During development, neurons extend thin protrusions that must choose between alternative routes. A study of this process in fruitflies unites two previously disparate protein families. In many ways, the developing nervous system resembles a city at rush hour, with large numbers of neurons ― up to 10~(12) in humans ― extending thin protrusions called axons that take highly specific routes to reach their destinations. This ability of axons to choose one particular path from many alternatives is essential for precise wiring of the nervous system. On page 583 of this issue, Yoshikawa and colleagues present the next chapter in the unfolding story of how axons find their way. The central nervous system of the fruitfly (Drosophila melanogaster) embryo provides a simple example of pathway selection, as axons growing across the midline of the nervous system ― thereby connecting the left and right halves of the animal ― choose between two alternative routes. Roughly half the axons in each body segment choose the anterior route, establishing an anterior axon bundle (commissure); the other half take the posterior path, establishing a posterior commissure.
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