Mesenchymal stem-like cells expressing both CD146 and PDGF-Rb are located perivascularly in the Sulfameter functionalis and basalis layers of the human endometrium. Very recently, Garry et al. demonstrated that endometrial surface epithelial regeneration takes place as a consequence of cellular differentiation from stromal cells and not by Ofloxacin direct extension from the residual basal epithelial glands. These findings collectively suggest that ESP cells present in the vascular endothelium are one of the most likely candidates for endometrial stem/progenitor cells that may reside not only in the basalis but also in the functionalis endometrium. The percentage of SP cells derived from cultured epithelialenriched fraction was significantly greater during menstruation than at any other cycle stage, whereas the rate of SP cells freshly isolated from endometrial samples was significantly greater in the proliferative phase than in the secretory phase. In this study we showed that the proportion of ESP cells to the EMP + ERP fraction freshly isolated from human endometria was the highest at the early proliferative phase among all the phases of the menstrual cycle, which is in agreement with previous report, and it decreased gradually until its nadir in the late secretory phase. Given the stem cell-like properties of ESP cells as presented here and reported elsewhere, it is tempting to speculate that ESP cells may generate committed progenitor cells through asymmetrical cell division, which, in turn, may lose the SP phenotype, further behave as transient amplifying cells, and eventually propagate through symmetrical cell division. In this context, the gross number of ESP cells may be almost unchanged, but the number of non-SP cells present particularly in the functionalis endometrium may increase, presumably resulting in the decline in the proportion of ESP cells from menstruation towards the late secretory phase. There are several explanations for the low efficiency of reconstitution. First, the local environment at the xenotransplantation site may lack necessary factors for the regeneration of the entire endometrium. Second, ESP cells may require a specific ����niche���� provided by other endometrial cell components to reconstitute the entire endometrium in vivo as well as in vitro culture.