Fig. 9. The effects of BDNF during the development of retinotectal synaptic connectivity and their potential significance. (A) BDNF increases both arbor complexity and the density of presynaptic specializations in RGC axons. (B) BDNF increases the density of postsynaptic specializations on tectal neurons without influencing dendritic branch complexity. (C) BDNF, by increasing RGC axon arbor size and complexity, allows individual presynaptic RGC axon arbors to establish synaptic contacts with increased numbers of postsynaptic neurons. By modulating synapses directly, BDNF increases the density of synaptic innervation per tectal neuron. In a competitive scenario, the limiting amounts of BDNF would provide those presynaptic arbors directly exposed to BDNF a synaptic advantage and a greater opportunity to establish contacts with multiple postsynaptic partners.