Biological processes are regulated by chemical and mechanical signals, yet how these signalling modalities interact remains poorly understood. Here we identify a crosstalk between tissue stiffness and long-range chemical signalling in the developing Xenopus laevis brain. Targeted knockdown of the mechanosensitive ion channel Piezo1 in retinal ganglion cells or in the brain tissue surrounding retinal ganglion cells causes pathfinding errors in vivo. In the brain parenchyma, Piezo1 downregulation ...