a, Stages of development from the unfertilized oocytes to the 2-cell stage embryo. Oocytes in meiosis and zygotes in mitosis are suitable for nuclear transfer, but not zygotes in interphase or even 2-cell stage embryos in interphase. Whether 2-cell stage embryos in mitosis can be used for transfer of a genome from a more differentiated cell is addressed here. b, 2-cell stage embryo in interphase 54h post hCG (human chorionic gonadotropin, a hormone stimulating ovulation). c, Blastomeres in mitosis 55h post hCG. d, One blastomere had the genome removed in mitosis. e, One of the two blastomeres transferred with mouse ES cells expressing H2B-cherry. f, A 4-cell stage embryo 12h post transfer. g, Morula at 28h post transfer, composed of 8 cells, 4 of which are derived from the transferred blastomere. H) Blastocyst at 48h post transfer. PB= polar body.

a, Contribution of cells derived from the transferred blastomere marked by H2B-cherry to ICM and TE lineages at the blastocyst stage. Oct4 and H2B-cherry double positive cells are marked with a asterisk. b, quantification of H2B-cherry cells in TE and ICM (red columns). Total number of cells is indicated above each column. The total number of Oct4 positive cells as a marker of the ICM fate is 23% (blue column) at the expanded blastocyst stage. Bars indicate standard deviations. c, Contribution to full-term development of blastomeres transferred in mitosis. d, Contribution of cells derived from transferred (red) and nontransferred blastomere (dark) to a portion of the intestinal tube and the lung. e, Stem cells derived from chimeric blastocysts. P1= passage1 after manual picking of the ICM outgrowth. f, Karyotype of stem cells, showing a normal mouse karyotype of 40 chromosomes. Each chromosome is indicated with an arbitrary number of 1–40. g, Oct4 expression in embryonic stem cells derived after chromosome transfer, alkaline phosphatase staining, and SSEA-1 expression. h, chimeric mouse after injection of ES cells into BDF2 blastocysts. Tissue derived from the ES cell is brown, Tissue derived from the injected embryo is dark grey.

A) Schematic representation of the experiment: 2-cell stage embryos are fused at interphase to form a tetraploid 1-cell stage embryo. These chromosomes will assemble in a spindle at the next mitosis, which can be removed and replace with a diploid genome. These embryos then cleave and develop to the balstocyst stage (B).

a, Mitotic CD4+ T-cell population. Donor T-cells do not show expression of an Oct4::GFP transgene. b, Upon transfer of a mitotic T cell into a blastomere in mitosis, development occurs and the Oct4::GFP transgene is reactivated within hours after transfer. Developmental progression to the blastocyst stage.