Murine mammary epithelial stem cells: discovery, function, and current status.

An entire mammary epithelial outgrowth, capable of full secretory differentiation, may comprise the progeny of a single cellular antecedent, i.e., may be generated from a single mammary epithelial stem cell. Early studies showed that any portion of an intact murine mammary gland containing epithelium could recapitulate an entire mammary epithelial tree on transplantation into an epithelium-free mammary fat pad. More recent studies have shown that a hierarchy of mammary stem/progenitor cells exists among the mammary epithelium and that their behavior and maintenance is dependent on signals generated both locally and systemically. In this review, we have attempted to develop the scientific saga surrounding the discovery and characterization of the murine mammary stem/progenitor cell hierarchy and to suggest further approaches that will enhance our knowledge and understanding of these cells and their role in both normal development and neoplasia.