Characterization of side-population cells in human normal endometrium

Background: It has been proposed that the human endometrium may contain a population of adult stem cells that are responsible for its remarkable regenerative capability. Recently, a subset of stem cells or progenitor cells in adult tissue has been identified as side-population cells (SP cells) displaying low staining with Hoechst 33342 by fluorescence-activated cell sorter (FACS) analysis. In this study, we isolated SP cells from the human endometrium and analysed their properties. Method: Endometrial cells were obtained using enzymatic digestion from uterine hysterectomy for the treatment of uterine myoma and stained with Hoechst 33342 dye either alone or in combination with verapamil. The cells were then analysed using FACS. Results: SP cells were present among normal human endometrial cells. Most SP cells were enriched in the CD9^-CD13^- fraction. These SP cells showed long-term repopulating properties and produced gland (CD9⁺)- and stroma (CD13⁺)-like cells. CD9^-CD13^- cells isolated from the endometrium also generated gland- or stroma-like cells. Conclusions: SP cells in the human endometrium can function as progenitor cells. This is the first report of the phenotype of SP cells from normal human endometrial cells.