Endometrial cells with high ALDH activity contribute to uterine development and regeneration

Adult stem cells are thought to drive the regenerative potential of the endometrium and contribute to the pathogenesis of endometriosis; however, their identity and defining features remain to be characterized. Here, we used in vivo and in vitro approaches to demonstrate that cells with high aldehyde dehydrogenase 1 activity (ALDH^HI cells) were long-lived progenitors in the endometrium with a higher organoid formation capacity, long-term passaging potential, and stemness gene signatures. Using lineage tracing with an Aldh1a1^creERT2/+; Rosa26^LSL-tdTomato reporter mouse, Aldh1a1⁺ epithelial cells expanded during postnatal development, Aldh1a1⁺ stromal cells expanded during estrous cycling, and both populations of Aldh1a1⁺ cells were present during postpartum repair. In response to ovariectomy or exogenous estradiol, we found that ALDH1A1⁺ cells localized to glandular crypts of the endometrium or throughout the luminal epithelium, respectively, indicating that their spatial localization is hormone-sensitive. Functionally, we found that selective ablation of ALDH1A1⁺ cells in Aldh1a1^creERT2/+; Rosa26^LSL-DTR mice decreased endometrial gland number and FOXA2 expression. These findings were recapitulated in the human endometrium, where endometrial epithelial organoids with high ALDH activity (ALDH^HI cells) showed a higher organoid formation capacity than ALDH^LO cells and displayed unique transcriptomes with fewer luminal-like ciliated cells. Overall, our studies indicate that ALDH1A1⁺ cells are hormone-sensitive adult stem cells in the endometrium with regenerative potential that are critical for endometrial development and function.