How low can you go with oxygen concentration?.

The concentration of oxygen in the female reproductive tract gradually decreases from 5-7% in the fallopian tube to 2% in the uterus. A reduction in oxygen from atmospheric to physiologic levels improves cell number, decreases DNA fragmentation, and induces less oxidative stress on the blastocyst. However, studies investigating benefits of ultralow oxygen concentrations in vitro have shown inconsistent findings2,4–8. We sought to determine if culturing from single-cell stage embryos at various ultralow oxygen concentrations improved embryo development in the mouse model. Basic science Cryopreserved single-cell stage mouse embryos (B6C3F1 X B6D2F1 strain; Embryotech, Haverhill, MA) were thawed and randomly divided into 5 oxygen treatment arms: 0.7-0.8% (n=173), 1.5% (n=176), 2% (n=166), 3% (n=171), and 5% (n=171, control arm). Embryos were cultured to the blastocyst stage in groups of 6 in 25μl droplets of Continuous Single Culture Complete (CSCC, Irvine Scientific) in an incubator (Heracell 150i) at 8% CO2 and a pH of 7.3. Embryo development was evaluated by quantifying cell number over a 5-day period. Chi square test and logistic regressions were used to test associations where appropriate. A p-value of <0.05 was considered statistically significant. Mouse embryos cultured at ultralow oxygen concentrations of 1.5%, 2%, and 3% had similarly high rates of blastocyst development, whereas those cultured at 0.7-0.8% had the lowest rate of blastocyst development (Table). Compared to cleavage-stage embryos cultured at 5%, mouse embryos cultured at 2% and 3% oxygen concentrations were more likely to become blastocysts, (OR 4.1 and OR 3.7, p<0.05, respectively), and those cultured at 0.7-0.8% were least likely to develop into blastocysts (OR 0.2, 95% CI 0.12-0.34, p <0.001; Table). We demonstrated a unimodal distribution for optimal oxygen concentrations in vitro. Too low, 0.7-0.8%, resulted in poor blastulation, and the standard 5% did not improve embryo development. Oxygen concentrations at 2% and 3% resulted in optimal blastocyst development. These findings shed light on the potential benefit of ultralow oxygen concentration, particularly at 2-3%.TableMouse embryo development at ultralow oxygen concentrationsOxygen Tension0.7-0.8%1.50%2%3%5%Number of single-cell embryos on Day 1*173176166171171Number of cleavage* (%)171 (98.8)176 (100)156 (94.0)163 (95.3)170 (99.4)Number of blastocyst* (%)94 (54.3)159 (90.3)150 (90.4)156 (91.2)146 (85.4)Odds ratio of a cleavage stage mouse becoming a blastocyst0.20*1.54.1*3.7*1.0 (ref)Logistic regression used; *significance p<0.05 Open table in a new tab