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 f or dissecting the molecular and developmental bases of functional comp artmentalization 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 affe rent projection and by a number of classical biochemical markers that are distributed in a pattern of 'zebra stripes'. Compartmentation alon g both the mediolateral and rostrocaudal axes might be linked mechanis tically to segmentation in the fruit fly, since early cerebellar devel opment is especially dependent upon the expression of mammalian homolo gs of Drosophila segmentation genes. In addition, as has been demonstr ated in the retinotectal system, some of these genes are likely to con trol 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 subcel lular or micro level. This can be visualized by differential patterns of mRNA distribution within the sore cerebellar efferent system, the P urkinje cell, defining within such cells a number of distinct subcellu lar 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 cerebel lar research will be to unravel the true nature of such a relationship , and its relevance to function and behavior.