Uniquely among mammalian organs, the mammary gland of the female undergoes repeated cycles of expansion and regression during pregnancy, suckling and weaning. The tissue must therefore be equipped with hormone-responsive mechanisms for cell proliferation, differentiation and subsequent destruction. Various populations of progenitor cells have been identified and characterised, but our understanding of their properties and lineages is still incomplete. Here (p. ), Elizabeth Robertson and colleagues identify the transcriptional repressor Blimp1 as a marker of a progenitor population in the luminal compartment of the mouse mammary gland – rare in virgin females but expanding dramatically in pregnancy. They further demonstrate that Blimp1 is required for cell proliferation, polarity establishment and maintenance, and for the efficient production and secretion of milk components. While the mechanisms by which Blimp1 exerts these effects have yet to be elucidated, these data identify an important new regulator of mammary gland development and a new subpopulation of progenitor cells in the mammary epithelium.
