Birth of mice after transplantation of early cell-cycle-stage embryonic nuclei into enucleated oocytes.

The present study was conducted to investigate the influence of cell cycle stage of the donor nucleus on chromatin structure and development of mouse embryonic nuclei transplanted into enucleated oocytes. Donor cell-cycle stage was controlled in order to examine, in addition, the developmental potential of nuclei from 2-, 4-, and 8-cell-stage embryos. The cell cycle stage of donor nuclei was classified as early, middle, or late. After nuclear transfer, electrofusion, and activation, early-stage transplants formed a single pronucleus-like structure, but middle-stage transplants formed very irregular types of structures and late-stage transplants extruded a polar body. A high proportion of development to the blastocyst stage (77.8%) and an increased cell number (62.1 cells) were obtained from the early 2-cell-stage transplants as opposed to the middle- (0%) and late-stage (20.8%, 37.0 cells) transplants (p < 0.001). With transplantation of early-stage nuclei, high proportions of development to the blastocyst stage and of offspring were obtained from nuclear transplant embryos with a nucleus from a 2-, 4-, or 8-cell-stage embryo. The results confirm that the donor cell-cycle stage critically affects the chromatin structure and development of nuclear transplant embryos. The results also demonstrate that the nuclei from 2-, 4-, and 8-cell-stage mouse embryos in the early stage of each cell cycle can be reprogrammed when transplanted into enucleated mature oocytes.