IMPROVED DEVELOPMENT TO BLASTOCYST OF OVINE NUCLEAR TRANSFER EMBRYOS RECONSTRUCTED DURING THE PRESUMPTIVE S-PHASE OF ENUCLEATED ACTIVATED OOCYTES

The timing of pronuclear formation and the initiation and duration of the DNA synthetic period (S-phase) were determined during the first ce ll cycle of electrically activated ovine oocytes matured in vivo. Reco nstructed embryos were produced by electro-fusion-mediated nuclear tra nsfer of unsynchronized single blastomeres. These were derived from em bryos produced in vivo at the 16-cell stage (Day 4) and transferred to enucleated metaphase II oocytes at the time of activation or to enucl eated activated oocytes during early, mid, and late stages of the pres umptive S-phase. The frequency of development to blastocyst was greate st in embryos reconstructed during the presumptive S-phase of enucleat ed activated oocytes than in embryos reconstructed at the time of acti vation (mean 55.4% vs. 21.3%). No significant differences were observe d when embryos were reconstructed during early, mid, or late stages of the presumptive S-phase (61.3%, 45.7%, and 57.7%, respectively). The results indicate that the use of enucleated activated oocytes as cytop lasts for embryo reconstruction can increase the frequency of developm ent to blastocyst of embryos reconstructed from unsynchronized donor b lastomeres.