Low oxygen tension during in vitro embryo production improves the yield, quality, and cryotolerance of bovine blastocysts.

Mammalian oocytes undergo maturation and fertilization in the low-oxygen (O 2 ) environment of the oviduct. To evaluate the effect of O 2 tension during in vitro maturation and fertilization on embryo yield, quality, cryotolerance, and gene expression, we matured and fertilized bovine cumulus-oocyte complexes under low (5%) or high (20%) O 2 tension. Presumptive zygotes from both groups were cultured at 5% O 2 for 8 days. Blastocysts were vitrified, and then warmed, and cultured for further 24 h to assess their cryotolerance. Our findings indicate that low O 2 during maturation and fertilization enhances embryo development and cell count in both fresh and vitrified/warmed blastocysts. In this study, the interaction of O 2 tension and status (fresh or vitrified/warmed) affected the transcript abundance of SOD2, AQP3, and BAX in blastocysts. These results highlight the role of low O 2 tension during bovine maturation and fertilization and provide support to using 5% O 2 throughout all stages of bovine in vitro embryo production.