11. Possible compensatory mechanisms of follicle protection in the retained ovary after unilateral oophorectomy in mouse models

Background In women who undergo unilateral oophorectomy (UO), some studies have shown that spontaneous pregnancy rates are similar to those who still have both ovaries intact. Furthermore, women who underwent UO entered menopause approximately 1 year earlier than those who have both ovaries, suggesting a compensatory mechanism. Since these effects are lower than what would be expected from the loss of half of the ovarian reserve, the aim of this study was to investigate possible compensatory mechanisms of follicle protection in the retained ovary after UO in mouse models. Methods 4–5 week-old B6 albino female mice underwent UO (n = 5) or sham surgery (n = 5) and were sacrificed at 10 weeks old. At time of sacrifice, the remaining left ovary was removed from the mice that underwent UO (Ooph) and both the right and left ovaries were removed from the mice that underwent sham surgery (Sham - Left Ovary, Sham - Right Ovary). Ovary(ies) were collected for gross and histological analysis. Grossly, ovary diameters and weights were measured and averaged. Histologically, ovarian follicles were counted, classified, and averaged in evenly spaced sections throughout the ovary. Growing follicles were defined as any follicle larger than a primordial. Statistical significance was determined using unpaired t-tests. Results Ovary diameters, weights, and follicles were analyzed for 15 mice. The remaining ovary in the mice that underwent UO weighed significantly more than the bilateral ovaries that underwent sham surgery (p = 0.001) (Figure 1a). Additionally, there was a significantly higher number of primordial follicles (PMFs) in the oophorectomy group compared to the sham surgery group (p = 0.02) (Figure 1b). While the number of growing follicles and the PMF:Growing ratio was greater in the oophorectomy group, it was not statistically significant (Figures 1c, 1d). Conclusions These experiments have demonstrated the existence of differences in size and histology in the remaining ovary in mice post-oophorectomy compared to the bilateral ovaries post-sham surgery. Further studies are necessary to discern the ovarian morphological differences leading to the size discrepancy as well as to determine the mechanisms leading to increased numbers of PMFs post-oophorectomy. RNA sequencing experiments comparing the remaining ovary post-oophorectomy and the bilateral ovaries post-sham surgery are currently in progress.