Increased vesicle number in BDNF −/− synapses is restricted to vesicles located farthest from the active zone. The number of vesicles within consecutive 25 nm bins was plotted as a function of distance from the synaptic cleft. Within the first 100 nm from the synaptic cleft there is no difference in the number of vesicles between the wild-type mice (▪) and BDNF −/− mice (■) in either longitudinal sections (A) or cross sections (B). However, beyond 100 nm, BDNF −/− mice exhibit a significant increase in the number of vesicles compared with WT mice. * D ≪ D_0.01by Kolmogorov–Smirnov.