Cerebellar morphogenesis occurs through a complex interplay of cell 
proliferation and migration that in mouse and rat begins about midgestation and 
ends in the third postnatal week. Cerebellar cells derive from germinative 
matrices in the ventricular zone and the external granular layer. Like granule 
cells, unipolar brush cells (UBCs) are excitatory interneurons situated in the 
granular layer of the cortex and innervated by mossy fibers. While granule cells 
are produced from the external granular layer, the generation of UBCs is still 
controversial. We utilized the reeler mutant mouse, which has widespread 
misplacement of neurons due to lack of Reelin protein, to ascertain the origin 
of UBCs. In the reeler cerebellum, which is small and lacks foliation, Purkinje 
cells are greatly reduced in number and in large part are located ectopically in 
deep cerebellar masses. Granule cells are also reduced in number and form an 
irregular granule cell layer. In this study we demonstrate that the reeler 
mutation influences the positioning of UBCs and also significantly reduces their 
number. Both subsets of UBCs identified in normal mouse, the calretinin-positive 
and the metabotropic glutamate receptor 1alpha-positive subsets, are affected in 
the reeler. About 40% of the calretinin-positive UBCs are ectopically situated 
in the deep cerebellar regions and the immediate vicinity of the ependyma of the 
fourth ventricle. Ectopic UBCs have discrete, although somewhat looser brushes 
than granular layer UBCs, but form synaptic junctions with complex axon 
terminals, possibly belonging to mossy fibers and UBC axons, like their normally 
situated counterpart. The observed displacement of UBCs in the reeler suggests 
that they originate from the ventricular zone.