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 su rrounding 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 ch ick and mouse tissues. Basal and alar plates of the neural tube are de void of activity, as is the posterior-half-sclerotome, which repels in a contact-dependent manner. When ganglia are sandwiched between dermo myotome and notochord placed at a distance, axon growth is channeled i n a bipolar trajectory. These results show that gradients of diffusibl e repulsion molecules flanking axon pathways can generate linear patte rns of axon growth. We suggest that such ''surround repulsion'' may fu nction generally, in concert with contact-dependent guidance mechanism s, to guide axons in the developing nervous system.