Fig. 6. Model of EphB proteins acting as inhibitory axon guidance cues governing dorsal retinal axon pathfinding to the optic disc. In wild-type retina (left), the high-ventral to low-dorsal gradient of EphB protein expression (based primarily on EphB2 protein expression) is depicted in purple. Dorsal RGC axons growing ventrally towards the optic disc encounter an increasing gradient of EphB protein extracellular domains serving as inhibitory axon guidance cues. It is proposed that this inhibition leads to tight axon fasciculation as RGC axons approach the optic disc and that the high EphB protein expression in the ventral retina prevents inappropriate dorsal axon growth into the opposite half of the retina. Both these constraints are removed in the EphB2 EphB3 null mutant mice (right) resulting in the observed dorsal RGC axon defasciculation defects around the optic disc and the aberrant axon growth into the ventral half of the retina.
