Conventional and high resolution field emission scanning electron microscopy of vertebrate cerebellar parallel fiber-Purkinje spine synapses.

Purkinje dendrite-parallel fiber spine synapses of human, teleost fishes, Rhesus monkey and mouse cerebellar cortex have been studied by means of conventional scanning electron microscopy (SEM) using ethanol-cryofracturing technique and by high resolution field emission scanning electron microscopy (HRFESEM) for studying the outer and inner surface morphology of pre- and postsynaptic endings. Transmission electron microscopy of mouse cerebellar cortex either by means of ultrathin sections and freeze-etching replicas have been complementarily used for proper identification and comparative observations. Normal teleost fishes showed short neck and neckless Purkinje spines with exhibited spread or extended postsynaptic densities. In pathological human cerebellum, the ethanol cryofracturing technique exposed the outer surface of edematous flat and invaginated Purkinje spine synapses. In fractured presynaptic endings HRFESEM showed in Rhesus monkey cerebellar cortex a homogeneous extravesicular material binded to the synaptic vesicles and joining them to the presynaptic membrane. HRFESEM partially resolved the synaptic cleft as currently observed in high magnification transmission electron microscopy. Round subunits, 25-35 nm in diameter, were observed associated to postsynaptic membrane, apparently corresponding to the localization and distribution of E face postsynaptic intramembrane particles, which suggest that such subunits correspond to the domains of neurotransmitter postsynaptic receptors.