Ephrin-B regulates the ipsilateral routing of retinal axons at the optic chiasm

In Xenopus tadpoles, all retinal ganglion cells (RGCs) send axons contralat erally across the optic chiasm. At metamorphosis, a subpopulation of EphB-e xpressing RGCs in the ventrotemporal retina begin to project ipsilaterally. However, when these metamorphic RGCs are grafted into embryos, they projec t contralaterally, suggesting that the embryonic chiasm lacks signals that guide axons ipsilaterally. Ephrin-B is expressed discretely at the chiasm o f metamorphic but not premetamorphic Xenopus. When expressed prematurely in the embryonic chiasm, ephrin-B causes precocious ipsilateral projections f rom the EphB-expressing RGCs. Ephrin-B is also found in the chiasm of mamma ls, which have ipsilateral projections, but not in the chiasm of fish and b irds, which do not. These results suggest that ephrin-B/EphB interactions p lay a key role in the sorting of axons at the vertebrate chiasm.