Abstract
The present study was conducted to establish a cloning system by nuclear transfer technique in mice in which nuclear transfer into 2-cell emrbyos and Metaphase II oocytes were examined and the developmental property of the reconstituted eggs was investigated in vitro and in vitro. The results were summerized as follows. 1) By transfer of a single nucleus from 2-and 4-cell embryos into one of blastomeres of the enucleated 2-cell embryo, identical triplet and twin mice were produced after in vitro culture and transfer to recipients.2) In the nuclear transfer into enucleated oocytes, the chromosomes of oocytes at telophase of the first meiotic division were successfully removed using standard micromanipulation and a differential interference micros-copy. The enucleated oocytes (ooplasts) wewe able to use as recipient ooplasm for nuclear transfer after culture for 4-6h in vitro.3) When the ooplasts received a nucleus from the late 2-cell embryos, 23% developed to blastocysts in vitro, and the offspring were obtained after transfer to recipients. Transfer of a inner cell mass (ICM) cell and a thymocyte from fetuses and new born mice into the ooplasts also resulted in development into blastocyst stage. 4) It was shown that the timing of fusion of donor nucleus with a ooplast and activation of it is essential factor for regulation of the remodelling of the donor nucleus in nuclear transferred oocytes. This study shows the possibility that not only early embryonic nucleui but also more advanced nuclei are reprogrammed when they are transferred into the ooplast in mice.
