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 c ycle 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 embry os. The cell cycle stage of donor nuclei was classified as early, midd le, or late. After nuclear transfer, electrofusion, and activation, ea rly-stage transplants formed a single pronucleus-like structure, but m iddle-stage transplants formed very irregular types of structures and late-stage transplants extruded a polar body. A high proportion of dev elopment 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) transp lants (p < 0.001). With transplantation of early-stage nuclei, high pr oportions 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-cycl e stage critically affects the chromatin structure and development of nuclear transplant embryos. The results also demonstrate that the nucl ei from 2-, 4-, and 8-cell-stage mouse embryos in the early stage of e ach cell cycle can be reprogrammed when transplanted into enucleated m ature oocytes.