Studies on somatic cell nuclear transfer in sheep

The present study investigated the effects of oocyte activation compounds such as 6-dimethylaminopurine (6-DMAP) and cycloheximide (CHX) on sheep parthenogenesis, and production of nuclear transfer (NT) embryos and fetuses. The effects of these chemical compounds on generating chromosomal abnormalities in parthenogenetic and cloned embryos and their long-term effects on the development of NT embryos in utero after embryo transfer into recipient ewes were also evaluated. The results revealed that the blastocyst rate of parthenogenetic embryos activated using 6-DMAP was significantly higher (P < 0.05) than those treated with CHX. In contrast, the blastocyst rates of the NT embryos did not differ significantly (P > 0.05) between treatment groups. All the parthenogenetic day-6 embryos derived from 6-DMAP activation were chromosomally abnormal whereas in the CHX treated embryos it was significantly lower. However, the proportion of chromosomally abnormal NT embryos did not differ significantly between two treatment groups. In addition, the results revealed that when NT embryos were transferred to recipient ewes, the late embryonic losses (between day 21--35 in pregnancy) were significantly higher (P < 0.05) in the 6-DMAP treated embryos than those treated with CHX.; The present study also assessed the telomere lengths of cloned sheep and their progeny resulted from mating between clones. The results revealed that the telomere lengths of sheep clones derived from adult somatic cells or late-passaged fetal fibroblasts had significantly lower (P < 0.05) telomere lengths compared to their age-matched controls. The telomere lengths of their offspring did not differ significantly when compared to age-matched controls. In conclusion, the present study demonstrated the importance of oocyte activation protocols on chromosomal abnormalities in NT embryos, and their long-term effects on the embryo survival after embryo transfer. In addition the study showed that telomere length is not reset in sheep clones when derived from adult somatic cells or long-term cultured fetal fibroblasts.