The effect of recipient oocyte volume on nuclear transfer in cattle.

This study compared the developmental potential of bovine nuclear transfer embryos with varying amounts of cytoplasm. Embryos formed from single cytoplasts fused to blastomeres by a single electrical pulse or from double cytoplasts using a double electrical pulse resulted in reconstituted embryos containing 75% and 150% of the original oocyte volume. No differences in fusion, cleavage, or development rates to blastocysts were observed between the groups. Mean cell numbers 2 days after fusion were significantly lower in single-cytoplast clones. Cell numbers of resulting blastocysts were likewise significantly lower in single-cytoplast clones. Embryos formed by fusion of blastomeres with single cytoplasts using a single electrical pulse or from double cytoplasts using either a single or a double pulse resulted in reconstituted embryos containing 50%, 100% and 100% of the original oocyte volume. Again, no differences in fusion or cleavage rates were observed between groups, but the development to blastocysts at day 7 was significantly higher in double cytoplasts constructed with one fusion pulse than in single cytoplasts (P < 0.05). Mean cell numbers 2 days after fusion were significantly lower in single-cytoplast clones (P < 0.05), but at the blastocyst stage, no statistically significant differences in cell numbers were observed. The results of this study show that cytoplasmic volume plays a role in the development of nuclear transfer embryos. When using crude enucleation methods such as oocyte bisection, normal cytoplasmic volumes can be achieved by fusing double cytoplasts with embryonic blastomeres.