The fine structure of freeze-fractured presynaptic membranes

Electron micrographs of presynaptic membrane faces were obtained from replicas of aldehyde-fixed, freeze-fractured neuropil of mammalian and avian C.N.S. There is strong evidence that the presynaptic membrane is split and that the exposed membrane faces represent the inner and outer membrane leaflet respectively. Presynaptic sites on the inner face appear as patches adhering to postsynaptic elements and include a distinctly indented area. They bear clusters of membrane particles and a varying number of small protuberances. The latter apparently occur in presynaptic membranes exclusively and form hexagonal patterns corresponding to the ‘presynaptic vesicular grid’. Evidence is presented that the protuberances are temporary attachment sites of transmitter vesicles to the presynaptic membrane. These contacts may be identical with the sites of transmitter release.