NOT ONLY INNER CELL MASS CELL-NUCLEI BUT ALSO TROPHECTODERM NUCLEI OFMOUSE BLASTOCYSTS HAVE A DEVELOPMENTAL TOTIPOTENCY

The nuclei of mouse trophectoderm cells were found to have development al totipotency like inner cell mass cells after serial nuclear transfe r. Single inner cell mass or trophectoderm cells from expanded blastoc ysts synchronized with the cell cycle by treatment with nocodazole and aphidicolin to the G1 stage were injected into the perivitelline spac e of enucleated metaphase II oocytes together with Sendai virus. All o ocytes were given three electrical pulses to induce fusion and activat ion (first nuclear transfer). Aphidicolin was present in all media use d until fusion. When reconstituted oocytes developed to the two-cell s tage, the nuclei of the reconstituted eggs were fused with the enuclea ted blastomeres of fertilized two-cell embryos by inactivated Sendai v irus (second nuclear transfer). The reconstituted embryos were culture d in vitro and transferred to recipients. After the second nuclear tra nsfer, 23-64% (for inner cell mass cells) and 32-62% (for trophectoder m cells) developed to morula or blastocyst stage. Better development o f second nuclear transfer embryos was observed when oocytes fused with trophectoderm nuclei did not extrude a polar body after the first nuc lear transfer. After transfer of morulae and blastocysts to recipients , four males were obtained, two from inner cell mass and two from trop hectoderm nuclei. These findings indicate that the nucleus of inner ce ll mass and trophectoderm cells of mouse blastocysts can be reprogramm ed within the cytoplasm of unfertilized oocytes and then in fertilized embryos.