Differential effects of type I, III, and V collagens on mammary epithelial development in vitro

During pregnancy, mammary epithelial cells (MECs) and myoepithelial cells proliferate and develop ductal networks and alveolar bud structures that extend into the surrounding stroma. In the mammary stroma, type I, III, and V collagens form fibrillar structures and dynamically alter their expression patterns. While type I collagen is known to influence ductal elongation and alveolar bud formation, the specific roles of type III and V collagen in mammary epithelial morphogenesis remain unclear. This study aimed to create three-dimensional (3D) cultures by embedding mammary organoids in Matrigel with type I, III, and V collagen to explore their distinct roles in mammary duct elongation and alveolar bud formation in vitro. We prepared 3D organoid cultures of MECs and myoepithelial cells within type I, III, and V collagen gels to reproduce ductal elongation and alveolar bud formation. Type I collagen promoted ductal elongation and alveolar bud formation in a density-dependent manner. In contrast, type III collagen suppressed these morphogenetic processes compared to type I collagen. Type V collagen induced the formation of narrow ducts and swollen alveolar-like structures. Type III and V collagens reduced cell proliferation and attenuated Akt and ERK signaling relative to type I collagen. Myoepithelial cells were absent from ductal and alveolar regions in type I and V collagen gels but encircled the organoids in type III collagen gels. Scanning electron microscopy revealed distinct fibrillar architectures among the three collagen types. These findings demonstrate that collagens I, III, and V exert distinct regulatory effects on mammary epithelial morphogenesis during pregnancy.