Solitary membrane associated particles in normal and experimentally enlarged synaptic vesicles

Abstract Freeze-etch replicas of synaptic vesicles were obtained from the vertebrate central nervous system and from the frog motor endplate. Large solitary particles were found to be associated mainly to the cytoplasmic fracture face of the vesicle membrane. They are especially conspicuous in the initial stages of Wallerian degeneration when the synaptic vesicles are abnormally large. The finding of these particles bears a striking resemblance to the previous observations of cytochemically verified vesicular calcium binding sites. The possible role of these particles for control by Ca 2+ of transmitter release is discussed.