Optimization of oxygen concentration for growing bovine oocytes in vitro: constant low and high oxygen concentrations compromise the yield of fully grown oocytes.

The oxygen environment in cell culture has a significant impact on the health and performance of cells. Here, we compared the effects of reduced (5%) and ambient (20%) oxygen concentrations on bovine oocyte-granulosa cell complexes, each containing a growing oocyte 90-102 µm in diameter, cultured for 14 days. Both oxygen concentrations showed some advantages and disadvantages; in 5% oxygen, the survival rate of oocytes was significantly higher than in 20% oxygen, but the resulting oocytes were significantly smaller, which was a serious disadvantage. During the first 4 days of culture, the growth and viability of oocytes were satisfactory using 5% oxygen. This observation led us to examine the effect of changing the oxygen concentration from 5% to 20% on Day 4 in order to minimize the expected disadvantages of constant 5% and 20% oxygen. The largest population of fully grown oocytes was obtained from cultures in which the oxygen concentration was changed in this way, which also led to higher oocyte viability than in constant 20% oxygen. A similar tendency was found in the frequency of oocytes becoming blastocysts after in vitro fertilization. Surviving oocytes eventually became located within an enlarged dome-like structure, and although the 5% oxygen environment may have been appropriate for oocyte growth in the early stages, 20% oxygen may have been necessary for the growth of oocytes in the dome-like structure. These results indicate an effective way of modulating oxygen concentration according to the growth of oocyte-granulosa cell complexes in vitro.