β-Mercaptoethanol in culture medium improves cryotolerance of in vitro-produced bovine embryos

The objective of this study was to investigate the effects of adding β-mercaptoethanol (βME) to culture medium of bovine in vitro-produced (IVP) embryos prior to or after vitrification on embryo development and cryotolerance. In Experiment I, Day-7 IVP blastocysts were vitrified and, after warming, cultured in medium containing 0, 50 or 100 μM βME for 72 h. Embryos cultured in 100 μM βME attained higher hatching rates (66.7%) than those culture in 0 (47.7%) and 50 (52.4%) μM βME. In Experiment II, IVP embryos were in vitro-cultured (IVC) to the blastocyst stage in 0 (control) or 100 μM βME, followed by vitrification. After warming, embryos were cultured for 72 h (post-warming culture, PWC) in 0 (control) or 100 μM βME, in a 2 × 2 factorial design: (i) CTRL-CTRL, control IVC and control PWC; (ii) CTRL-βME, control IVC and βME-supplemented PWC; (iii) βME-CTRL, βME-supplemented IVC and control PWC; or (iv) βME-βME, βME-supplemented IVC and βME-supplemented PWC. βME during IVC reduced embryo development (28.0% vs. 43.8%) but, following vitrification, higher re-expansion rates were seen in βME-CTRL (84.0%) and βME-βME (87.5%) than in CTRL-CTRL (71.0%) and CTRL-βME (73.1%). Hatching rates were higher in CTRL-βME (58.1%) and βME-βME (63.8%) than in CTRL-CTRL (36.6%) and βME-CTRL (42.0%). Total cell number in hatched blastocysts was higher in βME-βME (181.2 ± 7.4 cells) than CTRL-CTRL (139.0 ± 9.9 cells). Adding βME to the IVC medium reduced development but increased cryotolerance, whereas adding βME to the PWC medium improved embryo survival, hatching rates, and total cell numbers.