Growth anisotropy of the extracellular matrix drives mechanics in a developing organ
The final size and shape of organs results from volume expansion by growth and shape changes by contractility. Complex morphologies arise from differences in growth rate between tissues. We address here how differential growth drives epithelial thickening and doming during the morphogenesis of the growing Drosophila wing imaginal disc. We report that 3D morphology results from elastic deformation due to differential growth between the epithelial cell layer and its enveloping extracellular matrix (ECM). Furthermore, the ECM envelope exhibits differential growth anisotropy (i.e. anisotropic expansion in 3D), growing in-plane on one side, but out of plane on the other side. The elasticity, anisotropy and morphogenesis is fully captured by a mechanical bilayer model. Moreover, differential expression of the Matrix metalloproteinase MMP2 controls growth anisotropy of the two ECM layers. This study shows that the ECM is a controllable mechanical constraint whose intrinsic growth anisotropy directs tissue morphogenesis in a developing organ.