Fig. 3. Shh expression at the chiasm border defines a barrier within the ventral midline that is implicated in guiding RGC axons. Retinal ganglion cell (RGC) axons growing towards the diencephalic ventral midline project either contralaterally or ipsilaterally in response to guidance cues at the optic chiasm. (A) In wild-type animals, downregulation of Shh expression allows RGC axon decussation and formation of the chiasm. (B) Ectopic expression of Shh in the ventral midline region interferes with RGC axon growth and prevents them from crossing the midline. The Shh expression domain is shown in blue. Shh can inhibit retinal axons in vitro, suggesting that in vivo it might act on axons directly rather than by altering the expression of distinct guidance cues in the chiasm, although conclusive evidence for this guidance function in vivo remains to be obtained. A, anterior; P, posterior; POA, pre-optic area; VH, ventral hypothalamus.