Impacts of Oxygen Tension on Developmental Competence of Preimplantation Embryos.

Oxygen (O2) tension is a critical factor influencing in vitro development of pre-implantation embryos. The in vivo environment has lower O2 tension (2-10%) than atmospheric air (~20%), along the female reproductive tract, from the oviducts (8-10%) to the uterus (2-5%), supporting development of early-stage embryos. As the female reproductive tract is inherently hypoxic, replicating low-O2 conditions in vitro may enhance embryo development. In contrast, culturing embryos under non-physiological O2 tension may impair stress adaptation and reduce developmental competence. Optimal O2 tension likely varies with species and embryo stage, suggesting a single uniform O2 tension throughout in vitro culture may not be ideal; conditions beneficial at one stage may be detrimental at another. Although atmospheric O2 harms embryo development and redox balance, specific advantages of low (5%) or ultra-low (≤2%) O2 remain uncertain, despite many studies documenting improved development under hypoxia. This review examines the current literature on effects of atmospheric, low, and ultra-low O2 tension during in vitro embryo culture, emphasizing impacts on in vitro fertilization (IVF) outcomes, and the regulation of transcription and epigenomics during pre-implantation embryo development.