fig1

Figure 1. Historical model of the mammary epithelial cell differentiation hierarchy. Multipotent mammary stem cells (MaSCs) have the ability to self-renew and give rise to all downstream lineages. Bipotent stem cells give rise to committed unipotent progenitor populations that maintain basal/myoepithelial and luminal lineages. These unipotent cells are responsible for maintaining mature basal/myoepithelial and luminal lineages in the postnatal gland. Two luminal progenitor populations have been reported, one giving rise to alveolar cells during pregnancy and the other responsible for maintaining the mature ductal cells