Effect of Wnt4 on the proliferation capacity of basal stem/progenitor cells after early pregnancy in mice. (A) Recombinant Wnt4 rescues the parity-induced in vitro proliferation defect in basal mammary epithelial cells. Selected mammary epithelial cells from parous mice were cultured in the absence or presence of recombinant Wnt4. Three independent experiments were performed. Data represent the mean ± SEM of colonies per well, with six to nine wells assessed per cell type. *P ≤ 0.02 (two-tailed unpaired Student t test). (B) Mechanistic model illustrating the parity-induced decrease in hormone-sensing and Wnt4-secreting luminal cells on the Wnt/Notch signaling pathways and the proliferation/differentiation potential in basal stem/progenitor cells.