Characterization of clonal and regenerative perivascular stem cells in human endometrium

Decidualization denotes the transformation of endometrial stromal cells into specialised secretory decidual cells, a process indispensable for pregnancy. Decidualization of the human endometrium is not dependent on an implanting embryo but initiated during the mid-luteal phase of the cycle by elevated progesterone levels and local paracrine signals. Consequently, decidualization is a reiterative process directly linked to menstrual repair and rapid oestrogen-dependent growth. The extraordinary regenerative ability of the endometrium depends on endometrial mesenchymal stem cells (eMSCs) with inexhaustible self-renewing and differentiation capacity. Cyclic regeneration and rapid proliferation also render the stroma intrinsically heterogeneous, harbouring not only eMSCs but also endometrial transit amplifying (eTAs), mature, and senescent fibroblast subpopulations. Several lines of evidence presented in this thesis demonstrated that imbalance in these subpopulations is associated with reproductive failure. Quantification of clonal (eMSCs/eTAs) cells in mid-luteal biopsies obtained in consecutive cycles revealed increased levels in the 2nd biopsy obtained from miscarriage but not infertile patients, indicating that the tissue response to injury (i.e. the 1st biopsy) differs between patient groups. Further, in-depth characterization of primary stromal cell cultures prior to in vitro fertilisation (IVF) treatment showed that disordered temporal changes in the secretome of decidualizing cultures are associated with subsequent implantation failure. Additional characterization of perivascular eMSCs, which drive endometrial regeneration, highlighted the unique properties of these cells in terms of gene expression, metabolism, clonogenic and angiogenic potential. Importantly, eMSCs also formed 3D structures that resemble the uterine mucosa when cultured in Matrigel. These novel organoids termed endometrial regenerative bodies (ERBs), epithelialize when co-cultured and decidualize in response to differentiation cues. In sum, I provided evidence that dyshomeostasis between stromal subpopulations, which may be caused by eMSCs deficiency or dysfunction, precedes reproductive failure. Further, the ability of eMSCs to form ERBs provides a powerful new tool to study physiological and pathological implantation events in vitro.