Effects of preactivation of ooplasts or synchronization of blastomere nuclei in G1 on preimplantation development of rabbit serial nuclear transfer embryos

Blastomeres from eight-cell-stage rabbit embryos have been fused with enucl eated metaphase II oocytes (ooplasts) or with ooplasts that were preactivat ed before fusion. Preactivation of ooplasts before nuclear transfer (NT) ra ises the rate of preimplantation development from 15% to 56%, which remains elevated in the next series of NT (48.6% and 47.2% in the second and third rounds, respectively). Transfer of eight-cell embryos from the third round to the recipient resulted in the birth of normal young. Synchronization of blastomere nuclei in the G1 phase with nocodazole before fusion results in 42% morula/blastocyst formation. However, in the second generation of NT e mbryos, the yield drops to as low as 17%, indicating deleterious effects of the second nocodazole treatment on blastomeres. The calculated number of c lones per one round of cloning was 4.5, 3.9, and 3.8 in subsequent series; the highest number of morulae and blastocysts that developed from individua l donor embryos after three rounds were 26 and 27, respectively.