Enucleation after fusion and activation enhances the development of reconstructed bovine embryos

Effects of enucleation timing on enucleation rates, development and methylation levels of reconstructed bovine embryos were investigated. However, the enucleation rate of reconstructed embryos produced by the enucleation before fusion and activation (EBFA) was higher than that by the enucleation after fusion and activation (EAFA) procedure (80.7% vs. 59.1%, P < 0.05). The blastocyst rate of reconstructed embryos cloned with ear fibroblasts in EBFA group was reduced (P < 0.05) in comparison with that of EAFA group (24.6% vs. 34.4%). Two out of 11 recipients were pregnant and gave birth to two viable calves after transfer of 20 reconstructed EBFA embryos. Two out of seven recipients were pregnant and also gave birth to two calves, with one surviving, after transfer of 12 reconstructed embryos produced by EAFA procedure. Finally, the methylation level of satellite I gene of donor cells (69.8%) and reconstructed embryos in EBFA group (64.7%) were similar, which were both higher (P < 0.05) than that of the reconstructed embryos in EAFA group (44.4%). The methylation level of satellite I gene of the reconstructed embryos in the IVF embryos (31.9%) was lower (P < 0.05) than those in all other treatments. In conclusion, the reconstructed bovine embryos produced by the EAFA procedure revealed a better developmental competence with a lower methylation rate of satellite I gene than those produced by the EBFA procedure.