Melatonin improves the in vitro growth of bovine oocytes collected from early antral follicles by maintaining oocyte-cumulus cell communication.

Purpose: In vitro, oocyte development is susceptible to oxidative stress, which leads to endoplasmic reticulum (ER) stress. This study investigated whether the antioxidant melatonin attenuates ER stress and maintains oocyte-cumulus cell communication during the in vitro growth (IVG) of bovine oocytes.

Methods: Oocyte-granulosa cell complexes (OGCs) were harvested from slaughterhouse-derived ovaries and grown in vitro for 5 d at 38.5°C in 5% CO₂ humidified air. Melatonin (10^-7, 10^-9, or 10^-11 M) was added to the culture medium.

Results: Oocyte diameter increased on day 5 from its initial value in all groups. The antrum formation rate was significantly higher in the 10^-9 M melatonin-treated group than in the control. The melatonin-treated group showed reduced oxidative stress and increased gap junction communication compared with the control. ER stress-related genes in OGCs were significantly downregulated in the 10^-9 M melatonin-treated group compared with those in the control. No significant changes were found in subsequent maturation among groups; however, 10^-9 M melatonin treatment during IVG and IVM increased the maturation rate compared with that in the control.

Conclusions: Melatonin reduces oxidative stress, which attenuates ER stress in OGCs during IVG of bovine oocytes and may improve IVG efficiency in assisted reproductive technology.