FGF signaling regulates cytoskeletal remodeling during epithelial morphogenesis.

Changes in the cytoskeletal architecture underpin the dynamic changes in tissue shape that occur during development. It is clear that such changes must be coordinated so that individual cell behaviors are synchronized; however, the mechanisms by which morphogenesis is instructed and coordinated are unknown. After its induction in non-neural ectoderm, the inner ear undergoes morphogenesis, being transformed from a flat ectodermal disk on the surface of the embryo to a hollowed sphere embedded in the head. We provide evidence that this shape change relies on extrinsic signals subsequent to genetic specification. By using specific inhibitors, we find that local fibroblast growth factor (FGF) signaling triggers a phosphorylation cascade that activates basal myosin II through the activation of phospholipase Cgamma. Myosin II exhibits a noncanonical activity that results in the local depletion of actin filaments. Significantly, the resulting apical actin enrichment drives morphogenesis of the inner ear. Thus, FGF signaling directly exerts profound cytoskeletal effects on otic cells, coordinating the morphogenesis of the inner ear. The iteration of this morphogenetic signaling system suggests that it is a more generally applicable mechanism in other epithelial tissues undergoing shape change.