Early in the development of invertebrate and vertebrate nervous systems,leader neurons lay down an axonal scaffold that follower axons track along. On p. 4999, Bak and Fraser describe an important behavioural difference between leader and follower axons in the developing zebrafish forebrain. By using timelapse fluorescence microscopy to analyse the growth of commissural axons in a living brain, they show that only the leader axons, and not the follower axons,slowed down at the midline. When the leader axons were ablated by laser irradiation, follower axons adopted leader behavioural characteristics. Bak and Fraser also report that the growth cones of leader axons were short and wide, whereas those of followers were more streamlined. They propose that the midline dynamics of commissural axons are determined by both the level of exposure to midline cues and the presence of other axons as a substrate.

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