Fig. 1.

The intricate structure of the mammary gland can be recapitulated in 3D laminin-rich gels and is context-dependent. a The human mammary gland is composed, in part, of a bilayered epithelium consisting of luminal epithelial cells lining the duct and myoepithelial cells lining the basal surface. This tree-like structure is separated from the surrounding stroma by a basement membrane (BM) rich in laminins and collagens other than type I. Three-dimensional culture of murine mammary gland explants allows us to study primary branching or the formation of alveoli (alveologenesis), depending on the context (e.g., the type of ECM present). In a laminin-rich ECM (such as Matrigel), organoids undergo, b alveologenesis or c form acini depending on the soluble factors present. Keratin (K) staining reveals that proper polarity is achieved in this culture model: K8 (green), an epithelial marker, stains throughout these structures while K14 (red), indicative of myoepithelial cells in vivo, is confined to the basal surface (* denotes lumen). On the other hand, culturing organoids within a type I collagen matrix (d), upon stimulation by any of a number of growth factors, yields branched, primary duct-like structures with hollow lumens (inset). b and c were reproduced, with permission from Elsevier, from Fata et al. (2007). d was reproduced, with permission from The Company of Biologists Ltd., from Simian et al. (2001)