Purpose Studies in bovine and porcine have indicated that melatonin (MT) could induce meiotic maturation of immature oocytes in vitro. The object of the current study was to investigate if MT could ameliorate human oocytes maturation during rescue in vitro maturation (IVM). Methods Two hundred seventy eight germinal vesicle (GV) oocytes and 451 (MI) metaphase I oocytes were vitrified, thawed and then matured in vitro. All the oocytes were randomly allocated into six groups in which the oocytes were cultured in medium supplemented with different concentrations of MT (0, 10 −2 , 1, 10 2 , 10 4 , 10 6 nM) and nuclear maturation was evaluated at 6 h, 12 h, 18 h, 24 h and 48 h of culture. Results The optimal MT concentration for both GV and MI oocytes was 1 nM. At 24 h of culture, nuclear maturation rate of MI oocytes cultured in 1 nM MT medium was significantly higher than other groups (P<0.05); Nuclear maturation rate of GV oocytes cultured in 1 nM MT medium was also significantly higher than the control group (P<0.05). On the other hand, decreased nuclear maturation rate was observed in the high MT concentration group (10 6 nM). Conclusions The current study demonstrated that low concentration of exogenous MT could ameliorate nuclear maturation of human oocyte during rescue IVM, while high concentration of MT presented negative effects.

This finding reinforced the association between telomere abnormalities and PCOS. Given the importance of telomere length in cellular proliferation, our findings provided novel insights into the pathophysiology of PCOS that abnormalities in telomere length possibly disturb folliculogenesis and subsequently result in PCOS.

For patients with high risk of fertilization failure, there could be a risk of compromising the rate of available embryos, if fertilization is judged by the presence of 2 PB by cumulus cell removal only 4 h post-insemination. Therefore, this strategy is not recommended to all IVF cycles and future studies are needed to confirm its reliability.