Oxygen concentration during in vitro culture has a significant effect on the physiology of embryos, altering metabolic profile and developmental outcome. Although atmospheric oxygen has been used routinely for the culture of ovarian follicles, oxygen concentration may also be critical for follicle growth but the optimal concentration has not been determined. In this study, mechanically isolated primary and secondary follicles (80-140 µm diameter) from adult mouse ovaries were cultured in serum-free conditions for 8 days in either 5 or 20% oxygen to determine growth (follicular diameter), morphology and viability. For each oxygen concentration, half of the medium was replaced on Days 2, 4 and 6 or on Day 4 only. In the latter group, metabolic analysis of spent follicular culture media was performed by (1)H-NMR. The proportion of viable, growing follicles was significantly (P < 0.0001) higher in 5% than in 20% oxygen (59% versus 8%). Reducing the frequency of medium replacement during culture in 5% oxygen resulted in significantly (P < 0.001) more viable follicles (79 versus 46%). In 20% oxygen, poor follicular viability was observed irrespective of the frequency of medium replacement (8 and 10% respectively). Metabolic profiles showed marked differences in amino acid and carbohydrate utilization with respect to both oxygen concentration and between Days 4 and 8 of development. Metabolites which significantly discriminated between oxygen concentration at both time points were glucose consumption, lactate utilization, alanine, alanyl-glutamine, leucine and proline. In conclusion, the poor in vitro follicular development previously observed in minimal culture conditions may reflect the use of 20% oxygen. Frequent medium replenishment is not necessary and does not overcome the detrimental effect of high oxygen on follicle viability. Further optimization of culture conditions would benefit from metabolic analyses and the use of 5% oxygen should be tested further for impact on functional aspects of follicle culture such as steroid production which is currently unknown.
Keywords: follicles; granulosa cells; metabolism; murine; oocyte; oxygen.