Parity reduces the progenitor potential of mammary epithelial cell subpopulations. (A) Representative images of individual wells with colonies formed by the specified cell subpopulations from age-matched virgin and parous mice. Scale bar, 4 mm. (B) Bar graph comparing the colony-forming capacities of myoepithelial cells, basal stem/progenitor cells, luminal Sca1-, and luminal Sca1+ cells of age-matched virgin and parous mice. Data are from three independent experiments and represent the mean ± SEM of colonies per well; 18 wells were assessed per cell type. *P < 0.015, NS: not significant (P = 0.08), by using two-tailed unpaired Student t test. (C) Representative images for the immunophenotyping of 5-day-old colonies grown from myoepithelial cells, basal stem/progenitor cells, luminal Sca1-, and luminal Sca1+ cells. The colonies were stained for luminal Krt18 (green) and basal Krt14 (red) expression. Hoechst 33342 (blue) was used to distinguish nuclei and to label feeder cells (negative control). Scale bar, 200 μm.