Evaluation of the Impact of Reduced Oxygen Concentration on Embryonic Development

Significant progress has been made in characterizing the optimal environment for a developing embryo in culture. These efforts have been based on the premise that clinical embryo culture should mimic the in vivo environment. To this end, investigators have gone to great lengths to recreate every aspect of the natural setting to which the early embryo is exposed. This focused approach has led to significant modifications of the embryo culture system in the modern in vitro fertilization (IVF) lab and ultimately to improvements in pregnancy rates.

One area that has been subject to significant scrutiny is the relationship between incubator oxygen concentration and early embryonic development. Oxygen plays a central role in embryonic metabolism. The mechanism governing its utilization is dependent on the stage of embryonic development. During the first 3 days of development, oxygen reaches the embryo via passive diffusion and its concentration gradient is regulated by oxygen consumption during oxidative phosphorylation. Inefficiencies in this process - due to compromised integrity of the inner mitochondrial membrane or alterations in substrate availability - can result in excessive production of harmful reactive oxygen species which can cause significant damage to cellular machinery and ultimately lead to embryonic arrest.

The concentration of oxygen that the embryo in culture is exposed to can also impact this delicately balanced system and alter the metabolic health of an embryo. Historically, atmospheric oxygen concentration (approximately 20%) was exclusively used in human IVF laboratories for embryo culture. However, multiple investigations subsequently found that the physiologic concentration of oxygen within the female reproductive tract is well below atmospheric levels, being consistently measured at <10%. These observations led to multiple trials comparing atmospheric oxygen concentrations to 5% oxygen in embryo culture. These studies demonstrated significant perturbations in gene expression, protein secretion, and suboptimal utilization of amino acids and carbohydrates in embryos cultured in atmospheric oxygen. The same comparisons were made in clinical IVF studies and demonstrated that embryos cultured in 5% oxygen consistently resulted in an increase in clinical pregnancy rate and live birth rate. A meta-analysis of this topic suggested that a clinic with a baseline live birth rate of 30% could expect an improvement as great as 13% when culturing embryos at 5% O2.

As a result of these compelling data, most modern IVF programs now exclusively culture embryos at 5% oxygen concentration. However, some have proposed that the oxygen concentration to which the embryo is exposed after day 3 of development is actually lower than 5%. These data originate from the idea that the embryo crosses the utero-tubal junction on day 3 of development in vivo. Multiple studies have demonstrated that the oxygen concentration in the uterus is actually lower than that in the fallopian tube at approximately 2%. Thus, the most physiologic embryo culture system would culture embryos in 5% oxygen until day 3 and then decrease the oxygen concentration to 2% until transfer or cryopreservation on day 5 or 6.

A change in the optimal oxygen concentration for an embryo on day 3 would fit with a general shift in metabolic requirements of embryos seen at this stage of development. Activation of the embryonic genome occurs on day 3 which prompts a significant increase in biosynthetic activity. The metabolic behavior of embryos also shifts substantially during this time. The embryo changes its metabolic strategy from oxidative phosphorylation to glucose based metabolism in the form of the aerobic glycolysis and the citric acid cycle. During this process, termed compaction, embryos exhibit greatly increased oxygen consumption.