Dormant reporter-marked basal stem/progenitor cells contribute to mammary lobuloalveoli formation.

The Mammary gland undergoes complicated epithelial remodeling to form lobuloalveoli during pregnancy, in which basal epithelial cells remarkably increase to form a basket-like architecture. However, it remains largely unknown how dormant mammary basal stem/progenitor cells involve in lobuloalveolar development. Here, we show that expression marks a rare population of mammary epithelial cells with the majority being basal epithelial cells. reporter-marked basal epithelial cells are relatively dormant mammary stem/progenitor cells. Although reporter-marked basal epithelial cells have limited contribution to the homeostasis of mammary epithelium, they divide rapidly during pregnancy and contribute to lobuloalveolar development. Furthermore, reporter-marked basal epithelial cells are preferentially used for multiple pregnancies. Using single-cell RNA-seq analysis, we identify multiple functionally distinct clusters within the reporter-marked cell-derived progeny cells during pregnancy. Taken together, our findings underscore reporter-marked basal cells as dormant stem/progenitor cells that contribute to mammary lobuloalveolar development during pregnancy.