We used EM tomography to examine the fine structure of the apparently
amorphous electron dense material that is seen at active zones of axon
terminals when viewed by conventional 2D electron microscopy. Serial
1-nm optical slices from 3D reconstructions of individual thin tissue
sections reveal that the material is composed of an interconnecting ne
twork of elongate components directly linked to synaptic vesicles and
the presynaptic membrane. Each vesicle at the active zone that lies ad
jacent to the presynaptic plasma membrane has several such connections
. Information provided by reconstruction data may be useful in generat
ing experiments aimed at understanding the mechanisms involved in the
docking of synaptic vesicles and their exocytosis during synaptic tran
smission. ((C) Elsevier, Paris).