Wang, K.; Wolf, S.; Zelinski, M.; Krieg, A.

In vitro culture of cryopreserved ovarian follicles has the potential to extend fertility options for young women seeking to preserve ovarian tissue prior to undergoing cancer treatments. Successful implementation of this strategy has been elusive and likely requires a more complete understanding of the microenvironment of the developing ovarian follicles, including ovarian oxygen concentrations. The oxygen tension within the reproductive tract plays a crucial role in follicular development and oocyte maturation. While in vitro culture systems often use atmospheric oxygen (21%), the native environment in vivo is significantly lower, ranging from 1.5% to 8.7%. This study aimed to investigate the effects of reduced oxygen tensions (3% and 5%) on follicle survival, growth, antrum formation, and hormone production in cultured secondary follicles from rhesus macaques (macaca mulata). A total of 300 follicles were isolated from 7 animals and cultured under three oxygen conditions: 3%, 5%, and 21% O2. Follicle survival and antrum formation were assessed weekly by microscopy and Kaplan-Meier survival analysis, while growth dynamics and hormone levels (estradiol, progesterone, AMH, and inhibin B) were monitored throughout the culture period. Results demonstrated that follicles cultured at 3% and 5% oxygen exhibited significantly higher survival rates and antrum formation compared to those cultured at 21% O2. No significant differences in survival were observed between the 3% and 5% oxygen groups. Growth dynamics revealed distinct patterns, with both low oxygen groups promoting more robust and sustained follicle growth, while atmospheric oxygen led to rapid degeneration. Hormonal analysis showed that follicles in 21% O2 had elevated early hormone production but exhibited reduced long-term viability. In contrast, 3% and 5% oxygen delayed hormone production, reflecting a more stable and sustained follicular environment. These findings underscore the importance of low oxygen tensions in mimicking the physiological conditions of the reproductive tract, improving follicular development, and supporting optimal hormonal function in vitro. This study suggests that further reducing oxygen levels to 3% may offer additional advantages for long-term follicle viability and function in reproductive technologies.