Mammary stem cells

The defining anatomical characteristic of mammals is the mammary gland. Comprising of a ductal epithelial network and surrounding stroma, it is a remarkably adaptive organ whose development is closely related to physiological requirement. During pregnancy, the number of mammary epithelial cells increases dramatically and these undergo terminal differentiation to enable lactation after birth. After weaning, the number and differentiation status of these cells returns to their pre-pregnant state, maintaining a similar developmental potential for future pregnancies. Strong evidence now exists to support the reliance of this adaptability on mammary stem cells (MaSCs). Through in vivo mammary reconstitution experiments, primarily using the mouse as a model system, it has been demonstrated that MaSCs reside within mature mammary epithelium as single cells that can be prospectively identified. This discovery, along with the identification of other progenitors with limited proliferation capacity, has indicated that mammary epithelium is organized in a hierarchical fashion, in which MaSCs give rise to mature, functional epithelium via intermediate, lineage-restricted progenitors. The ability to prospectively identify and isolate functionally distinct mammary epithelial subpopulations has been an important development in mammary biology and opened the way for a more detailed understanding of the molecular regulation of normal and neoplastic development of this organ. In addition, it should enable therapeutic strategies that target specific mammary cell subtypes to be explored.