O-046 Implantation and beyond, the endometrial perspective

text Early pregnancy failures are spread throughout the first few weeks, suggesting that impairment of normal biological processes can occur at sequential stages: epithelial attachment and breaching, early stromal and then decidual invasion, glandular and vascular invasion. Biopsy-based transcriptomics and proteomics have failed to demonstrate a reproducible or interpretable molecular signature for endometrial receptivity, but single cell RNAseq suggests a step change in the epithelial transcriptome in the mid secretory phase, consistent with the appearance of new cell phenotype(s). Previously unseen heterogeneity in both epithelial and stromal cell populations has become evident, but gene signatures have not yet achieved a level of resolution that allows insights clear enough to decode the biology of the receptive state. However, methodology for propagating and recombining endometrial cell populations into 3D engineered tissue models has advanced, so that new mechanistic questions can be asked. Initial acquisition of receptivity to implantation is followed by the development of a supportive environment for embryos that possess the capacity to progress, with maternal cell populations (epithelial, stromal, immune and vascular) acting cooperatively within a remodelling extracellular matrix (ECM). A balance must be achieved in the ECM between breakdown, with opening of hydrated spaces to allow expansion of the embryonic sac while maintaining a substrate for stable physical attachment to allow invasion by extravillous trophoblast. The extent and nature of cellular trafficking to and from the uterus is important before and during early pregnancy. There is evidence that regulation of a resident senescent cell subpopulation by uterine NK cells occurs in concert with ECM remodelling to achieve a functionally supportive environment in the early first trimester. Access of bone marrow-derived cells to vessel walls is regulated by placental-endothelial signalling to initiate remodelling for the increased blood flow to the conceptus that is required in later pregnancy. Thus ‘receptivity’ and ‘supportiveness’ require normal cell proportions and functional phenotypes in multiple endometrial cell populations and their physical environment in order to allow a well-calibrated sequential developmental response in the conceptus.