Electric-field-induced blastomere fusion was studied in 2-cell rabbit embryos. Field strengths (1 to 3 kV cm−1) and durations (35 to 1000 μs) were chosen so as to provide the right balance between fusion rate, viability and developmental capacity of embryonic cells. Maximum plasma membrane tolerance of 2-cell rabbit embryos was observed at about 3 vk cm−1 for 1000 μs. All surviving ‘fused’ embryos were able to develop in vitro and most of them formed expanded blastocysts. Observation of ‘fused’ embryos immediately after fusion and during the whole cell cycle showed that 27·7% of the two diploid nuclei remained separated in the hybrid cell. More than one metaphase plate was formed at the onset of mitosis causing direct cleavage into three or four ‘cells’. In the remaining embryos the two diploid nuclei seemed to form a common metaphase plate and cleaved into two equal blastomeres. After transfer to recipient does, 54·4% of these tetraploid embryos developed beyond implantation. Between day 11 and 20, ten live and morphologically fully normal embryos were recovered. Nine embryos were uniformly tetraploid and one recovered on day 18 was a diploid/tetraploid mosaic. The remaining implantation sites contained either abnormal, very retarded embryos or indefinable embryo remnants. After transfer of ‘nonfused’ embryos treated with 3 kV cm−1, 49% gave birth to normal live young. These results suggest that the electric field can be applied successfully in a relatively wide strength and duration range without causing any visible teratogenic effect on treated embryos. Thus, tetraploid embryos can develop normally at least until two-thirds of pregnancy, but the question whether they are able to survive till term remains open.

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