Cytoplasm mediates both development and oxidation-induced apoptotic cell death in mouse zygotes

Eggs must be the major locus of reproductive aging in women, because donati on of eggs from younger to middle-aged women abrogates the effects of age o n 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 mu M H2O2 that induced mitochondrial dysf unction and developmental arrest, followed by delayed cell death, consisten t with apoptosis. We reconstructed zygotes with nuclei and cytoplasm from t reated or untreated zygotes, then followed development and apoptotic cell d eath in the reconstituted embryos. Pronuclear exchange between untreated, n ormal zygotes served as nuclear transfer controls. Rates of cleavage and de velopment 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, reconstitu ted zygotes displayed TUNEL staining at a similar rate to that of H2O2-trea ted controls, suggesting that apoptotic potential could be transferred cyto plasmically. On the other hand, rates of cleavage and development to morula and blastocyst of the reconstituted zygotes, derived from stressed pronucl ei and untreated cytoplasm, were significantly increased (P < 0.05), compar ed to those of H2O2-treated, control zygotes, indicating that healthy cytop lasm could partly rescue pronuclei from oxidative stress. Although oxidatio n stressed both nuclei and cytoplasm, cytoplasm was more sensitive than nuc lei to oxidative stress. It is suggested that cytoplasm, most likely mitoch ondria, plays a central role in mediating both development and apoptotic ce ll death induced by oxidative stress in mouse zygotes.