In an overwhelming majority of experiments, both mammalian embryonic and somatic cloning have relied on introducing exogenous nuclei into enucleated metaphase II (MII) oocytes. Since attempts at cloning using interphase zygotes as recipient cells have failed, these cells were – until quite recently – commonly regarded as poor recipients for nuclear transfer. However, we have recently shown that interphase zygotes can be successfully used as recipients of embryonic nuclei. In a previous study, we used our original method of selective enucleation (SE), in which the pronuclear envelope with attached chromatin is removed while the liquid pronuclear contents and nucleoli in the zygote’s cytoplasm are left intact, to obtain fertile mice upon transfer of 8-cell (1/8) nuclei into SE zygotes. Here we report that 16-cell (1/16) nuclei can also support full-term development. Additionally, full pre-implantation development, albeit to a limited degree, was obtained after transfer of embryonic stem (ES) cell and foetal fibroblast (FF) nuclei (2.4% and 2.5%, respectively). Sporadically, SE zygotes reconstructed with FF nuclei were able to implant, but they never developed beyond midpregnancy. Our results clearly indicate that SE zygotes can be successfully used as competitive recipients of embryonic nuclei from, at least, the 16-cell (morula) stage. However, their use as recipients of ES cell and somatic cell nuclei seems to be questionable.
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