Staggerer chimeras: Intrinsic nature of purkinje cell defects and implications for normal cerebellar development

The site of gene action of the Staggerer mutation of mice was investigated with Staggerer↔wild-type chimeras. Homozygous Staggerer mice show severe locomotor difficulties due to cerebellar abnormalities which include degeneration of virtually all granule cells and cytological defects in Purkinje cells. Although the locomotor deficits of the mutant were not present in the chimeras, the presence of Staggerer cells affected cerebellar structure. The size and the extent of foliation of the chimeric cerebella were intermediate between wild-type and homozygous Staggerer. A normally proportioned granule cell layer was present. Using β-glucuronidase as an independent determinant of a cell's genotype, it was found that the genotypically Staggerer medium-to-large neurons expressed all of the light microscopic defects observable in these cells in the homozygous mutant. These defects include: (1) smaller size; (2) usually ectopic location; and (3) regional variation in the cytological appearance of the perikaryon. By contrast, all Purkinje cells which were genotypically wild-type appeared normal in size, in location and in their cytological appearance. Their density, however, was much reduced from wild-type. The effects of the Staggerer mutation on the granule, stellate and basket cells could not be directly assessed as the glucuronidase marker is not suitable for use with these cells. The Staggerer gene thus acts directly on Purkinje cells rather than via extracellular environmental changes. The findings are discussed in terms of their implications for normal cerebellar development.