FIGURE 1.

DCVs dock on and fuse with the presynaptic membrane alongside SVs at active zones. (A) A 3 nm thick virtual slice from a reconstructed 110 nm thick tissue section cut in the transverse plane of an active zone at a resting neuromuscular junction. The active zone ridge (AR) of the presynaptic membrane and main body of AZM (^∗), which extends from the ridge into the cytosol are flanked by a DCV (left) and SV (right). Scale bar = 50 nm. (B) Surface models of structures in (A) generated from 42 serial virtual slices (18 nm total) through the reconstructed volume. Both vesicle types are in contact with the presynaptic membrane and are connected to AZM macromolecules. Thus, they are docked. The AZM macromolecules connected to the vesicles include ribs, spars, and booms in the main body of the AZM and pins away from the main body. The ribs, spars, and booms are also connected to a beam, step, and mast, respectively, at the midline of the main body. Pegs link the ribs to the presynaptic membrane. Pins link the vesicles directly to the presynaptic membrane. (C–F) Surface models generated as in (B) from active zones fixed during repetitive synaptic transmission. A former docked DCV and SV that had fused with the presynaptic membrane are shown in (C,D) respectively. Each is at a stage of flattening into the presynaptic membrane where the AZM macromolecules connected to docked vesicles are still attached. A docking DCV and SV are shown in (E,F), respectively. Both are within 15 nm of the presynaptic membrane and are connected to the same assortment of AZM macromolecules as docked vesicles (B). (G) Color code for components in (B–F). All images are from muscles fixed with aqueous glutaraldehyde and stained with aqueous solutions of osmium tetroxide and uranyl acetate. Non-AZM macromolecules connected to the vesicle were not included in the surface models. (D,F) Taken from Szule et al. (2012).