Effect and possible mechanisms of melatonin treatment on the quality and developmental potential of aged bovine oocytes.

After reaching the metaphase II (MII) stage, unfertilised oocytes undergo a time-dependent process of quality deterioration referred to as oocyte aging. The associated morphological and cellular changes lead to decreased oocyte developmental potential. This study investigated the effect of exogenous melatonin supplementation on in vitro aged bovine oocytes and explored its underlying mechanisms. The levels of cytoplasmic reactive oxygen species and DNA damage response in bovine oocytes increased during in vitro aging. Meanwhile, maturation promoting factor activity significantly decreased and the proportion of morphologically abnormal oocytes significantly increased. Melatonin supplementation significantly decreased quality deterioration in aged bovine MII oocytes (P<0.05). Additionally, it decreased the frequency of aberrant spindle organisation and cortical granule release during oocyte aging (P<0.05). In the melatonin-supplemented group, mitochondrial membrane potential and ATP production were significantly increased compared with control. Furthermore, melatonin treatment significantly increased the speed of development of bovine oocytes to the blastocyst stage after in vitro fertilisation and significantly decreased the apoptotic rate in the blastocysts (P<0.05). The expression of Bax and Casp3 in the blastocysts was significantly reduced after treatment with melatonin, whereas expression of Bcl2 significantly increased (P<0.05). In conclusion, these findings suggest that supplementation of aged bovine oocytes with exogenous melatonin improves oocyte quality, thereby enhancing the developmental capacity of early embryos.