Characterization of Human Mammary Epithelial Stem Cells

Previous studies have demonstrated that normal mammary tissue contains a hierarchy of cell types that are ultimately and continuously derived from a self-sustaining mammary stem cell population. However, a suitable assay for detecting, quantifying and characterizing such cells in humans has been lacking. Here we show that histologically normal, bi-layered mammary epithelial structures containing the complete repertoire of luminal and myoepithelial cells as well as their in vitro clonogenic progenitors are regenerated in a quantitative fashion in human mammary stem cell-containing collagen gels implanted for 4-6 weeks under the kidney capsule of hormone-treated immunodeficient mice. Using this assay, we show that these human mammary stem cells have a CD49fEpCAM phenotype and functional self-renewal potential. The development and validation of this assay sets the stage for investigations of normal human mammary stem cell regulation and their relationship to human breast cancer stem cells.