Timing and uniformity of embryonic gene activation affect subsequent pre-implantation development of cloned bovine embryos

In the present study, we examined the timing of onset, intensity, and mosaicism of embryonic gene expression in bovine nuclear transfer (NT) embryos. The relationship between gene expression and early embryonic development was also examined. To monitor the gene expression of NT embryos, we produced NT embryos with bovine transfected fibroblasts carrying a firefly luciferase gene under the control of a chicken beta-actin promoter, an expression system that has previously been shown to be representative of embryonic gene expression in mice. Photon count imaging showed that luciferase luminescence began in NT embryos with fibroblasts 48 hours post fusion (hpf) and reached a plateau at the 4- to 8-cell stage at 60 hpf. Only 4- to 8-cell NT embryos luminescent by 60 hpf developed to the blastocyst stage. At 60 hpf, strongly luminescent embryos developed to the blastocyst stage at a higher rate (P<0.05) than embryos with weak or absent luminescence. However, embryos with mosaic luminescence developed at a much lower rate (P<0.05) than those with whole-embryo luminescence, even if the embryos exhibited strong luminescence. Our results indicate that precise and uniform embryonic gene expression at the 4- to 8-cell stage at 60 hpf may be closely related to development of bovine NT embryos to the blastocyst stage.