blastocyst development rate (low donor, P < 0.05). At 4 and 9 h after IVM, GSH concentrations gradually decreased in oocytes from both donors. The GSS expression levels at 0, 4, and 9 h after IVM were significantly higher in COCs from the high donor than in COCs from the low donor (P < 0.05). The expression levels of GPX, a marker of oxidative stress, at 4 and 9 h after IVM were significantly higher in COCs from the low donor than in COCs from the high donor. Finally, adding cysteine into the IVM medium of oocytes collected in the same OPU session from the low donor significantly increased oocytes GSH concentrations and blastocyst development rates (P < 0.05). In conclusion, we showed that oocyte GSH concentrations were related to the differences in embryo development between individual donors. Our results suggested that increasing the GSH concentration in IVM oocytes could reduce the individual differences in embryo production between donors.