The unipolar brush cells are excitatory, cerebellar granular layer interneurons 
that receive mossy fiber input on their dendritic brushes in the form of a giant 
glutamatergic synapse. We investigated the postnatal development of the brush of 
the unipolar brush cell in lobules IX and X by light microscopy and defined the 
maturation of mossy fiber-unipolar brush cell synapses and mossy fiber-granule 
cell synapses by electron microscopy using calretinin immunocytochemistry to 
identify unipolar brush cells. During the first postnatal week, unipolar brush 
cells possessed one or two short, branched dendrites. The brush differentiated 
primarily during the successive 21 postnatal (P) days, during which it underwent 
progressive maturation. This developmental process was subdivided into stages 
1-4, which were descriptively termed protodendritic unipolar brush cell (P2-12), 
filopodial brush (P12-16), intermediate brush (P16-21), and dendriolar brush 
(P21-28) stages. Electron microscopic measurements of individual mossy 
fiber-unipolar brush cell and mossy fiber-granule cell synaptic junctions were 
made at P12, 16, 21, and 28. While the average length of mossy fiber-unipolar 
brush cell synapses increased during development, that of mossy fiber-granule 
cell synapses decreased. Comparisons of the lengths of mossy fiber-unipolar 
brush cell and mossy fiber-granule cell synapses demonstrated that mossy 
fiber-unipolar brush cell synapses were longer on average than mossy 
fiber-granule cell synapses for all ages. Frequency distribution histograms also 
showed that the percentage of mossy fiber-unipolar brush cell synapses longer 
than 0.5 microm was lower in the pooled P12-P16 groups than in the pooled 
P21-P28 groups (8 versus 20%). In contrast, mossy fiber-granule cell synapses 
longer than 0.5 microm were a small minority at P12, 16, and 21, and occurred 
rarely at P28. The present study indicates that mossy fiber-unipolar brush cell 
synapses increase in length with the differentiation of the brush dendrioles, 
while that of mossy fiber-granule cell synapses decrease with differentiation of 
the granule cell dendritic claws. The finding that mossy fiber-unipolar brush 
cell synapses were generally longer than mossy fiber-granule cell synapses may 
indicate that the properties of the postsynaptic targets play a major role in 
shaping synaptic appositions within cerebellar glomeruli.