Main actors behind the endometrial receptivity and successful implantation

Cells release into the extracellular environment diverse types of membrane vesicles of endosomal and plasma membrane origin called exosomes and microvesicles, respectively. These extracellular vesicles (EVs) represent an important mode of intercellular communication by serving as vehicles for transfer between cells of membrane and cytosolic proteins, lipids, and RNA. Deficiencies in our knowledge of the molecular mechanisms for EV formation and lack of methods to interfere with the packaging of cargo or with vesicle release, however, still hamper identification of their physiological relevance in vivo. In this review, we focus on the characterization of EVs and on currently proposed mechanisms for their formation, targeting, and function.

T-helper (Th) cells play a central role in modulating immune responses. The Th1/Th2 paradigm has now developed into the new Th1/Th2/Th17 paradigm. In addition to effector cells, Th cells are regulated by regulatory T (Treg) cells. Their capacity to produce cytokines is suppressed by immunoregulatory cytokines such as transforming growth factor (TGF)-beta and interleukin (IL)-10 or by cell-to-cell interaction. Here, we will review the immunological environment in normal pregnancy and complicated pregnancy, such as implantation failure, abortion, preterm labor, and preeclampsia from the viewpoint of the new Th1/Th2/Th17 and Treg paradigms.

In humans, the endometrium, the uterine mucosal lining, undergoes dynamic changes throughout the menstrual cycle and pregnancy. Despite the importance of the endometrium as the site of implantation and nutritional support for the conceptus, there are no long-term culture systems that recapitulate endometrial function in vitro. We adapted conditions used to establish human adult stem cell-derived organoid cultures to generate 3D cultures of normal and decidualised human endometrium. These organoids expand long-term, are genetically stable and differentiate following treatment with reproductive hormones. Single cells from both endometrium and decidua can generate a fully functional organoid. Transcript analysis confirmed great similarity between organoids and the primary tissue of origin. On exposure to pregnancy signals, endometrial organoids develop characteristics of early pregnancy. We also derived organoids from malignant endometrium, and so provide a foundation to study common diseases, such as endometriosis and endometrial cancer, as well as the physiology of early gestation.