Epithelial tubes are crucial for the function of numerous organs, and their utility relies on attaining the correct length and width during development. When tubes elongate without widening, such anisotropic growth relies on a break in symmetry in the growth of the epithelium. This raises the questions of which chemical and/or mechanical signals provide such a symmetry break and where these signals come from (for example, the epithelia surrounding smooth muscle or signalling gradients in the epithelium). Now, Roman Vetter, Dagmar Iber and colleagues analyse growth biases in mouse lung and kidney tubes. In cultured bronchi and ureteric branches, biased elongation occurs in the absence of mesenchyme, and is unaffected by chemical inhibition of either the FGF receptor or matrix metalloproteinases. In embryos, cross sections reveal collapsed tubular morphologies with narrow luminal spaces, but computational simulations reveal that these morphologies do not underlie the biased elongation. Live imaging reveals fluid flow through these narrow tubes, allowing prediction of the level of shear stress on the apical surface caused by this flow. With the aid of a cell-based tissue model, apical shear forces can account both for the cell shapes observed in vivo and for the biased tube elongation. Shear stress thus emerges as an important driver of anisotropic tube growth.
