Ovarian steroid hormones: what's hot in the stem cell pool?

The vital role of ovarian hormones in the development of the normal breast foreshadowed their importance in mammary stem cell regulation. Two recent papers reveal that 17β-estradiol and progesterone control the size and repopulating ability of the mammary stem cell compartment. This likely occurs via paracrine signaling from steroid receptor-positive luminal cells to steroid receptor-negative stem cells. These findings illuminate roles for the female sex steroids in mobilizing the stem cell pool in the normal breast, and also provide a crucial link between the known hormonal risks of breast cancer and the potential stem cell origin of this disease.