Nuclear reprogramming in embryos generated by the transfer of yak (Bos grunniens) nuclei into bovine oocytes and comparison with bovine–bovine SCNT and bovine IVF embryos

Although inter-species SCNT may be useful for increasing and preserving populations of endangered species, there are many reports that inter-species nuclear transfer embryos only develop to the blastocyst stage. In this study, yak–bovine SCNT blastocysts were successfully implanted in the surrogate bovine uterus but failed to develop to term or aborted. To clarify the reasons, we examined yak–bovine SCNT blastocyst development, total cell number, inner cell mass (ICM) number, trophoblast (TE) cell number and relative gene expression in yak fibroblast cells and yak–bovine SCNT embryos at various stages. The potential for development of yak–bovine SCNT embryos to blastocysts was 30 ± 5.7% (mean ± S.E.M.); the total cell number was 85.3 ± 16.3, fewer than in IVF bovine embryos (106.2 ± 18.2) but within the reported range (60–300). The yak–bovine SCNT blastocysts had a lower ratio of TE cells to total cells (43.9 ± 8.7%) than bovine IVF embryos (59.4 ± 3.4%; P < 0.05) or bovine–bovine SCNT (69.5 ± 5.4%; P < 0.05). Also, several yak–bovine SCNT embryos had abnormal initiation of expression of both Mash2 and IL6. However, expression of vimentin, collagen, Cx43 and PSMC3 were normal in yak fibroblast cells and yak–bovine SCNT embryos. In conclusion, we inferred that the normal allocation of ICM and TE cells in yak–bovine SCNT embryos and embryo-specific gene reprogramming may be important for successful inter-species animal cloning.