Topographic spinocerebellar mossy fiber projections are maintained in the lurcher mutant.

A variety of recent studies of cerebellar development have focused attention on the role of Purkinje cells as organizing elements for the topography of afferent fiber connectivity in the cerebellum. We have investigated the involvement of Purkinje and granule cells in the maintenance of topographic spinocerebellar mossy fiber projections by analyzing the distribution of spinocerebellar mossy fiber terminals in lurcher (+/Lc) mutant mice. Purkinje cells in the +/Lc mutant degenerate starting after the first week of postnatal development because of an intrinsic genetic defect. The loss of their Purkinje cell targets also results in the death of 90% of the granule cells. We examined the distribution of spinocerebellar mossy fiber terminals in the juvenile and adult +/Lc mutant to determine how the pattern of afferent projections is affected by the loss of Purkinje cells shortly after innervation of the cerebellum. Labeling of spinocerebellar mossy fiber terminals with WGA-HRP in the P38 and adult +/Lc mutant showed that, despite the loss of almost all Purkinje cells and 90% of the granule cells, spinocerebellar mossy fibers project to the appropriate folia and segregate into relatively normal parasagittal bands. While we cannot rule out the possibility that Purkinje cells may be involved in the initial establishment of topographic maps, our results indicate that Purkinje cells are not necessary for the maintenance of the normal spinocerebellar mossy fiber topographic map.