Ovine oocytes vitrified at germinal vesicle stage as cytoplast recipients for somatic cell nuclear transfer (SCNT)

The development of embryos produced by somatic cell nuclear transfer (SCNT) using vitrified oocytes as cytoplast recipients has been reported in cattle but not in sheep. This study investigated the parthenogenetic development of ovine oocytes vitrified and thawed at the germinal vesicle (GV) stage, matured in vitro, and then activated using two activation protocols. The optimal activation protocol was then used to assess development when vitrified oocytes were used as cytoplast recipients for SCNT. No blastocysts were obtained from vitrified oocytes activated by CA+CHX/CB (calcium ionophore A23187 + cycloheximide, and cytochalasin B); in contrast, vitrified oocytes activated by Sr/CB (strontium chloride (SrCl(2)) + cytochalasin B) developed to blastocyst, although the number was significantly lower (p < 0.05) than in toxicity and control groups (3.8 vs. 20.0 and 27.3%, respectively). In SCNT embryos, cleavage at both 24 and 48 h postactivation (31.0 vs. 55.1% and 48.0 vs. 85.0%) was significantly lower (p < 0.05) in vitrified oocytes compared to controls. However, no significant differences were observed in the frequency of development to blastocyst (13.0 vs. 23.4%), the number of hatched blastocysts (7.0 vs. 10.3%), total cell numbers (90.3 ± 4.9 vs. 97.6 ± 4.6), number of apoptotic nuclei (13.1 ± 0.9 vs. 13.2 ± 1.4), or the proportion of diploid embryos (60.0 vs. 75.0%). This study demonstrates for the first time that ovine oocytes vitrified at the GV stage can be used successfully as recipient cytoplasts for SCNT.