Effects of melatonin in the maturation medium on developmental competence of bovine oocytes exposed to heat shock and on embryo quality

Abstract

Heat stress has been related to reduced fertility in cattle, which may reflect damage to developing oocytes. The deleterious effects of heat stress on oocytes may involve an increased production of reactive oxygen species (ROS). The addition of antioxidants, such as melatonin, to the in vitro maturation medium of heat-stressed bovine oocytes may be advantageous for reducing ROS concentrations, thereby enhancing the subsequent fertilisation and development to the blastocyst stage. Aims: The present study evaluated the in vitro production and quality of bovine embryos derived from oocytes matured under heat-shock conditions with increasing melatonin concentrations. Methods: Cumulus–oocyte complexes from Bos taurus × Bos indicus cows were matured under heat shock (12 h at 41°C, followed by 12 h at 38.5°C) in medium without melatonin (control) or with 10−12, 10−9, 10−6 and 10−3 mol/L melatonin. Logistic regression equations were adjusted when the main effect of melatonin was observed. Key results: The cleavage rate on Day 3 of culture, the production of blastocysts on Days 7 and 8, the number and proportion of apoptotic cells in expanded blastocysts, and the proportion of apoptotic cells in the inner cell mass (ICM) were not affected by melatonin. The proportion of 8- to 16-cell-stage embryos on Day 3 and the proportion of ICM cells in expanded blastocysts were influenced by the melatonin concentration. Through logistic regression, it was estimated that the proportion of 8- to 16-cell-stage embryos increased linearly as the melatonin increased until 10−3 mol/L, and that the proportion of ICM cells in blastocysts was maximised between 10−6 and 10−4 mol/L melatonin, and decreased thereafter. Conclusions: Melatonin at 10−3 mol/L in the in vitro maturation (IVM) of heat-shocked oocytes stimulated the first embryo cleavages but reduced the ratio of ICM cells in blastocysts. The embryo quality may be improved by melatonin between 10−6 and 10−4 mol/L. Implications: The addition of melatonin to IVM medium has the potential to improve the quality of bovine embryos from heat-stressed oocytes. Among the many challenges, the increase in global temperature is a prominent interference in reproductive processes in dairy cattle, since it has deleterious effects on gametes. Therefore, strategies such as the use of substances that reduce the damage caused by thermal stress have been widely used in scientific experiments. In the present study, the use of the antioxidant melatonin in the in vitro maturation medium of heat-stressed bovine oocytes was able to improve embryo quality.