The effect of low and ultra-low oxygen tensions on mammalian embryo culture and development in experimental and clinical IVF

Over the last forty years, many trials have been performed using mammalian embryo cultures with reduced oxygen tension (O-2) to encourage proper embryo development and increase the success rate for in vitro fertilization (IVF) outcome. Even if the use of atmospheric O-2 (20%) affects in vitro embryo development and intracellular redox balance, the use of low (5% O-2, physiologic) and ultra-low (close or less to 5% O-2) O-2 applied to in vitro embryo culture is still under debate. Numerous studies in various mammalian species have shown that embryo development improves when culturing embryos under low O-2, although culture conditions are not the only factors involved in the success of IVF. This article reviews the literature data of the last four decades and discusses the current evidence on the use of low and ultra-low O-2 in embryo culture, and examines the impact of multiple factors on IVF outcomes.