RESUMEN
This study aims to investigate the effects of melatonin on follicular growth, viability and ultrastructure, as well as on the levels of mRNA for antioxidant enzymes, reactive oxygen species (ROS) and meiotic progression in oocytes from in vitro cultured bovine early antral follicles. To this end, isolated early antral follicles (500-600 µm) were cultured in TCM-199+ alone or supplemented with 10-6 , 10-7 or 10-8 M melatonin at 38.5°C with 5% CO2 for 8 days. Follicle diameters were evaluated at days 0, 4 and 8 of culture. At the end of culture, ultrastructure, chromatin configuration, viability (calcein-AM and ethidium homodimer-1 staining), and the levels of ROS and mRNA for catalase (CAT), superoxide dismutase (SOD) and peroxiredoxin 6 (PRDX6) and glutathione peroxidase (GPx) were investigated in oocyte-granulosa cell complexes (OGCs). The results showed that early antral follicles cultured with 10-6 and 10-8 M melatonin had a progressive and significant increase in their diameters throughout the culture period (p < .05). Additionally, oocytes from follicles cultured with 10-7 or 10-8 M melatonin had increased fluorescence for calcein-AM, while those cultured with 10-6 or 10-7 M had reduced fluorescence for ethidium homodimer-1. Different from follicles cultured in other treatments, those cultured with 10-8 M melatonin had well-preserved ultrastructure of oocyte and granulosa cells. Melatonin, however, did not influence the levels of ROS, the mitochondrial activity, oocyte meiotic resumption and expression mRNA for SOD, CAT, GPX1 and PRDX6. In conclusion, the presence of 10-8 M melatonin in culture medium improves viability and preserves the ultrastructure of oocyte and granulosa cells of early antral follicles cultured in vitro.