Effect of ooplast activation on the development of oocytes following nucleus transfer in cattle.

We assessed the effect of ooplast (enucleated oocytes) activation prior to receiving a donor nucleus on the development of nucleus transferred oocytes in cattle. The ooplasts were activated by electric stimulus at 30, 33, 36 and 39 h after being placed in culture medium for meiotic maturation. The activated ooplasts were further cultured in vitro, for a total 42 h from the beginning of maturation, 16- to 32-cell stage embryos produced by in vitro fertilization were used as donor embryos. The nucleus transferred oocytes were co-cultured with bovine oviductal epithelial cells in vitro. The fusion rate was not different between the activated (90%) and aged (94%) ooplasts 42 h after culture. Activated ooplasts receiving a donor nucleus showed a higher developmental rate than the aged ooplasts. Maximal development of the oocytes was obtained if the ooplast was activated at 9 h prior to receiving a donor nucleus. Thirty-nine percent developed to morulae and 24% to blastocysts. This compares (P<0.01) with 13% of the aged ooplasts developing to morulae and 8% to blastocysts. Of the activated ooplasts at 3, 6 and 12 h prior to fusion with a donor blastomere, 12, 16 and 13% developed to blastocysts, respectively. Of the 17 recipient cows receiving nucleus transferred embryos, 9 (53%) were diagnosed pregnant by palpation per rectum examination, and 3 normal offspring were obtained.