Murine implantation chamber formation precedes natural and artificial decidualization

During pregnancy uterine stromal cells undergo a mesenchymal to epithelial cell transition termed decidualization. In humans initiation of decidualization occurs in the absence of an embryo resulting in a need to identify embryo-independent molecular cues that can initiate decidualization. Although, similar to humans, decidualization in the mouse can be induced in the absence of an embryo, whether an implantation event is prerequisite for such decidualization is not known. In this study using different models of estrogen-dependent implantation, including natural (embryo) and artificial (sesame oil, agarose only beads, and Concanavalin A coated agarose beads) we determined that implantation chamber formation precedes decidualization. We show that focal stimuli, including the embryo, Concanavalin A coated bead, and oil droplets, induce V-shaped implantation chambers that lead to sub-epithelial PTGS2 expression and decidualization. Unfertilized eggs and uncoated agarose blue beads fail to form an implantation chamber and do not initiate decidualization. Further, we show that lectins that share sugar binding properties with Concanavalin A can also induce a V-shaped implantation chamber. Finally, using second harmonic generation we show that during decidualization collagen fibers spread radially away from the implantation chamber irrespective of the focal signal used for inducing the chamber. Thus, in the mouse artificial decidualization also initiates at the site of implantation chamber formation. These findings are critical when separating physical stimulus-dependent, embryo-dependent and embryo-independent mechanisms of decidualization that underlie a successful pregnancy.