To more specifically define which progenitor cell populations were responsive to hormones, we grew sorted MEC on CGP 57380 collagen gels. Consistent with a prior report, when grown as 3D structures on collagen, ML/LPC cells were enriched in acinar colony formation, while MB/BPCs were enriched in ductal colony formation. In response to hormones, however, an increase in acinar and ductal progenitor activity was observed only when P4 was present. Taken together, these findings suggest that both luminal and basal progenitor cells are sensitive to hormones. While the presence of both E2 and P4 together increases progenitor cell numbers, E2 favors acinar progenitor activity, and P4 favors both ductal and acinar progenitor activity. To determine whether the hormonal responsiveness of progenitors requires WNT signaling, LRP6 was inhibited in primary MECs dissociated from reduction mammoplasty tissues. shLRP6MECs were grown on collagen in the presence of vehicle and evaluated for progenitor activity. While E2 treatment Apoptosis Activator 2 stimulated acinar colony formation in control cells, knock down of LRP6 abolished this activity. Similarly when MECs were grown as floating colonies or mammospheres prior to plating on collagen, shLRP6 attenuated acinar colony formation. This indicates that LRP6 is necessary for the E2-mediated increase in luminal acinar progenitor activity. The epithelium of the breast undergoes dramatic developmental changes, and cues for mammary epithelial morphogenesis shift from the localized diffusion of growth factors to systemic regulation by ovarian steroids and circulating hormones. Consistent with this, here we show that that mammary progenitor cells in human breast tissues are regulated by both hormonal and nonhormonal mechanisms. However, in contrast to rodents where estrogen promotes ductal elongation and its inhibition can deplete stem cells, we found that estrogen directly enhances acinar progenitor activity in human cells through paracrine secretion of WNT ligands.In addition, unlike in mice where progesterone increases alveolar proliferation and expansion, progesterone stimulated human ductal progenitor cells and ductal morphogenesis through autocrine WNT signaling.