Freeze-fracture study of the postsynaptic membrane of the cerebellar mossy fiber synapse in the frog.

We have examined the postsynaptic membrane of the synaptic junctions of frog cerebellar mossy fibers by electron microscopy of freeze-fracture replicas and thin sections. The intramembranous particles (imps) in the E fracture face of the postsynaptic membrane are approximately 10 nm in size and form conspicuous aggregates which we classified as macular, annular, or anastomotic in form, according to the occurrence and placement of imp-free "windows" within the aggregate. The size and shape of the aggregates appear related in that the area of macular aggregates is consistently smaller than the area of annular or anastomotic aggregates. Measurements of aggregate area range from 0.06 to 0.75 micrometer2. The variable size and shape of the imp aggregate in the postsynaptic membrane sets it apart from other excitatory synapses in the central nervous system, where macular aggregates are usually described. Examination of serial thin sections suggests that the shape of the postsynaptic density is equivalent to that of the imp aggregate observed in the postsynaptic membrane by freeze-fracture. This supports the notion that the region of postsynaptic membrane associated with the postsynaptic density in thin sections corresponds to the particle-rich regions of E face membrane observed in freeze-fracture replicas.