Abnormal chromosome segregation at early cleavage is a major cause of the full-term developmental failure of mouse clones.

To clarify the causes of the poor success rate of somatic cell nuclear transfer (SCNT), we addressed the impact of abnormalities observed at early cleavage stages of development on further full-term development using 'less-damage' imaging technology. To visualize the cellular and nuclear division processes, SCNT embryos were injected with a mixture of mRNAs encoding enhanced green fluorescent protein coupled with α-tubulin (EGFP-α-tubulin) and monomeric red fluorescent protein 1 coupled with histone H2B (H2B-mRFP1) and monitored until the morula/blastocyst stage three-dimensionally. First, the rate of development of SCNT embryos and its effect on the full-term developmental ability were analyzed. The speed of development was retarded and varied in SCNT embryos. Despite the rate of development, SCNT morulae having more than eight cells at 70h after activation could develop to term. Next, chromosomal segregation was investigated in SCNT embryos during early embryogenesis. To our surprise, more than 90% of SCNT embryos showed abnormal chromosomal segregation (ACS) before they developed to morula stage. Importantly, ACS per se did not affect the rate of development, morphology or cellular differentiation in preimplantation development. However, ACS occurring before the 8-cell stage severely inhibited postimplantation development. Thus, the morphology and/or rate of development are not significant predictive markers for the full-term development of SCNT embryos. Moreover, the low efficiency of animal cloning may be caused primarily by genetic abnormalities such as ACS, in addition to the epigenetic errors described previously.