Spontaneous fusions seen with fast freezing of hippocampal synapses. A, A quick-freezing image of a hippocampal synapse. Arrows indicate the presence of three docked vesicles at the active zone. Right, Quantitative data from morphological analysis of the total recycling pool and docked pool (n = 23 serially reconstructed synapses). B, C, Spontaneous omega figures. Notice the occurrence of omega figures with narrow (B) or more widely open (C) fusion necks. Spontaneous fusions were always topographically restricted to the active zone.D, Examples of two omega figures simultaneously present at the same active zone. These always occurred in close spatial proximity. E, Example of a synapse with multiple coated vesicles. These vesicular structures were always localized at the periphery of presynaptic terminals. Scale bars: A, 0.2 μm;B–D, 0.1 μm; E, 0.15 μm.