Abstract
During development, axons grow through complex tissue environments to wire the brain. Axon growth speeds measured in cell culture are incompatible with the distances axons must cover in vivo. Yet how axons achieve the growth speeds required to establish long-range connectivity within a narrow embryonic developmental window, and by what mechanism, remains unknown. Here, using long-term live imaging in brain sections, mouse genetics, molecular and pharmacological perturbations, together with AxoVivo, a newly developed intravital imaging platform that resolves subcellular structures, including growth cones and cytoskeletal dynamics, in living mouse embryos, we show that axons of cortical projection neurons (CPNs) employ a low-adhesion mechanism to grow. Even as growth cones change shape and turn, axons grow an order of magnitude faster than in vitro. The actin motor myosin II controls the growth cone actin cytoskeleton and its morphology but not axon growth itself, which is instead promoted by microtubule stabilisation. Finally, super-resolution shadow imaging (SUSHI) revealed that growth cones do not pull on the surrounding extracellular matrix. Taken together, this low-adhesion mechanism driven by protruding microtubules ensures the rapid growth necessary for long-distance wiring of the developing nervous system.