Mammary stem and progenitor cells: Tumour precursors?

Several groups have proposed that mammary epithelial cell (MEC) populations, in common with other epithelia, have stem and progenitor sub-populations that are long-lived and provide most of the growth potential during ductal (and perhaps lobuloalveolar) outgrowth. In this review, we describe what is known about normal development, particularly with respect to the growth potential and regenerative capacity of mouse MEC populations. We have developed a theoretical model in order to understand how the activity of the somatic stem/progenitor cell compartment during mammary gland development could affect the demographic of adult MEC populations. This demographic is likely to be key to understanding tumour risk, since long-lived cells provide great advantages in the process of cancer development.