Organization of spinocerebellar projection map in three types of agranular cerebellum: Purkinje cells vs. granule cells as organizer element

The organization of the spinocerebellar projection was analysed by the anterograde axonal WGA‐HRP (horseradish peroxidase‐wheat germ agglutinin conjugate) tracing method in three different types of agranular cerebellar cortex either induced experimentally by X‐irradiation or occurring spontaneously in weaver (wv/wv) and staggerer (sg/sg) mutant mice. The results of this study show that in the X‐irradiated rat and weaver mouse, in both of which the granule cells are directly affected and die early in development, the spinal axons reproduce, with few differences, the normal spinocerebellar pattern. Conversely, in staggerer mouse, in which the Purkinje cells are intrinsically affected and granule neurons do not seem to be primarily perturbed by the staggerer gene action, the spinocerebellar organization is severely modified. These findings appear somewhat paradoxical because if granule cells, the synaptic targets of mossy spinocerebellar fibers, were necessary for the organization of spinocerebellar projection, the staggerer cerebellum would exhibit a much more normal projectional map than the weaver and the X‐irradiated cerebella. It is, therefore, obvious that granule cells, and even specific synaptogenesis, are not essential for the establishment of the normal spinocerebellar topography. On the other hand, the fact that the Purkinje cells are primarily affected in the unique agranular cortex in which the spinocerebellar organization is severely modified suggests that these neurons could be the main element in the organization of the spinocerebellar projection map. This hypothesis is discussed in correlation with already‐reported findings on the zonation of the cerebellar cortex by biochemically different clusters of Purkinje cells.