Consistency of oxygen tension during IVM and IVF is critical for epigenetic integrity and redox homeostasis in bovine preimplantation embryos.

Preimplantation embryos display acute microenvironmental sensitivity, dynamically coordinating metabolic pathways and epigenetic remodeling to maintain homeostasis. However, because early chromatin-modifying enzymes are vulnerable to external fluctuations, abrupt environmental shifts across major cellular transitions risk disrupting the substrate availability and redox synchronization required for faithful reprogramming. To address this problem, this study investigated the biological significance of environmental continuity during the critical transitions of in vitro maturation (IVM) and in vitro fertilization (IVF), examining how oxygen tension shapes the interplay between embryonic redox balance and the epigenome. Using a bovine model across a 2 × 2 factorial design (5% vs. 20% O2), we demonstrate that oxygen tension during IVM and IVF affects blastocyst yield, redox balance, and epigenetic mark profiles as distinct, independently regulated outcomes. Maintaining a stable 20% O2 atmosphere (M20F20) maximized blastocyst yields (50.63 ± 4.55%) and was associated with the highest intracellular glutathione (GSH) levels and lowest reactive oxygen species (ROS), indicating superior antioxidant capacity. Conversely, shifting from high-oxygen IVM to low-oxygen IVF (M20F5) resulted in the lowest blastocyst rates, elevated ROS, and reduced GSH, suggesting that an abrupt oxygen transition imposes a compounded redox burden that exceeds the embryo's antioxidant buffering capacity. At the molecular level, repressive histone marks (H3K9me2/3) were higher in groups with consistent oxygen environments across IVM and IVF (M5F5 and M20F20) than in mismatched groups, regardless of the absolute oxygen level. Global DNA methylation (5 mC) was primarily driven by IVM oxygen tension, whereas 5-hydroxymethylcytosine (5hmC) responded to both IVM and IVF conditions. Notably, the M20F5 group exhibited elevated 5hmC levels and the poorest developmental outcomes, a pattern consistent with 'epigenetic stalling', in which TET-mediated demethylation may be initiated but fails to proceed to completion under redox-imbalanced conditions. We cross-validaed these oxygen-dependent epigenetic responses in a somatic bovine fibroblast model, where oxygen tension modulated KDM6A expression. Together, these findings demonstrate that oxygen tension during the IVM and IVF stages independently shapes embryonic redox balance and epigenetic mark profiles, outcomes that are not predicted by blastocyst yield alone, and that an abrupt downward oxygen transition at IVF represents a specific point of vulnerability for epigenetic fidelity.