Peripheral neurons depend on CNS-derived guidance cues for proper navigation during leech development.

In leech, major nerve pathways are pioneered by CNS neurons and evidence from dye-injection and antibody experiments suggest that they serve as guides for later differentiating neurons. In this study we have directly tested this hypothesis by examining the consequences of CNS ablation on the navigation in the periphery of a well-defined population of afferent sensory neurons. We show that in the absence of CNS-derived axons the axonal growth cones of this population of peripheral neurons extend with little directionality and instead of forming orderly projections, default into forming circular fasciculated pathways with each other. This suggests that CNS-derived guidance cues are absolutely required for the correct navigation of these peripheral sensory neurons.