Comparison of two systems of in vitro cattle embryo production: reduced oxygen versus atmospheric oxygen content

Currently, two culture systems for in vitro embryo production (IVP) of mammals are used. Each of these systems has own pluses and minuses. Culture at atmospheric oxygen level (20 %) requires conventional CO2 incubator and co-culture with a feeder cells, which involves procedures for maintaining these cells in culture. Culture at reduced oxygen level (5 %) requires special incubating system making IVP procedures more expensive than using conventional technique. The aim of this study was to examine, which of these culture systems can provide higher blastocyst yield after in vitro fertilization of fresh or vitrified oocytes. Bovine oocytes derived from the ovaries of slaughterhoused cows were matured in vitro, then part of them were vitrified by an ultra-rapid vitrification technique. After warming, vitrified (n= 482) and fresh oocytes (n= 558; control) were fertilized in vitro using frozen bull semen. Following 20 hours, presumptive zygotes, developed from both vitrified and fresh oocytes, were divided into two groups and incubated either under atmospheric oxygen (AOC; 20 % O2) in a conventional SANYO CO2 incubator (MCO-15AC) at 38.5 °C and 5 % CO2, or under reduced oxygen content (ROC; 5 % O2) in a MIRI multiroom incubator (ESCO Medical) at 38.5 °C and 6.1 % CO2 until day 8. Experiments were performed in 5 replicates. Zygotes, obtained from fresh oocytes, reached higher cleavage (76.72 %) and blastocyst (39.34 %) rates, when cultured in ROC compared to AOC (65.6 % and 29.25 %, resp.). Similarly, in vitrified oocytes, cleavage (71.9 %) and blastocyst (22.8 %) rates were significantly higher (p < 0.05) in ROC than in AOC (56.2 % and 14.0 %, resp.). Embryo cell number also was significantly higher in blastocysts of both fresh (165.5 ± 8.9) and vitrified (154.54 ± 7.8) groups cultured under ROC, when compared to the AOC level (108.67 ± 3.6 and 98.04 ± 5.82, resp.). In conclusion, the culture system with a reduced oxygen showed to be more efficient for bovine IVP than the system with an atmospheric oxygen in a conventional CO2 incubator. This finding is important when in vitro produced embryos are derived from cryopreserved oocytes.