Disruption of arl13b impairs the development of cerebellar granule cells. A Expression of the granule cell progenitor marker, zic1, in the cerebellum of wild-type embryos at 48 hpf revealed by in situ hybridization. B, C Expression of zic1 is reduced in the URL of both arl13b mutant and morphant embryos compared with wild-type embryos. Note that zic1 expression is severely reduced in the dorsomedial subregions of the URL (arrows). D The expression of the differentiated granule cell marker, reelin, in the cerebellum of wild-type embryos at 3 dpf. E, F Expression of reelin is reduced in the cerebellum of both arl13b mutant and morphant embryos. Note that in some embryos reelin expression is almost absent in the dorsal medial subregions of the cerebellum (colored ovals). G–G’’ In Tg(neurod1:eGFP) transgenic embryos, GFP+ granule cells are grouped into three clusters, the dorsomedial (dashed ovals), dorsoposterior, and ventrolateral subdivisions. H–H’’ The pattern of GFP+ granule cells is dramatically altered in the cerebellum, and particularly in the dorsomedial cerebellar subdivisions (dashed ovals) are severely affected in arl13b morphants. The parallel fibers connecting the two hemispheres are disrupted in arl13b morphants. Dorsal views of the embryos are shown. I–L Malformations of the dorsomedial cerebellar subdivisions (dashed ovals) and parallel fibers are also present in arl13b mutants both at 3 dpf and 4 dpf. cb, cerebellum; PF, parallel fiber. A–F, Scale bar 100 μm. G–L, Scale bar 50 μm.