Does the oxygen concentration in the culture medium [either physiologic (5%) or atmospheric (20%)] affect mitochondrial ultrastructure and function in preimplantation mouse embryos generated by IVF? SUMMARY ANSWER: Embryos cultured in 20% oxygen show increased mitochondrial abnormalities compared to embryos cultured in 5% oxygen.WHAT IS KNOWN ALREADY: ART are widely used and have resulted in the birth of more than 8 million children. A variety of media and oxygen concentrations are used to culture embryos. Embryos cultured under physiological O 2 tension (5%) reach the blastocyst stage faster and have fewer alterations in gene expression when compared with embryos cultured under atmospheric oxygen conditions (20%). The mechanisms by which oxygen tension affects preimplantation development remain unclear, but mitochondria are believed to play an important role. The aim of this study was to evaluate how mitochondrial ultrastructure and function in IVF embryos were affected by culture under physiologic (5%) or atmospheric (20%) oxygen concentrations.STUDY DESIGN, SIZE, DURATION: Zygotes, 2-cell, 4-cell, morula and blastocyst were flushed out of the uterus after natural fertilization and used as controls. IVF was performed in CF1 x B6D2F1 mice and embryos were cultured in Potassium simplex optimized medium (KSOM) with amino acids (KAA) under 5% and 20% O 2 until the blastocyst stage. Embryo development with the addition of antioxidants was also tested.PARTICIPANTS/MATERIALS, SETTING, METHODS: Mitochondrial function was assessed by measuring mitochondrial membrane potential, reactive oxygen species (ROS) production, ATP levels, and the expression of selected genes involved in mitochondrial function. Mitochondria ultrastructure was evaluated by transmission electron microscopy (TEM).MAIN RESULTS AND THE ROLE OF CHANCE: Embryos cultured under 20% O 2 had fewer mitochondria and more vacuoles and hooded (abnormal) mitochondria compared to the other groups (P < 0.05). At the blastocyst stage the mitochondria of IVF embryos cultured in 20% O 2 had lower mtDNA copy number, a denser matrix and more lamellar cristae than controls. Overall IVF-generated blastocysts had lower mitochondrial membrane potential, higher ROS levels, together with changes in the expression of selected mitochondrial genes (P < † Co-first authors.

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