Breast cancer, stem cells and sex hormones: part 1. The impact of fetal life and infancy.

Like other organs, the breast contains rare somatic stem cells (SCs) that are long-lived and slowly dividing. In the adult breast, they are closely regulated in areas located along the breast ducts called SC niches. Breast SCs can produce offspring that become ductal, alveoli or myoepithelial cells. In fetal life, SCs form the primitive breast ducts and up to 30 weeks of gestational age, this process appears to be largely independent of estrogen. Early life risk factors for breast cancer include birth weight, rapid growth during infancy and diet. The impact of these risk factors may be mediated through SC number. These somatic breast SCs persist into adult life and so they are exposed to oncogenic influences for much longer than the short-lived differentiated breast ductal and alveolar cells. As such, it is likely that the breast SC is a prominent target for carcinogenesis and so SC number may be an important determinant of breast cancer risk later in life.