Mechanical Cues Regulate Estrogen and Progesterone‐Induced Nascent <scp>ECM</scp> Deposition by Human Endometrial Stromal Fibroblasts

The endometrium, the mucosal lining of the uterus, is a highly regenerative tissue that undergoes cyclic remodeling guided by tightly regulated levels of estrogen and progesterone. Stromal cells, including fibroblasts, are embedded within the connective tissue of the endometrium and contribute to the rapidly changing extracellular matrix (ECM). During the secretory phase, high levels of progesterone induce decidualization of endometrial fibroblasts, which changes their morphology and protein secretion. While it has been shown that the mechanical properties of endometrial tissue, such as the elastic modulus, also contribute to tissue homeostasis and pathology, the interplay between hormones and tissue modulus in contributing to ECM remodeling remains unknown. To address this, we used hydrogels of varying elastic moduli (5 and 15 kPa) to induce decidualization of endometrial fibroblasts. Using metabolic labeling of glycosylated nascent ECM proteins, we then visualized and measured the deposition of newly secreted (nascent) ECM proteins during decidualization. In addition, we designed an automated ImageJ-based workflow for unbiased quantification of nascent ECM deposition. Our results demonstrate that both 5 and 15 kPa hydrogels support decidualization of endometrial stromal fibroblasts as shown by an increase in cell flattening and prolactin secretion. While increased hydrogel modulus alone enhances nascent ECM deposition, decidualization produces an additional increase that converges to similar levels regardless of the initial hydrogel modulus. Collectively, these findings demonstrate that endometrial stromal fibroblasts deposit nascent ECM that is enhanced during decidualization. These observations may provide new insights toward future studies addressing the mechanisms of ECM remodeling in endometrial tissue.