Abstract

The objective of this study was to investigate the effects of two parameters on mouse embryo development in vitro. These parameters were the effect of oocyte age on activation and the effect of O₂ concentration in culture. In the first experiment, oocytes were recovered from superovutated mice at 15 h (group 1) or 20 h (group 2) after human chorionic gonadotropin (HCG) injection. All oocytes were activated for 6 h with 10 mM Sr²⁺ in Ca²⁺ free medium in the presence of 5 µg/mL of cytochalasin B. After activation, embryos were cultured in KSOM.aa medium for 4.5-5.5 days. Zygotes from naturally bred mice were used as control. Differences in blastocyst formation rate and blastocyst cell number among treatments were analyzed by one-way ANOVA after arcsin square transformation. In the first experiment, blastocyst formation rate in the first group was higher than in the second group (62.6% vs. 47.1%; P < 0.05). In addition, blastocyst cell number was also higher in the first group than in the second one (69.4 ± 3.2 vs. 52.4 ± 2.2; P < 0.05). However, both values were higher in control group (80%, 76.2 ± 1.2; P < 0.05) than in the experimental groups. These results showed that young oocytes were activated more effectively than aged oocytes. In the second experiment, mouse zygotes were cultured in a humidified atmosphere of 5% CO₂ in air (group 3) or 5% CO₂, 5% O₂, and 90% N₂ (group 4). Blastocyst formation rate and blastocyst cell number of zygotes cultured in low O₂ concentration (group 4) for 4.5 days were higher than for group 3 (76.3% vs. 56.4 and 69.0 ± 3.4 vs. 52.8 ± 2.3; P < 0.05). There was a significant difference in blastocyt formation rate of embryos for 5.5 days between the two groups (25.8% for group 4 vs. 14.4% for group 3; P < 0.05). This suggests that the embryos developed more slowly in high O₂ concentration. These results showed that low O₂ concentration provided a more suitable environment for mouse embryo development in vitro. The same experiment was repeated with parthenogenetic embryos recently in our laboratory.