Surround Repulsion of Spinal Sensory Axons in Higher Vertebrate Embryos

We have tested whether the orientation of axons sprouting from bipolar dorsal root ganglion neurons is influenced by diffusible cues from surrounding tissues. Surface ectoderm, dermomyotome, and notochord exert strong chemorepulsion on axons growing in collagen gels, operating at separations beyond those found in vivo and active in cocultures of chick and mouse tissues. Basal and alar plates of the neural tube are devoid of activity, as is the posterior-half-sclerotome, which repels in a contact-dependent manner. When ganglia are sandwiched between dermomyotome and notochord placed at a distance, axon growth is channeled in a bipolar trajectory. These results show that gradients of diffusible repulsion molecules flanking axon pathways can generate linear patterns of axon growth. We suggest that such "surround repulsion" may function generally, in concert with contact-dependent guidance mechanisms, to guide axons in the developing nervous system.