Hypoxia Leads to Diminished Ovarian Reserve in an Age-Dependent Manner

Abstract

<b><i>Objectives:</i></b> Perinatal hypoxia causes premature activation and initiation of growth in dormant follicles, leading to diminished ovarian reserve. An indirect mechanism such as the release of stress-related hormones may influence ovarian follicle recruitment under hypoxic conditions. We wanted to determine whether hypoxic ovarian damage results from increased follicle growth and “burnout” or from increased apoptosis and whether this damage is age-dependent. <b><i>Design:</i></b> Animal study was conducted. <b><i>Participants/Materials, Setting, Methods:</i></b> Using adult 6-week-old (<i>n</i> = 8) and one-day-old newborn (<i>n</i> = 20) ICR (CD-1) female mice, ovarian follicular counts were conducted on H&amp;E-stained sections. <b><i>Methods:</i></b> Immunohistochemistry was performed on sections stained with Ki-67, anti-Caspase 3, and anti-FOXO3A. <b><i>Results:</i></b> Exposure to hypoxia resulted in significantly reduced proportion of primordial follicles versus normoxia in both adult dams and newborn pups (3.17 ± 2.75 vs. 17.89 ± 4.4%; <i>p</i> = 0.004; 40.59 ± 14.88 vs. 81.92 ± 31.56%, <i>p</i> = 0.001, respectively), concomitant with increased growing-primary and secondary follicles, and more pronounced in adult dams versus 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. <b><i>Limitations:</i></b> The current study has several limitations; first, the sample size for each group is relatively small, which could limit the generalizability of the findings. Second, the study used an ex vivo culture system, which may not fully capture the complex interactions that occur in the whole animal. Third, the exposure to hypoxia only lasted for 3 h, which may not be long enough to observe all the potential effects. In addition, the study only analyzed specific markers of apoptosis in a few cell types, and other cell types or apoptotic pathways might be involved. Lastly, the study provides evidence for accelerated follicular activation and decreased ovarian reserve, but the underlying mechanisms are not fully explored. <b><i>Conclusions:</i></b> Direct tissue hypoxia led to premature activation and initiation of growth in dormant follicles leading to diminished ovarian reserve. Hypoxic damage is age-dependent, with adult ovaries more susceptible than newborn ovaries. These findings support the possibility of follicular “burn out” as a potential mechanism responsible for hypoxia-induced loss of ovarian reserve.