Successful proliferation of ESP cells in the presence of EMP cells in conventional media suggests that EMP cells may provide a ����niche���� appropriate for activation of ESP cells. Moreover, successful proliferation of ESP cells alone in EGM2MV medium prompts us to postulate that EMP cells alone and/ or cell-to-cell interaction between ESP and EMP cells may produce bioactive substances such as EGF, VEGF, bFGF, and/or insulin-like growth factor-I that EGM-2MV medium contains. Microenvironments under the kidney capsule of NOG mice may also provide some but not sufficient ����niche����, which may be at least in part attributed to low efficiency of reconstitution of organized endometrial tissue from ESP cells. We have previously demonstrated that the single cell suspension without any cell selection which consisted of ESP cells and non-SP cells could reconstitute well-organized endometrial tissue at almost 100% frequency, which is much greater than the rate of ESP cells alone. These findings suggest that EMP cells are required as a ����niche���� provider for in vivo tissue reconstitution but EMP cells alone may not be able to generate the organized endometrial tissues in vivo. Third, ESP cells might not contain a sufficient number of stem/progenitor cells for Metamizole sodium hydrate generating glandular cells. In this study, we collected endometrial tissues by scraping strongly the Lafutidine uterine cavity with the back edge of a scalpel. A portion of the endometrium, particularly the deep basalis layer, however, penetrates into the myometrium where it will not be collected by scraping. Thus, it is possible that some of the ESP cells present in the deep basalis layer may not be included in the starting endometrial materials in this study. In this study, we have shown that at least some ESP cells were localized to the endometrial endothelial wall, predominantly expressed several EC markers, preferentially proliferated and differentiated in vitro in an EC-specific medium, and displayed high migratory and angiogenic potential. These results suggest that ESP cells have endometrial stem cell-like properties as well as EC or EPC-like characteristics.