No studies have yet explored the in vivo regenerative capacity of these putative endometrial stem/progenitor cells. Candidate tissuespecific stem cells have been identified in several tissues based on the SP phenotype. This characteristic is due to the unique ability of the primitive cells to pump out the DNA binding dye Hoechst 33342 via the ATP-binding cassette transporter G2. Primitive hematopoietic precursors from bone marrow were the first SP cells identified with this technique. We recently demonstrated that SP cells isolated from the human uterine myometrium regenerate human myometrial tissues in vivo when xenotransplanted into the uteri of NOG mice. In the present study, we adapted our in vivo regeneration assay and SP isolation procedure to characterize the properties of human endometrial SP cells. These cells were able to differentiate into endometrium-like tissue and a variety of endometrial cell components when xenotransplanted into NOG mice. This is the first in vivo evidence in support of the existence of stem/progenitor cells in the ESP. Human and primate endometrium regenerates from the lower basalis layer, a germinal compartment that persists after menstruation to give rise to the new upper functionalis layer. The surface epithelium develops primarily through the proliferation of epithelial cells from the tips of the gland stumps. The findings presented here strongly support the idea that the basalis of the endometrium harbors stem/progenitor cells responsible for endometrial regeneration during menses as well as after parturition in both women and menstruating non-human primates. It Permethrin remains possible, however, that endometrial stem/progenitor cells also exist in the functionalis of the endometrium. Indeed, the ABCG2 + population, which presumably includes ESP cells having endometrial stem cell-like properties, is localized exclusively in the endothelium of both the functional and basal layers of the human endometrium.A Sibutramine HCl relatively small number of dispersed human endometrial cells containing ESP cells can generate functional endometrial tissue comprising glands, stroma, immune cells and vascular components when they are transplanted under the kidney capsule of severely immunodeficient mice.