Role of staggerer gene in determining cell number in cerebellar cortex. I. Granule cell death is an indirect consequence of staggerer gene action

Mouse chimeras were made by aggregating two embryos at the 8-cell stage of development. These experimental mosaic animals are useful in assessing whether the effect of a mutant gene in a given cell type is direct (intrinsic) or indirect (extrinsic). The results reported are an analysis of the involvement of the staggerer gene in the phenotype of granule cell death observed in staggerer mutant mice. The ichthyosis mutation has been used as an independent cell marker. This mutation produces no known neurological defects but does cause a characteristic clumping of heterochromatin, especially in small cells. Analysis of two staggerer ↔ ichthyosis chimeras leads to the conclusion that staggerer granule cells, that would have died in the mutant, are rescued in the mosaic cerebella. The death of granule cells in the homozygous staggerer mutant is thus an indirect, epigenetic consequence of direct gene action in other cells. The implications and limitations of this conclusion are discussed.