From zebra stripes to postal zones: deciphering patterns of gene expression in the cerebellum.

The analysis of patterned gene expression has been an important tool for dissecting the molecular and developmental bases of functional compartmentalization in the mammalian cerebellum. In particular, sagittally-oriented cellular aggregates arranged along the mediolateral axis are the patterning element most commonly invoked to illustrate cerebellar compartmentalization, and these are revealed both by patterns of afferent projection and by a number of classical biochemical markers that are distributed in a pattern of'zebra stripes'. Compartmentation along both the mediolateral and rostrocaudal axes might be linked mechanistically to segmentation in the fruit fly, since early cerebellar development is especially dependent upon the expression of mammalian homologs of Drosophila segmentation genes. In addition, as has been demonstrated in the retinotectal system, some of these genes are likely to control positional information required for the sagittal organization of cerebellar afferent projections. However, in contrast to these global or macro zones, the cerebellum is also compartmentalized at the subcellular or micro level. This can be visualized by differential patterns of mRNA distribution within the sole cerebellar efferent system, the Purkinje cell, defining within such cells a number of distinct subcellular domains or 'postal zones'. The global versus subcellular levels of cerebellar compartmentalization are related since they both appear to be linked to patterns of afferent innervation.A major goal of cerebellar research will be to unravel the true nature of such a relationship, and its relevance to function and behavior.