The unmasking of novel unipotent stem cells in the mammary gland

Mammary stem cells (MaSCs) have been prospectively isolated and shown to reconstitute a fully functional mammary gland in the mouse at the single cell level. These cells are self-renewing and can differentiate into all mature mammary epithelial cell types, thus fulfilling the defining characteristics of a stem cell. In a recent issue of Nature, Blanpain and colleagues (Van Keymeulen et al, 2011) add a new layer of complexity to the prevailing model of the mammary epithelial hierarchy through the recognition of novel unipotent stem cells that appear to maintain normal homeostasis of postnatal mammary epithelium. These data point to the existence of an unexpectedly complex hierarchy of stem cells that include multipotent as well as more restricted stem cells.