Hypoxia leads to diminished ovarian reserve in an age dependent manner
Authors: Gutzeit Ola , Bachar Gal , Nebenzahl-Sharon Keren , Weiner Zeev , Beloosesky Ron , Fainaru Ofer 
Published: April 2023

Objective
In a previous study, we demonstrated that perinatal hypoxia leads to premature activation and initiation of growth in dormant follicles, leading to diminished ovarian reserve. Other reports have also established a connection between intrauterine deprivation, premature delivery, small for gestational age newborns, and decreased ovarian reserve, attributed to premature recruitment and depletion of ovarian follicles. However, it cannot be dismissed that an indirect mechanism, such as the release of stress-related hormones, may influence ovarian follicle recruitment under hypoxic conditions. Therefore, we conducted this study to determine whether hypoxic ovarian damage results from increased follicle growth and "burnout" or from increased apoptosis and whether this damage is dependent on age.
Subjects
Animal studies were carried out using adult 6-week-old (n = 8) and one-day-old newborn (n = 20) ICR (CD-1) female mice. The mice were sacrificed, and ovaries harvested and immediately cultured in Leibovitz media supplemented with L-Glutamine and 10% Fetal Bovine Serum. Ovarian tissue from both dams and pups was subjected to hypoxic (1% O2 and 99% N2)- or normoxic (21% O2 and 5% CO2) conditions and cultured for three hours at 37oC. The tissues were subsequently fixed in 4% formaldehyde for further processing and analyses. Follicular counts were conducted on H&E-stained sections, while immunohistochemistry was performed on sections that were stained with Ki-67 (cell proliferation marker), anti-Caspase 3 and anti-FOXO3A (apoptosis markers).
Results
Exposure to hypoxia resulted in a significant reduction in the proportion of primordial follicles out of the total follicular pool as compared to normoxia in both adult dams and newborn pups (3.17 ± 2.75% vs. 17.89 ± 4.4%; p = 0.004 and 40.59 ± 14.88% vs. 81.92 ± 31.56%, p = 0.001, respectively). This decrease was concomitant with an increase in the proportion of growing- primary and secondary follicles. Notably, the impact was strikingly more pronounced in adult dams than in newborn pups (6-fold vs. 2-fold, respectively). Ki67 staining revealed higher scores of cell proliferation in follicular granulosa cells after exposure to hypoxia than normoxia. However, Caspase 3 and Foxo3A staining did not show any differences in these markers of apoptosis in oocytes, granulosa cells, theca cells, or stromal cells when exposed to hypoxia versus normoxia.
Conclusion
Our study demonstrates that direct tissue hypoxia leads to the premature activation and initiation of growth in dormant follicles leading to diminished ovarian reserve. This effect is associated with increased granulosa cell proliferation without concomitant changes in apoptosis. Hypoxic damage is age dependent, with adult ovaries exhibiting a more pronounced susceptibility than newborn ovaries. Collectively, these findings support the possibility of follicular "burn out" as a potential mechanism responsible for hypoxia-induced loss of ovarian reserve.