The early embryo response to intracellular reactive oxygen species is developmentally regulated

In vitro embryo production (IVP) suffers from excessive developmental failure. Its inefficiency is linked, in part, to reactive oxygen species (ROS) brought on by high ex vivo oxygen (O-2) tensions. To further delineate the effects of ROS on IVP, the intracellular ROS levels of early bovine embryos were modulated by: (1) varying O-2 tension; (2) exogenous H2O2 treatment; and (3) antioxidant supplementation. Although O-2 tension did not significantly affect blastocyst frequencies (P>0.05), 20% O-2 accelerated the rate of first cleavage division and significantly decreased and increased the proportion of permanently arrested 2- to 4-cell embryos and apoptotic 9- to 16-cell embryos, respectively, compared with embryos cultured in 5% O-2 tension. Treatment with H2O2, when applied separately to oocytes, zygotes, 2- to 4-cell embryos or 9- to 16-cell embryos, resulted in a significant (P<0.05) dose-dependent decrease in blastocyst development in conjunction with a corresponding increase in the induction of either permanent embryo arrest or apoptosis in a stage-dependent manner. Polyethylene glycol-catalase supplementation reduced ROS-induced embryo arrest and/ or death, resulting in a significant (P<0.05) increase in blastocyst frequencies under high O-2 culture conditions. Together, these results indicate that intracellular ROS may be signalling molecules that, outside an optimal range, result in various developmentally regulated modes of embryo demise.