Effects of Sublethal Mild Hypothermia on Kinetics, Transcriptome, and Cryotolerance of In Vitro Produced Bovine Embryos.

In this study, we evaluated the effect of a previous exposure of in vitro produced blastocysts to hypothermia (33°C) on their further cryotolerance. In Experiment 1, embryos were exposed to hypothermia for either 6 or 12 h on day 5 or 6 of development, and the blastocyst rates and kinetics were evaluated. In Experiment 2, the transcriptome of embryos exposed to hypothermia on either day 5 or 6 for 12 h was assessed. In Experiment 3, embryos were exposed to hypothermia for 12 h on day 6, vitrified-warmed, and their re-expansion, apoptotic index, and total cell number were assessed. Results showed that hypothermia had no effect on blastocyst rate but reduced the percentage of hatched blastocysts when applied for 12 h. Moreover, hypothermia was associated with changes in the transcriptional profile, particularly in embryos exposed on day 6, including increased expression of the cold-shock-related transcripts RBM3 and CIRBP. Finally, hypothermia on day 6 did not improve embryo re-expansion after cryopreservation but did reduce apoptosis in the surviving embryos. Our results provide insight into transcriptomic responses associated with mild hypothermia in bovine embryos and support the hypothesis that hormetic-like responses may contribute to improved embryo quality after cryopreservation.