Fig. 3. Luminal epithelial cells and myoepithelial cells sort themselves out inside rBM gels but form bilayered acini inside collagen gels. The spatial organization of myoepithelial cells in relation to luminal epithelial acinar backbones was analyzed in rBM (a) and hydrated collagen-I gel (b) and compared to that in normal breast (c). Gels were doublestained for Thy-1 to demonstrate myoepithelial cells (green) and propidium iodide to demonstrate nuclei (red). Note that whereas myoepithelial cells segregate from luminal epithelial acini in rBM, in vivo-like double-layered structures are formed in the collagen-I based acinus assay (bar, 25 µm).