Oxygen tension in IVM and IVF of bovine oocytes: effect on embryonic development and pregnancy rate

In order to evaluate the effect of O2 tension on the in vitro maturation (IVM) and fertilization (IVF) process, 11 replicates of OPU/IVP were performed using 1092 viable oocytes obtained from 48 Bos indicus embryo donors of commercial herds. Cumulus-oocyte complexes (COCs) from each donor were homogeneously and randomly distributed across two treatments. The IVM was performed in TCM-199 with ovine FSH, ovine LH, porcine estradiol, human EGF, porcine insulin, and FCS. The IVF was performed in Fert-TALP medium with heparin and PHE using Percoll–gradientselected spermatozoa from a Bos indicus bull. During IVM and IVF, the oocytes were incubated in 5% CO2 in air (Group 20% O2) or 5% CO2, 5% O2, and 90% N2 (Group 5% O2) for 18-24 h at 39C. The embryos were incubated in Synthetic Oviduct Fluid (SOF) with aminoacids and citrate at 39oC for 6-8 days with 5% CO2, 5% O2, and 90% N2. A number of COCs (n = 187) were evaluated until Day 9 when hatching rates were recorded. For the remaining embryos produced on Day 7 (n = 905), pregnancy rate was evaluated after the embryo transfer. There were no differences (P < 0.05) in the cleavage rate (69.6% and 70.4%), embryo development rate (37.3% and 38.4%), or Quality-I blastocyst percentage on Day 7 (30.8% and 30.4%) between the 5% O2 and 20% O2 groups, respectively. Considering the number of matured oocytes, the percentage of hatching blastocysts on Day 9 was greater in the 5% O2 group compared to the 20% O2 group (21.3% vs. 10.8%, respectively). The pregnancy rates were similar (P = 0.15) between 5% O2 (25.8%; 34/132) and 20% O2 (33.6%; 49/146) groups. The results of this study indicate the possibility of using a 5% oxygen tension during the entire IVP procedure for cattle.