Structural components in the synaptic cleft captured by freeze-substitution and deep etching of directly frozen cerebellar cortex.

Structural components in the synaptic cleft were examined in cerebellar excitatory synapses by conventional electron microscopy and by rapid freezing followed by freeze-substitution or deep etching. Two transverse components and one parallel element were identified in the clefts of rapidly frozen and freeze-substituted synapses: (i) bridging fibrils, 4-6 nm in diameter, that span the cleft; (ii) columnar pegs, 4-6 nm wide and 8-15 nm high, projecting from the postsynaptic surface; and (iii) intervening fine fibrils running parallel to the apposed synaptic membranes. These were more clearly visible in deep-etched synapses, although the postsynaptic pegs were difficult to distinguish from intramembrane particles in the cross-fractured postsynaptic membranes. Deep etching also revealed other fibrils on the cytoplasmic surface of the postsynaptic membrane. These appear to contact the membrane surface or the intramembrane particles. Freeze-substituted materials also displayed the fibrillar components in the postsynaptic dense fuzz, but failed to display the presynaptic dense projections typically observed in thin sections or deep-etched replicas of the conventionally fixed materials. The bridging fibrils are likely to play a mechanical role in holding the synapse together, while the short pegs may be integral parts of the receptor molecules.