Objective: To establish a suitable protocol for activating mouse somatic cell nuclear transfer embryos with strontium chloride (SrCl2).
Methods: We constructed and identified mouse nuclear transfer (NT) embryos. After nuclear injection, we activated the NT embryos using the following chemical activation methods: exposing the NT embryos to 5 and 10 mmol/L SrCl2 strontium for 1 -8 h, activating the NT embryos with 1-20 mmol/L SrCl2 strontium at 4 and 6 h, treating the NT embryos with 10 mmol/L SrCl2 strontium in different activating media, and exposing the NT embryos to 10 mmol/L SrCl2 strontium combined with 6-dimethylaminopurine (6-DMAP) and cytochalasin B (CB). After activation, the NT embryos were cultured in vitro in the cleavage medium.
Results: When the NT embryos were treated with SrCl2 at the concentration of 5 mmol/L, the cleavage rate was remarkably higher at 6 h (38.9%) than at 1 h (6.7%), 2 h (22.8%), 3 h (22.8%) and 4 h (25.6%) (P < 0.05), but with no significant differences from those at 5 h (28.9%), 7 h (34.4%) and 8 h (28.9%) (P > 0.05). When the NT embryos were treated with SrCl2 for 6 h, the rates of cleavage and blastulation were 68.9% and 7.2% at 10 mmol/L, markedly higher than at 1 mmol/L (28.3% and 0%), 2.5 mmol/L (35.6% and 0%), 5 mmol/L (37.8% and 1.1%), 7.5 mmol/L (60.6% and 2.2%), 15 mmol/L (51.7% and 1.1%), and 20 mmol/L (41.7% and 1.1%) (P < 0.05). The cleavage rate of the NT embryos cultured in the Ca2+ and Mg2+ KSOM medium was 27.8%, significantly lower than in the Ca(2+)-free KSOM (69.4%), Ca2+/Mg(2+)-free KSOM (66.1%), and Ca2+/Mg(2+)-free + EDTA KSOM (68.3%) (P < 0.05). The total cell blastocyst number was significantly larger in the NT embryos treated with SrCl2 + CB (45.40 +/- 2.23) than in those treated with SrCl2 (30.15 +/- 1.12), 6-DMAP (34.95 +/- 1.38), and 6-DMAP + CB (37.45 +/- 1.43) (P < 0.05).
Conclusion: Six-hour treatment with 10 mmol/L SrCl2 in Ca2+ alone or in combination with CB can well activate NT embryos in mice.