Side population cells contribute to the genesis of human endometrium.

Objective To investigate whether stem (progenitor) cells are found in human endometrial side population cells. Design Experimental laboratory study. Setting University-based laboratory in Japan. Patient(s) Normal endometrial tissue samples from 42 patients. Intervention(s) Side population cell analysis and staining were performed by using Hoechst 33342, CD31, CD34, EMA, CD105, CD146, and BCRP1/ABCG2. Main Outcome Measure(s) Human endometrial side population cells were isolated and characterized by fluorescence-activated cell sorter analysis. Stem cell activity was evaluated by colony-forming assays and cell cycle analysis. Result(s) Endometrial side population cells expressed not only the endothelial cell markers CD31 and CD34 and the epithelial cell marker EMA but also mesenchymal stem cell markers CD105 and CD146. Immunohistochemical analysis revealed that BCRP1/ABCG2, known as a marker of side population cells, was strongly expressed in the vascular endothelium and the epithelium of the basal layer of the endometrium. In cell cycle analysis, side population cells isolated directly from tissue were mainly in G0, whereas side population cells sorted after primary culture included populations in G1 and G2/M/S. These sorted side population cells showed greater colony-forming efficiency than non-side population cells and secreted PRL in an in vitro decidualization model. Conclusion(s) Human endometrial side population cells may include putative stem or progenitor cells.