Decidualization of endometrial stromal cells is a critical process for establishing uterine receptivity and successful embryo implantation, involving coordinated biochemical, cellular, and biomechanical remodeling. Emerging evidence indicates that this process is associated with activation of endoplasmic reticulum stress (ERS) and the development of distinct stromal subpopulations, including mature and senescent decidual cells. However, how these pathways integrate to regulate the mechanical properties of the endometrium remains unclear. Here, we investigated the impact of decidualization dysregulation on stromal cell nanomechanics and its contribution to implantation failure. Our first approach used an in vitro decidualization model of human endometrial stromal cells (cell line HESC), in which an excessive ERS was induced by thapsigargine. Then, decidualization, senescence and UPR were evaluated by molecular analyses, cellular nanomechanical properties by atomic force microscopy (AFM), and decidual functionality with a blastocyst-like spheroid implantation assay (using Swan71 cells). Decidualization induced a significant decrease in cellular stiffness (Young's modulus), consistent with a more permissive phenotype for embryo implantation. In contrast, exacerbated ERS -induced prior to decidualization- disrupted the balance between mature and senescent decidual cells and prevented this biomechanical softening, restoring stiffness to levels comparable to those on non-decidualized cells. Functionally, these alterations impaired trophoblast adhesion and expansion in vitro. Finally, to provide a physiologically relevant view of how decidualization operates in vivo, we studied endometrial samples from fertile women (N = 13) and from women with recurrent implantation failure (RIF, N = 11). These patients exhibited reduced expression of key decidualization and senescence markers evaluated by qPCR, similar to our results with exacerbated ERS, supporting the clinical relevance of our findings. Together, our results demonstrate that successful decidualization requires tight coordination between ERS, cellular senescence, and biomechanical remodeling. Disruption of this integrated program leads to a mechanically non-permissive endometrium, providing a novel mechanistic framework for implantation failure.
