Role of Stro1⁺/CD44⁺ stem cells in myometrial physiology and uterine remodeling during pregnancy

Regulation ofmyometrial functions during pregnancy has been considered the result of the integration of endocrine andmechanical signals. Nevertheless, uterine regeneration is poorly understood, and the cellular source within the gravid uterus is largely unexplored. In this study,we isolated and quantified themyometrial stem cells (MSC) population from pregnant female Eker rat uteri, by using Stro1⁺/CD44⁺ surface markers. We demonstrated that prior parity significantly increased the percentage of Stro1⁺+/CD44⁺+ MSC because of injured tissue response. Interestingly, we established that Stro1⁺+/CD44⁺+ MSCrespond efficiently to physiological cues when they were treated in vitro under different dose-dependent pregnant rat serum. Previous studies reveal strong regulatory links between O₂ availability and stem cell function. Based on these premises, cell proliferation assays showed that isolated Stro1⁺+/CD44⁺+ MSC possess a higher proliferative rate under hypoxic versus normoxic conditions. We also detected a total of 37 upregulated and 44 downregulated hypoxia-related genes, which were differentially expressed in Stro1⁺+/CD44⁺+ MSC, providing an alternative approach to infer into complexmolecular mechanisms such as energy metabolism, inflammatory response, uterine expansion, and/or remodeling. Since these cells preferentially grow under low oxygen conditions, we propose that the increase of the rat uterus during pregnancy involves myometrial oxygen consumption, thereby enhancing MSC proliferation. Moreover, pregnancy-induced mechanical stretching results in hypoxic conditions, ultimately creating an environment that promotes stem cell proliferation and further uterine enlargement, which is essential for a successful pregnancy. In summary, all of these data support that rat Stro1⁺+/CD44⁺+ MSC contribute to uterine enlargement during pregnancy.