Eggs must be the major locus of reproductive aging in women, because donation of eggs from younger to middle-aged women abrogates the effects of age on fertility. Oxidative stress, mitochondrial dysfunction, and apoptosis are associated with senescence. To develop an animal model of egg senescence, we treated mouse zygotes with 175 μM H2O2 that induced mitochondrial dysfunction and developmental arrest, followed by delayed cell death, consistent with apoptosis. We reconstructed zygotes with nuclei and cytoplasm from treated or untreated zygotes, then followed development and apoptotic cell death in the reconstituted embryos. Pronuclear exchange between untreated, normal zygotes served as nuclear transfer controls. Rates of cleavage and development to morula and blastocysts were significantly lower (P < 0.01) in zygotes reconstituted from untreated pronuclei and H2O2-stressed cytoplasts than those of nuclear transfer controls. Instead, the arrested, reconstituted zygotes displayed TUNEL staining at a similar rate to that of H2O2-treated controls, suggesting that apoptotic potential could be transferred cytoplasmically. On the other hand, rates of cleavage and development to morula and blastocyst of the reconstituted zygotes, derived from stressed pronuclei and untreated cytoplasm, were significantly increased (P < 0.05), compared to those of H2O2-treated, control zygotes, indicating that healthy cytoplasm could partly rescue pronuclei from oxidative stress. Although oxidation stressed both nuclei and cytoplasm, cytoplasm was more sensitive than nuclei to oxidative stress. It is suggested that cytoplasm, most likely mitochondria, plays a central role in mediating both development and apoptotic cell death induced by oxidative stress in mouse zygotes.
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