Figure 1.

Left, Two gradients EphA and ephrinA in the tectum activate axonal ephrinA in the rostral tectum and axonal EphA in the caudal tectum, respectively, to suppress branching. BDNF promotes branching uniformly, and branching occurs where axonal ephrinA and EphA activation reaches a minimum. Adapted from Figure 8B of Marler et al. (2008). Right, An alternative model in which only axonal EphA suppresses branching, while axonal ephrinA/TrkB promote branching. Tectal EphA (highest in rostral tectum) disrupts ephrinA/TrkB signaling, so ephrinA/TrkB activity increases from rostral to caudal. Branching occurs once an intracellular threshold is reached.