The effect of oxygen toxicity on the development of mammalian embryos was assessed by the use of superoxide dismutase (SOD), a potent scavenger of superoxide radicals. Mouse pronuclear embryos recovered 17 h after human chorionic gonadotropin (hCG) were cultured in medium BWW at 37 degrees C under an atmosphere of 5% CO2 in air. Culture of mouse pronuclear embryos in the presence of Cu.Zn-SOD (500 micrograms/ml) significantly increased the blastulation rate (44.6%) when compared with the control culture system (4.2%). Essentially the same effects were observed in SOD containing either Mn or Fe in the catalytic center. Heat treatment of the SOD preparation, and the addition of anti-SOD antibodies to the culture medium, significantly reduced the attenuation of the two-cell block by SOD, indicating that this effect is SOD dependent. SOD activity was detected in rabbit oviduct fluid (3.675 +/- 3.084 mIU/mg protein) by electron spin resonance. These results suggest that active oxygen is involved in the two-cell block phenomenon in mouse embryos exposed to air and that SOD in the oviduct may play an important role in the protection of embryos from superoxide radicals.

To examine the effects of oxygen toxicity on embryonic development, mouse pronuclear embryos were cultured under low oxygen conditions with or without superoxide dismutase (SOD), and the blastulation rate was compared with that of embryos cultured under standard conditions. The blastulation rate of mouse pronuclear embryos cultured under standard conditions was only 1.5% (2/131). This rate was increased significantly, to 28.5% (43/151), when the embryos were cultured under low oxygen conditions; and to 31.0% (35/113) when SOD (500 micrograms/ml) was added to the medium under standard conditions; the rate was increased to 75.2% (115/153) when the embryos were cultured under low oxygen conditions in the presence of SOD. The minimum effective concentration of SOD in the culture medium was 50 micrograms/ml under conditions of 5% O2. The blastulation rate was significantly decreased after 1-hr exposure of pronuclear embryos to room atmospheric oxygen concentration (20% O2), and subsequent culture under 5% O2 with SOD did not result in an improved blastulation rate. Culture with SOD under 5% O2 promoted the development of two-cell stage embryos to the blastocyst stage. When two-cell stage embryos were collected 48 hr after hCG and cultured for 66 hr, their blastulation rate was similar to that of embryos collected from mice 114 hr after hCG. These results suggested that embryonic development in vitro is greatly affected by atmospheric oxygen throughout the early embryonic stages and that this harmful effect can be prevented by culturing embryos under low oxygen conditions and in the presence of SOD.

Rat pronuclear embryos were cultured in hamster embryo culture medium-1 (HECM-1) or a modified Krebs-Ringer bicarbonate solution (mKRB). Embryo cultures in HECM-1 were also challenged with low oxygen concentrations. In HECM-1, 57.9% (70/121) of the pronuclear embryos developed into the 4-cell stage after 48 h of culture. The rates of 8-cell, morula and blastocyst formation were 32.2% (39/121), 17.4% (21/121) and 9.9% (12/121), respectively. On the other hand, in mKRB, rat pronuclear embryos showed developmental blockages at the 2-cell and 4-cell stages, and never developed beyond the 4-cell stage. The rate of blastocyst formation under a low oxygen concentration was 20.1% (43/214), showing a significant difference from the value of 5.5% (11/201) obtained under a standard oxygen concentration (P less than 0.005). This is the first report of successful culture of rat pronuclear embryos to the blastocyst stage. Furthermore, it is suggested that protection from oxidation stress is a prerequisite for rat embryo development in vitro.

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