A neglected type of neuron, termed the unipolar brush cell, was recently 
characterized in the granular layer of the mammalian cerebellar cortex with 
several procedures, including light and electron microscopic immunocytochemistry 
utilizing antibodies to calretinin and neurofilament proteins. Although certain 
features of the unipolar brush cells were highlighted in these studies, the 
internal fine structure was partially obfuscated by immunoreaction product. In 
this study, rat cerebella were prepared for electron microscopy after perfusion 
fixation and Araldite embedding, and folia of the vestibulo-cerebellum, where 
unipolar brush cells are known to be enriched, were studied by light microscopy 
in semithin (0.5-1 micron) sections and by electron microscopy in ultrathin 
sections. Unipolar brush cells were easily identified in semithin sections 
immunostained with antibodies to GABA and/or glycine, and counterstained with 
toluidine blue. The unipolar brush cells have a pale cytoplasm and are GABA and 
glycine negative, while Golgi cells are darker and appear positive for GABA and, 
for the most part, also for glycine. Sets of identification criteria to 
differentiate unipolar brush cells from granule and Golgi cells in standard 
electron micrographs are presented. The unipolar brush cells possess many 
distinctive features that make them easily distinguishable from other cerebellar 
neurons and form unusually conspicuous and elaborate synapses with mossy 
rosettes. The unipolar brush cell has a deeply indented nucleus containing 
little condensed chromatin. The Golgi apparatus is large and the cytoplasm is 
rich in neurofilaments, microtubules, mitochondria, and large dense core 
vesicles, but contains few cisterns of granular endoplasmic reticulum. In 
addition, unipolar brush cells contain an unusual inclusion, which invariably 
lacks a limiting membrane and is made up of peculiar ringlet subunits. The cell 
body usually emits a thin axon and is provided with a single, large dendritic 
trunk that terminates with a paintbrush-like bunch of branchlets. Numerous 
nonsynaptic appendages emanate from the cell body, the dendritic stem, and the 
branchlets. The appendages contain rare organelles and lack neurofilaments. The 
branchlets contain numerous mitochondria, neurofilaments, large dense core 
vesicles, and clusters of clear, small, and round synaptic vesicles. They form 
extensive asymmetric synaptic junctions with one or two mossy fibers, which 
indicates minimal convergence of excitatory inputs. Under the postsynaptic 
densities, the branchlet cytoplasm displays a microfilamentous web. Besides 
their contact with mossy rosettes, the branchlets form symmetric and asymmetric 
synaptic junctions with presumed Golgi cell boutons that contain pleomorphic 
synaptic vesicles, indicating that the unipolar brush cells receive an 
inhibitory modulation. Some of these junctions are unusually extensive.(ABSTRACT 
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