Figure 4.

Wnt and Shh are repellants for descending axon tracts of the brain. (A) In mouse, Shh repels axons of the raphe spinal tract (blue) multidirectionally. In the anterior spinal cord, Shh (red) is expressed in an anterior^high to posterior^low (A-P) gradient, and in a medial^high to lateral^low (M-L) gradient. Thus, RST axons are pushed to grow both posteriorly and laterally. (B) In mouse, a decreasing anterior^high to posterior^low Wnt gradient (red) in the roofplate of the dorsal spinal cord repels descending corticospinal tract axons (blue) posteriorly.