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Development in mammals

During development, the crossing of the optic nerves is guided primarily by cues such as netrin, slit, semaphorin and ephrin; and by morphogens such as sonic hedgehog (Shh) and Wnt. This navigation is mediated by the neuronal growth cone, a structure that responds to the cues by ligand - receptor signalling systems that activate downstream pathways inducing changes in the cytoskeleton. Retinal ganglion cell (RGC) axons leaving the eye through the optic nerve are blocked from exiting the developing pathway by Slit2 and Sema5A inhibition, expressed bordering the optic nerve pathway. Ssh expressed at the central nervous system midline inhibits crossing prior to the chiasm, where it is downregulated. The organization of RGC axons changes from retinotopic to a flat sheet-like orientation as they approach the chiasm site.