Developmental competence and oxidative state of mouse zygotes heat-stressed maternally or in vitro.

Mammalian preimplantation embryos are sensitive to maternal and direct heat stress. However, the mechanisms by which heat stress affects early embryonic development in vivo or in vitro are unknown. This study examined whether heat-stress-induced loss of developmental competence in mouse embryos was mediated by physiological changes in the maternal environment or by high temperatures alone. After fertilization, zygotes at the same stage were heat-stressed at 39.5 degrees C for 12 h either maternally (measured by maternal rectal temperature) or directly in culture. Zygotes in each group were cultured at 37.5 degrees C for a further 84 h to assess their developmental ability. Neither type of heat stress affected the first cleavage rate. However, the proportion of embryos that developed to morulae or blastocysts was significantly lower in the maternally heat-stressed group, but not in the directly heat-stressed group. Moreover, maternal heat stress significantly reduced intracellular glutathione concentrations and enhanced hydrogen peroxide concentrations in both zygotes and two-cell embryos that were recovered immediately after heat stress or 12 h later, respectively. In contrast, direct heat stress had little effect on concentrations of glutathione or hydrogen peroxide in cultured early embryos. These results demonstrate that maternal heat stress at the zygote stage reduces the developmental ability of mouse embryos via physiological changes in the maternal environment that lead to an increase in intracellular oxidative stress on the embryo.