It has been proposed that one of the causes for the retarded development of preimplantation embryos in culture compared to those developed in vivo is oxidative stress. Potential sources of such stress include the use of high oxygen tension (i.e., 20%), exposure to light, and the presence of transitional metals in the culture medium (162). Therefore, several studies have examined the effect of known antioxidants on preimplantation embryo development, although the data to date remain rather contradictory. Supplementation of medium with superoxide dismutase (SOD), which dis-mutases superoxide radicals, increased the development of mouse zygotes beyond the 2-cell block to the blastocyst stage (163,164). However, several studies have reported that SOD had no effect on either mouse (165), rabbit (166) or bovine (167) embryo development in vitro.
Similarly, Legge and Sellens (168) reported that addition of gluta-thione to the medium stimulated development of mouse zygotes in culture, whereas Nasr-Esfahani and Johnson (169) reported that the addition of glutathione to the medium did not increase embryo development in culture. Glutathione is present in fluid of the reproductive tract and, therefore, may have a role in embryo development (170). Moreover, the beneficial effects of the addition of cysteamine to the medium for bovine (171-174) and pig (175) oocyte development have been attributed to an increase in intracellular glu-tathione levels (176). Therefore, it is feasible to suggest that the maintenance of a high intracellular pool of glutathione may be important for high rates of development of the oocyte and early embryo.
The conflicting reports as to the benefits of adding antioxidants to culture media may in part be explained by their use in isolation and not as part of a more complete antioxidant system. For example, when SOD is present to dismutase superoxide radicals to hydrogen peroxide, then catalase and/or glu-tathione may be required to remove the peroxide formed. The presence of more than one antioxidant may facilitate the cycling of antioxidants back into the reduced forms. Alternatively, the generation of superoxide radicals will depend on the medium used for culture. Interestingly, however, it has been shown that pyruvate present in the culture medium is a powerful antioxidant (177) and readily decreases intracellular hydrogen peroxide levels within the embryo (178,179). As pyruvate is present in all media for embryo development, by default embryo culture media are supplemented with an antioxidant. Similarly, the amino acid taurine present in such media as G1 (38,40) and P1 (20), may also serve as an antioxidant (180). Finally, the addition of the water soluble antioxidant ascorbate has been shown to be highly beneficial when added to media used in slow freezing of embryos (181). Presumably, the presence of such an antioxidant is beneficial in reducing the impact of reactive oxygen intermediates generated during the freezing process.
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