Developmental potential of bovine nuclear transfer embryos and postnatal survival rate of cloned calves produced by two different timings of fusion and activation.

We compared developmental potential of somatic cell nuclear transfer (NT) embryos and postnatal survivability of cloned calves produced by two different fusion and activation protocols. As donor cells for NT, bovine cumulus cell-derived cultured cells of passage 5 were used following culture in serum-starved medium for 5–7 days. Enucleated oocytes were fused with donor cells at 21 or 24 hr post maturation. NT embryos fused at 21 hr were activated chemically 3 hr after fusion (DA group) and embryos fused at 24 hr were activated chemically immediately after fusion (FA group). Chemical activation was accomplished by calcium ionophore for 5 min and cytochalasin D + cycloheximide for 1 hr then cycloheximide alone for 4 hr. After in vitro culture in IVD101 medium for 7 days, embryo transfer was performed. Fusion rates were 86 and 84% in the DA and FA groups, respectively. Developmental rate to the blastocyst stage of NT embryos in the DA group was higher than in the FA group (42% vs. 28%). Pregnancy rate did not differ significantly between the DA and FA groups (11/13 and 5/7 at day 35), and 13 cloned calves (including 1 set of twins from a single embryo transfer) were born. High rates of postnatal mortality were observed in both groups. These results suggest that the DA method improves in vitro developmental potential of NT embryos, but the timing of fusion and chemical activation does not affect the pregnancy rate and the survivability of cloned calves. Mol. Reprod. Dev. 66: 264–272, 2003. © 2003 Wiley-Liss, Inc.