FGF SIGNALING AND TARGET RECOGNITION IN THE DEVELOPING XENOPUS VISUAL-SYSTEM

We report that the growth cones of Xenopus retinal ganglion cells expr ess fibroblast growth factor receptors (FGFRs) and that bFGF stimulate s neurite extension from cultured retinal neurons. Furthermore, bFGF i s abundant in the developing optic tract but is reduced in the optic t ectum. To test whether FGF signaling plays a role in axonal guidance i n vivo, bFGF was exogenously applied to the developing optic pathway i n ''exposed brain'' preparations. FGF-treated retinal axons navigate n ormally through the optic tract, but the majority veer aberrantly at t he tectal border and bypass the target. Our results implicate FGF sign aling in target recognition and suggest that diminished levels of bFGF in the tectum cause arriving axons to slow their growth.