Alveolar progenitors are enriched in luminal cell populations, while ductal progenitors are enriched in basal cell populations. (A) In vivo, primary epithelial cells isolated from reduction mammoplasty tissue form bilayered ductal or alveolar structures when grown in the humanized fat pads of NOD/SCID mice. These structures expressed luminal cytokeratin 18 (CK18) and basal smooth muscle actin (SMA). (B) In vitro, primary epithelial cells formed either ductal or alveolar structures when grown on collagen gels (n = 12; mean ± standard error of the mean (SEM)). (C) Ductal progenitor activity was enriched in the basal progenitor cell (BPC) population, while alveolar progenitor activity was enriched in mature luminal (ML) or luminal progenitor cell (LPC) populations. Epithelial cells were sorted using cell surface markers CD10 and EpCAM and grown on collagen gels. Differences were detected using analysis of variance (n = 3; mean ± SD). (D) Growth in suspension as mammospheres enriched for both alveolar and ductal progenitor activity, while growth in suspension as floating colonies over adherent plates enriched for alveolar progenitor activity. Epithelial cells were grown in suspension as floating colonies and mammospheres for 7 days then plated on collagen gels. Ductal and alveolar progenitor activity was quantified (n = 12; mean ± SEM). Scale bar = 100 μm. MB, mature basal.