Not only inner cell mass cell nuclei but also trophectoderm nuclei of mouse blastocysts have a developmental totipotency

The nuclei of mouse trophectoderm cells were found to have developmental totipotency like inner cell mass cells after serial nuclear transfer. Single inner cell mass or trophectoderm cells from expanded blastocysts synchronized with the cell cycle by treatment with nocodazole and aphidicolin to the G1 stage were injected into the perivitelline space of enucleated metaphase II oocytes together with Sendai virus. All oocytes were given three electrical pulses to induce fusion and activation (first nuclear transfer). Aphidicolin was present in all media used until fusion. When reconstituted oocytes developed to the two-cell stage, the nuclei of the reconstituted eggs were fused with the enucleated blastomeres of fertilized two-cell embryos by inactivated Sendai virus (second nuclear transfer). The reconstituted embryos were cultured in vitro and transferred to recipients. After the second nuclear transfer, 23-64% (for inner cell mass cells) and 32-62% (for trophectoderm cells) developed to morula or blastocyst stage. Better development of second nuclear transfer embryos was observed when oocytes fused with trophectoderm nuclei did not extrude a polar body after the first nuclear transfer. After transfer of morulae and blastocysts to recipients, four males were obtained, two from inner cell mass and two from trophectoderm nuclei. These findings indicate that the nucleus of inner cell mass and trophectoderm cells of mouse blastocysts can be reprogrammed within the cytoplasm of unfertilized oocytes and then in fertilized embryos.