Figure 3.

Shh and Wnt3 are axon guidance cues in the visual system. In the retina, Shh is bifunctional: low concentrations promote RGC axon outgrowth toward the optic disc, whereas high concentrations push axons into the optic nerve. Shh is also expressed along the border of the optic chiasm (red), where it acts as a repellant for RGC axons in the optic nerve. Ipsilaterally projecting RGC axons express Boc (Boc^ipsi), which mediates Shh repulsion at the chiasm. Wnt3 is expressed in a medial^high-to-lateral^low gradient in the tectum. At high concentrations, Wnt3 inhibits the growth of both dorsal and ventral RGC axons via Ryk as a receptor. At low concentrations, Wnt3 stimulates the growth of dorsal RGC axons to the lateral tectum in a Fz-dependent manner.