Figure 1.

Morphological subtypes of axospinous synapses from the rat CA1 stratum radiatum classified according to their PSD configuration. Electron micrographs of consecutive sections that, respectively, demonstrate a synapse with a fenestrated (a-f), horseshoe-shaped (h-m), or segmented (o-u) PSD, as well as two synapses (black and white arrows) with nonperforated PSDs (w-y). All micrographs containing PSD profiles are presented in each case. The synapses illustrated in a-f, h-m, or o-u belong to the perforated synaptic category because they exhibit a discontinuity(s) or perforation(s) in PSD profiles seen in consecutive sections (arrowheads). In contrast, nonperforated synapses show exclusively continuous PSD profiles in w-y. The presynaptic and postsynaptic elements of each synapse are labeled in a, h, o, and w by AT (axon terminal) and SP (spine). Two-dimensional reconstructions of PSD plates show their fenestrated (g), horseshoe-shaped (n), segmented (v), or nonperforated (z) configuration. Scale bars, 0.5 μm.