Abstract
We assessed the effect of the developmental stage of donor embryos on in vitro development of nuclear transplants in cattle. Nuclear donor embryos were obtained from in vitro fertilization and culture. The average cell numbers per donor embryo 68, 92, 116, and 140 h after insemination were 13.6, 13.5, 23.2, and 40.5, respectively. Enucleated oocytes were cultured 33 h for maturation and activated by electrical stimulation; activated oocytes were then cultured for an additional 9 h. Blastomeres from donor embryos were fused with enucleated oocytes by exposure to electric pulses. Nuclear transplants were then cultured with bovine oviductal epithelial cells. The fusion rate of 82-96% did not vary among the donor stages. The proportions of nuclear transplants that cleaved to the 2-and 8-cell stage were 73-87% and 22-53%, respectively. Maximal transplant development was obtained with donor embryos cultured for 116 h after insemination; 35% developed to blastocysts. This rate was significantly (P<0.01) higher than for donor embryos cultured for 68 h (13%), 92 h (3%), or 140 h (12%) after fertilization. These results indicate that developmental stage and cell cycle of donor embryos affect in vitro development of nuclear transplants.
