Sex-related differences in the developmental rate of in-vitro matured/in-vitro fertilized ovine embryos.

Sex determination of in-vitro matured/in-vitro fertilized ovine embryos cultured in synthetic oviduct fluid (SOF) medium was performed by the polymerase chain reaction amplification of specific Y DNA sequences so as to test the influence of sex on developmental growth during the preimplantation period. At 144 h post-insemination, embryos with a blastocoel were classified as the fast-developing group, whereas those showing a blastocoel only after this length of time were classified as the slow-developing group. At 144 h post-insemination, fast-developing embryos were cultured separately and some were classified according to the size of their blastocoel. At the end of culture (207 h post-insemination), all embryos were classified according to both their developmental stage and their morphological quality. The male:female sex ratio of fast-developing embryos was significantly higher than the expected ratio of 50%. More males were observed at the most advanced developmental stage at both 144 and 207 h post-insemination. The proportion of males did not differ between the good- and poor-quality groups, although a skewed sex ratio was observed with embryos of better quality at the most developed stage. In conclusion, embryos at the most developed stage were predominantly male and were derived mainly from the fast-developing group, raising the possibility of a deviation in the sex ratio after the transfer of in-vitro matured/in-vitro fertilized ovine embryos.