Figure 1.

Mammary stem cells/progenitor cells have three basic cellular division fates depending upon the physiological stage of the animal. 1) Expansion occurs when cells divide symmetrically to produce two daughter stem cells of similar potency; 2) Maintenance occurs when cells divide asymmetrically and produce one daughter stem/progenitor cell and one differentiated cell that may later undergo apoptosis; 3) Expansion occurs when cells divide symmetrically, but exhaust in the case of terminal differentiation which produces two differentiated cells, both of which may later undergo apoptosis; 4) Cells exhaust when they undergo apoptosis.