The somatic cell nuclear transfer technique has been applied to various mammals to produce cloned animals; however, a standardized method is not applicable to all species. We aimed here to develop optimum procedures for somatic cell cloning in nonhuman primates, using common marmosets. First, we confirmed that parthenogenetic activation of in vitro matured oocytes was successfully induced by electrical stimulation (three cycles of 150V/mm, 50μsec×2, 20min intervals), and this condition was applied to the egg activation procedure in the subsequent experiments. Next, nuclear transfer to recipient enucleated oocytes was performed 1h before, immediately after, or 1h after egg activation treatment. The highest developmental rate was observed when nuclear transfer was performed 1h before activation, but none of the cloned embryos developed beyond the eight-cell stage. To investigate the causes of the low developmental potential of cloned embryos, a study was performed to determine whether the presence of metaphase II (MII) chromosome in recipient ooplasm has an effect on developmental potential. As a result, only tetraploid cloned embryos produced by transferring a donor cell into a recipient bearing the MII chromosome developed into blastocysts (66.7%). In contrast, neither parthenogenetic embryos nor cloned embryos (whether diploid or tetraploid) produced using enucleated oocytes developed past the eight-cell stage. These results suggest that MII chromosome, or cytoplasm proximal to the MII chromosome, plays a major role in the development of cloned embryos in common marmosets.
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