Enucleation procedures applied in mammalian cloning remove not only the oocyte's chromosomes but presumably also the spindle-associated factors. If these factors are beneficial for reprogramming, alternative protocols that limit enucleation of factors in addition to the chromosomes may improve cloning efficiency. In this study, we evaluated the enucleation in combination with various activation protocols on clone development and gene expression. Clones produced by nuclear transfer into pre-activated bovine oocytes rather than non-activated oocytes can develop in vitro (Kurosaka et al. 2002 Biol. Reprod. 67, 643–647; Tani et al. 2003 Biol. Reprod. 69, 1890–1894). We produced bovine clones using four different nuclear transfer protocols and, in clones of all groups, examined timing of DNA synthesis in the first cell cycle, pre-implantation development, and gene expression at the blastocyst stage. Protocols applied were: (A) donor cells were fused with non-activated oocytes; (B) donor cells were fused with oocytes at 2 hours after activation with ethanol (7%, 7 min); (C) oocytes were enucleated after spindle disassembly with nocodazole treatment (0.3 μg/mL, 30 min) and donor cells were fused with non-activated oocytes; and (D) oocytes were enucleated after spindle disassembly and donor cells were fused with oocytes at 2 h after activation. Fused couplets in all treatment groups were treated with 10 μg/mL cycloheximide for 6 h, and cultured in vitro in SOF supplemented with fetal bovine serum at 39°C in an atmosphere of 5% CO₂, 5% O₂, and 90% N₂. The onset of DNA synthesis was determined by an immunofluorescence assay of 5-bromo-deoxyuridine incorporation at 6 and 9 h post-fusion (hpf). Oct-4 mRNA distribution in clone blastocysts was examined by whole mount in situ hybridization using a bovine Oct-4-specific antisense riboprobe. Data were statistically analyzed with Student's t-test. In the majority of clones DNA synthesis had not commenced 6 hpf but had initiated 9 hpf. Although the cell cycle of activated oocytes (protocols B and D) was 2 hours advanced compared to non-activated oocytes (protocols A and C), clones produced by all protocols had a similar onset of DNA synthesis at 6 to 9 h post-fusion. Developmental rates to the blastocyst stage of clones were not significantly different between the four protocols (48.5%–57.7%, P < 0.05). Oct-4 distribution in clones produced by all four protocols was not different from that of IVF embryos used as a control in that Oct-4 mRNA signal was typically restricted to the ICM (87.0%–100.0%, P < 0.05). We conclude that in bovine clones produced in this study, nocodazole-treated enucleation and activation status of recipient oocyte did not influence the pre-implantation development and spatial pattern of Oct-4 expression.

This work was supported by the Lalor Foundation.