Effects of Cell Cycle Stage of Donor Nuclei on the Development of Bovine Nuclear Transferred Embryos

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In vitro development and changes in the nuclei of reconstituted oocytes following fusion were investigated to determine the effects of cell cycle stage of donor nuclei on bovine nuclear transplantation. Enucleated oocytes, cultured for 44 to 46 h following maturation or preactivated for 9 h before fusion, were used as the source of recipient cytoplasm. Donor embryos were obtained from embryos matured, fertilized and cultured in vitro. Blastomeres of the embryos which had developed to the 4-cell stage at 39 h after insemination were separated. Each blastomere was cultured with aphidicolin for 2 to 4 h. These blastomeres, shortly after the next division (G1/S stage of the cell cycle), or those cultured for 6 h without aphidicolin after division (S stage of cell cycle), were fused with non-activated (Groups 1 and 3) or activated (Groups 2 and 4) oocytes. The proportion of embryos which developed into blastocysts were higher in Groups 2 (16%) and 4 (17%) than in Groups 1 (10%) and 3 (6%). The diameter of the nuclei after fusion with the recipient cytoplasm increased, but to a lesser extent in activated than in the non-activated cytoplasm. Some nuclei which had fused with non-activated cytoplasm showed nuclear envelop breakdown and premature chromosome condensation (PCC, 21%). No donor nuclei showed PCC when fused with activated cytoplasm.
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