Without using sperm, artificial oocyte activation is essential for current assisted reproductive technologies, particularly somatic cell nuclear transfer and round spermatid injection. Strontium has been widely used as an activator of oocytes especially in the mouse, by which efficient oocyte activation requires Ca2+-free medium. In this study, we examined whether Sr2+ can efficiently activate oocytes in Ca2+-containing culture media when calcium is chelated. Ethylene glycol-bis (β-aminoethyl ether) -N, N, N', N'-tetraacetic acid (EGTA) was added to three standard culture media (CZB, M16 and KSOM) for mouse embryos because it preferentially binds Ca2+ rather than Sr2+. We found that treatment with 5 mM Sr2+ and 2 mM EGTA left fewer than 1% of oocytes at the MII stage, which is comparable to that of Ca2+-free medium. As a result, addition of 2 mM EGTA along with 5 mM Sr2+ in either CZB, M16 or KSOM made more than 80% of available activated oocytes, which was comparable to or better than 72% in a Ca2+-free Sr2+ medium, since EGTA-Sr2+ activation led to significantly less oocyte degeneration than Ca2+-free Sr2+ activation. Furthermore, we demonstrated that this activation method can support the birth of cloned embryos. Thus, addition of EGTA to typical Ca2+-containing culture media can easily produce activation media that does not interfere with embryonic development.
2007 Society for Reproduction and Development