Epithelial organs are built through the movement of groups of interconnected cells.
We observed cells in elongating mammary ducts reorganize into a multilayered epithelium,
migrate collectively, and rearrange dynamically, all without forming leading cellular
extensions. Duct initiation required proliferation, Rac, and myosin light-chain kinase,
whereas repolarization to a bilayer depended on Rho kinase. We observed that branching
morphogenesis results from the active motility of both luminal and myoepithelial cells.
Luminal epithelial cells advanced collectively, whereas myoepithelial cells appeared
to restrain elongating ducts. Significantly, we observed that normal epithelium and
neoplastic hyperplasias are organized similarly, suggesting common mechanisms of epithelial