Intracytoplasmic glutathione concentration and the role of beta-mercaptoethanol in preimplantation development of bovine embryos

In vitro-matured/in vitro-fertilized bovine oocytes were cultured on cumulus cell layers in a serum-free medium (bovine embryo culture medium; BECM) supplemented with 3 mg/ml fatty acid-free BSA. The intracytoplasmic glutathione concentration of embryos was found to change significantly (P<0.008) during the preimplantation stages, beginning to increase at the 9- to 16-ceII stage (20.7 pM/embryo) and reaching the highest (P<0.03) level at the hatched-blastocyst stage (36.7 pM/embryo). A significantly (P<0.06) lower concentration of glutathione was obtained at the 2- to 8-cell stage (7.1 pM/embryo) than at any other stage. When inseminated oocytes were cultured in BECM supplemented with different concentrations of beta-mercaptoethanol (2-ME) to promote glutathione synthesis, higher (P<0.05) percentages of embryos developed to the 9- to 16-cell, morula and blastocyst stages at 96, 144 and 192 h post insemination, following the addition of 6.25 and 12.5 mu M than after no supplementation with 2-ME. However, when 16-cell embryos were cultured in BECM supplemented with 6.25 and 12.5 mu M of 2-ME, blastocyst formation was not significantly (P>0.9) increased. When the combined effects of 2-ME and/or cumulus cells were compared in a 2x2 factorial design, there was a significant (P<0.03) effect of 2-ME on the development of oocytes to blastocysts. The presence of cumulus cells significantly (P<0.001) affected development after the fourth cleavage (morula compaction and blastocyst formation), but there was no significant (P>0.11) interaction between 2-ME and cumulus cells. In conclusion, intracytoplasmic glutathione concentration of bovine embryos derived from in vitro-culture increases during preimplantation development. The glutathione synthesis promoter 2-ME exerts its embryotropic role on the development before the fourth cleavage, thus yielding an improvement in blastocyst formation.