[Show abstract][Hide abstract] ABSTRACT: Current models of chemotaxis during neuronal migration and axon guidance propose that directional sensing relies on growth cone dynamics. According to this view, migrating neurons and growing axons are guided to their correct targets by steering the growth cone in response to attractive and repulsive cues. Here, we have performed a detailed analysis of the dynamic behavior of individual neurons migrating tangentially in telencephalic slices using high-resolution time-lapse videomicroscopy. We found that cortical interneurons consistently display branched leading processes as part of their migratory cycle, a feature that seems to be common to many other populations of GABAergic neurons in the brain and spinal cord. Analysis of the migratory behavior of individual cells suggests that interneurons respond to chemoattractant signals by generating new leading process branches that are better aligned with the source of the gradient, and not by reorienting previously existing branches. Moreover, experimental evidence revealed that guidance cues influence the angle at which new branches emerge. This model is further supported by pharmacological experiments in which inhibition of branching blocked chemotaxis, suggesting that this process is an essential component of the mechanism controlling directional guidance. These results reveal a novel guidance mechanism during neuronal migration that might be extensively used in brain development.