Three-dimensional morphological analysis of nerve cells by scanning electron microscopy. A review

The present review provides a rational and new approach to study the three-dimensional morphology of nerve cells in situ at both cellular and macromolecular levels, by means of conventional (SEM) and high resolution scanning electron microscopy (HRSEM). The slicing and ethanol-cryofracturing methods, the freeze-fracture SEM method and tissue preparation for HRSEM have been described. Nerve cell outer surface, axon hillock and initial axon segment, axonal collateral ramifications and dendritic processes were visualized either by the slicing technique or the cryofracture method displaying neuronal geometry in situ. The cleavage plane occurred at the satellite neuroglial sheath exposing somatic hidden surfaces synaptic morphology. "En passant" axospinodendritic junctions, glomerular synapses and axosomatic contacts were examined in vertebrate cerebellar cortex. The SEM and cryofracture techniques could be applied as the Golgi light microscope technique to trace short neuronal circuits. The nerve cell inner surfaces were also studied by means of freeze-fracture SEM method and HRSEM. HRSEM provided information at both cellular and macromolecular levels. Topographic contrast of glycocalyx-like substance, synaptic junctions and myelin sheath was obtained. A comparison could be made between Au/Pd and chromium coated nerve cells. HRSEM provided SE-1 images of lipoprotein domains at the myelin sheath and globular subunits at the level of the postsynaptic membrane. This latter observation offers new potential areas for future studies on receptor morphology.