Junctional ultrastructure in isolated synaptic membranes

Summary

The ultrastructure of synaptic junctions in whole brain tissue and isolated synaptic membranes has been compared. Type I junctions are present in the isolated membranes, readily identified by the presence of dense-staining material associated with the postsynaptic membrane, but the dense projections present at the presynaptic membrane in intact tissue are absent. Type 2 junctions are not easily recognized because of the absence of prominent junctional densities, but apposed membranes with the appearance of type 2 junctions are seen in isolated membrane preparations. Junctions without dense-staining material are also seen among synaptosomes and survive the hypotonic conditions used during isolation of the membranes. It thus seems probable that both type 1 and 2 junctions are present in isolated synaptic membrane preparations. In type 1 junctions after isolation, the postsynaptic thickening and cleft substance are together seen to be composed of an array of 200 Å dense-staining subunits spanning the postsynaptic unit membrane. The relationship of this structure to the ultrastructure of the cleft substance and postsynaptic thickening in intact tissue is discussed.