The Temperature of the First Cleavage Impacts Preimplantation Development and Newborn Viability.

At the early developmental stage, embryos are susceptible to environmental factors, which modulate development trajectories. In our study, we examined how different incubation temperatures (35 °C, 37 °C, and 39 °C) in vitro during the first embryonic cleavage affect the morphology, cell division rate, and DNA methylation in two-, four-, and eight-cell embryos and the viability of these two-cell embryos transferred to recipient females. Embryos kept at 35 °C for the first 24 h after in vitro fertilization in two- and four-cell embryos at 37 °C showed enhanced variability in the size of blastomeres and DNA 5mC level among blastomeres, as compared to the groups kept at 37 °C and 39 °C. This was associated with the highest rate of embryo death in four- and eight-cell embryos and the highest viability of newborns. In contrast, incubation at 39 °C did not significantly impact developmental dynamics and viability in vitro but led to a notably higher rate of gestation failure compared to other groups. The indicators of the 37 °C group fell within an intermediate range. Therefore, we conclude that a decrease in temperature during zygotic genome activation (ZGA) highlights the adaptive potential of embryos during their initial cleavages, while an increase in temperature does not show clear effects on their fate.