Parthenogenetic activation of mouse oocytes by strontium chloride: A search for the best conditions

Strontium has been successfully used to induce activation of mouse oocytes in nuclear transfer and other experiments, but the optimum treatment conditions have not been studied systematically. When cumulus-free oocytes were treated with 10 mM SrCl2 for 0.5-5 h, activation rates (88.4 +/- 4.1 to 91.2 +/- 2.7%) did not differ (mean S.E.; P > 0.2), but rate of blastulation (57.3 +/- 3.5%) and cell number per blastocyst (45.0 +/- 2.4) were the highest after treatment for 2.5 h. When treated with 1-20 mM SrCl2 for 2.5 h, the activation rate and cell number per blastocyst were higher (P < 0.02) after 10 mM SrCl2 treatment than other treatments. The best activation and development were obtained :with Ca2+-free Sr2+ medium, but the activation rate was low (37.7 +/- 1.6%) in Ca2+-containing medium. Activation rates were the same, regardless of the presence or absence of cytochalasin B (CB) in the activating medium, but the blastulation rate was higher(P < 0.001) in the presence of CB. Only 70% of the cumulus-enclosed oocytes were activated and 10% blastulated after a 10 min exposure to 1.6 mM SrCl2, and many lysed, with increased intensity of Sr2+ treatment. The presence of CB in SrCl2 medium markedly reduced lysis of cumulus-enclosed oocytes. Media M16 and CZB did not differ when used as activating media. Only 10.5% of the oocytes collected 13 h post hCG were activated by Sr2+ treatment alone, with 34% blastulating, but rates of activation and blastulation increased (P < 0.001) to 94 and 60%, respectively, when they were further treated with 6-dimethylaminopurine (6-DMAP). The total and ICM cell numbers were less (P < 0.001) in parthenotes than in the in vivo fertilized embryos. In conclusion, the concentration and duration of SrCl2 treatment and the presence or absence of CB in activating medium and cumulus cells had marked effects on mouse oocyte activation and development. To obtain the best activation and development, cumulus-free oocytes collected 18 h post hCG should be treated for 2.5 h with 10 mM SrCl2 in Ca2+ free medium supplemented with 5 mu g/mL of CB. (c) 2005 Elsevier Inc. All rights reserved.