The branched architecture of the airway is physically shaped by the extracellular matrix and contractile airway smooth muscle during lung development*

The lung possesses a highly branched airway epithelial network, which is required for rapid gas exchange. To build the network of branches in the lung, airway morphogenesis requires three critical steps including branch site specification, branch elongation, and mesenchymal remodeling. Using comparative embryology, we have found several novel mechanisms of how airway branches are physically shaped. During airway morphogenesis, epithelial branches are wrapped by a sheath of extracellular matrix (ECM), known as the basement membrane (BM), and a layer of contractile airway smooth muscle (ASM). We hypothesized these tissue layers direct airway branching morphogenesis by constraining the growth of the airway epithelium. In the bird lung, early stages of branching morphogenesis occurs in the absence of ASM, permitting the investigation of how ECM remodeling influences branch shape. In birds, we found branch elongation rate and surface area expansion requires proteolytic turnover of the BM. As branches elongate, mesenchymal cells surrounding the tips of the airway branches become elongated and the mesenchyme begins to rearrange fluidly, resembling a mesenchymal unjamming transition. This facilitates the transport and assembly of new ECM components ahead of the growing branch, which may also influence growth and shaping of the extending branch. In the mammalian lung, ASM wraps the airway epithelium and is critical for specifying the location of newly formed branches. We found mechanotransduction signaling through focal adhesion kinase (FAK) controls the distribution of ASM wrapping around the growing airway epithelium. Inhibiting of FAK signaling results in altered ASM contractility, leading to changes in the branched architecture of the airway. Taken together, the ECM and ASM shape airways of the developing lung by constraining the growth of the epithelium to build a functional branched airway network.