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Fig. 2. Mammary gland stem/progenitor-cell fate. The degree of stemness potentially decreases from top to bottom: as the cell becomes more committed, the cell gradually loses its stemness. The stem cells are able to self-renew and proliferate within the niche, maintained in their un-differentiated state by cell-matrix and cell-cell interactions with the niche cells, involving integrins and cadherins, respectively. These cells can be distinguished by their long-term label-retaining cell (LT-LRC) properties, which are thought to reflect a state of quiescence. Responding to stimuli, stem cells exit the niche by becoming short-term (ST)-LRCs. These actively cycle and express stem cell markers such as p21^cip, Msi1 and CK19. As they become further committed, they become the transit-amplifying progenitors (TAs), comprising the side population (SP) that are able to efflux the Hoechst dye. The SP/TAs express bipotential markers, such as K18⁺ and K14⁺, or EMA^– CALLA^–, and may be steroid receptor positive. The SP/TA cells eventually give rise to more committed progenitors that are Sca1⁺. The Sca1⁺ population differentiates into luminal and myoepithelial cells. Stem cells are thought to possess many of the features that constitute the tumor phenotype, including self-renewal and unlimited replicative potential. Tumorigenic mutations are presumably sustained in the expanding SP/TA population. These cells give rise to tumorigenic progenitor cells. CD44⁺ CD24^– may be markers that distinguish tumorigenic progenitor cells from normal progenitor cells.

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