Cysteine Reduces ROS Production in Ovine Oocytes Subjected to Heat Stress During In Vitro Maturation

The current investigation aimed to study the effect of cysteine supplementation (100 µM) on the IVM medium of sheep oocytes matured under heat stress conditions. Sheep ovaries were obtained from local slaughterhouses and transported in a thermally isolated container filled with saline solution (35ºC) containing gentamycin at 50 µg/mL within two hours of slaughter. A total of 1,020 cumulus-oocyte complexes (COCs) were recovered from sheep ovaries using the slicing technique. COCs were distributed into six groups; the first group (Tr1) served as a control, in which oocytes were cultured at 38.5ºC in basic IVM medium without Cysteine. Tr2: oocytes matured at 38.5ºC with 100 µM of Cysteine. Tr3: oocytes matured at 40ºC without Cysteine. Tr4: oocytes matured at 40ºC with 100 µM of Cysteine. Tr5: oocytes matured at 41ºC without Cysteine. Tr6: oocytes matured at 41ºC with 100 µM of Cysteine. Clusters of 20-30 COCs were cultured in 100 µL drops of preconditioned (38.5, 40, and 41ºC, depending on the experimental group) IVM medium, covered with mineral oil, in a CO₂ incubator (5% CO₂, 95% humidity) for 24 h. Rates of maturation due to cumulus cell expansion and first polar body extrusion were assessed in addition to the intracellular reactive oxygen species (ROS) levels of matured oocytes. The results showed that the expansion of cumulus cells for the experimented groups was 83.67%, 89.64%, 72.22%, 91.8%, 68.86%, and 80.14%, while the polar body rates recorded 26.13%, 33.33%, 10.33%, 22.01%, 5.77%, and 10.79% for Tr1, Tr2, Tr3, Tr4, Tr5, and Tr6, respectively. The findings of ROS staining indicated that cysteine supplementation reduced (P>0.05) ROS production in treated groups. In conclusion, the addition of cysteine (100 µM) to the in vitro maturation medium of sheep oocytes has the ability to mitigate the impact of heat stress on maturation rate and reduce the levels of ROS production.