FGF signaling and target recognition in the developing Xenopus visual system.

We report that the growth cones of Xenopus retinal ganglion cells express fibroblast growth factor receptors (FGFRs) and that bFGF stimulates neurite extension from cultured retinal neurons. Furthermore, bFGF is abundant in the developing optic tract but is reduced in the optic tectum. To test whether FGF signaling plays a role in axonal guidance in vivo, bFGF was exogenously applied to the developing optic pathway in "exposed brain" preparations. FGF-treated retinal axons navigate normally through the optic tract, but the majority veer aberrantly at the tectal border and bypass the target. Our results implicate FGF signaling in target recognition and suggest that diminished levels of bFGF in the tectum cause arriving axons to slow their growth.