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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.
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