a | At the wild type neuromuscular junction (NMJ), Wingless (Wg; green) is secreted by, and localizes to, synaptic boutons (red). Inset: A single bouton at higher magnification, showing that Wg immunostaining (green) is localized both pre- and postsynaptically. Anti-horseradish peroxidase (HRP; red) is the neuronal marker. Arrow points to a synaptic bouton. b and c | At the ultrastructural level, the pre- and postsynaptic specializations seen in the wild type synapse (b) such as the active zone T-bars (arrow) — electron dense structures at Drosophila NMJs that are associated with presynaptic densities, and around which synaptic vesicles cluster — and subsynaptic reticulum (SSR), are completely lacking in the wg mutant (c).