Guidance cues from the embryonic dorsal spinal cord chemoattract dorsal root ganglion axons.

In the early stages, the dorsal root ganglion neurons extend their axons toward the dorsal spinal cord. We previously showed that surround repulsion by semaphorin 3A prevents sensory axons from straying from their paths. The finding, however, that sensory trajectories toward the dorsal spinal cord are almost normal in semaphorin 3A-deficient littermates raises the possibility that a chemoattraction-based mechanism also contributes to the formation of sensory axonal projections. By employing culture assays, we show that the dorsal spinal cord secretes chemoattractants for the dorsal root ganglion axons. Furthermore, we demonstrate that the activity of a dorsal spinal cord-derived cue is specific for early sensory axons. These results suggest that dorsal spinal cord-derived chemoattractants contribute to the formation of the initial trajectories of sensory axons.