Endometrial gland secretome maintains the homeostasis of decidualization

Decidualization of human endometrial stromal cells (hESCs) during the secretory phase is critical for implantation and pregnancy. In this study, we assessed 25 established in vitro decidualization markers using in vivo single-cell transcriptomic data and found that the expression levels of most of them, including PRL and IGFBP1, were comparable between the proliferative- and secretory-phase hESCs or between the decidualized and non-decidualized hESCs. We hypothesized that the lack of endometrial glands in vitro causes the discrepancy. However, the role of the endometrial glands in decidualization remains unclear. Using a co-culture model, we demonstrated that the secretome of human endometrial organoids attenuates decidualization by reducing canonical marker upregulation and hESC mesenchymal-epithelial transition. Transcriptomic analysis revealed the downregulation of key decidualization regulators CEBPA and EDN1. Consistent with the in vitro findings, an estradiol-stimulated mouse model with increased glandular formation showed reduced decidualization. Collectively, our results identify a novel homeostatic function of endometrial gland secretome in decidualization regulation.