Fig. 1. Luminal epithelial cells form aberrant acini in collagen gels. Cells were embedded in either rBM (a,c) or hydrated collagen-I gels (b,d). (a,b) Phase-contrast microscopy revealed almost identical spherical structures in collagen-I and rBM albeit with no visible central lumen in spheres in collagen-I. (c,d) Haematoxylin staining of cryostat sections of the gels showed lumen formation only in rBM (bar, 25 µm).