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Pattern formation in the striatum: developmental changes in the distribution of striatonigral neurons

The striatum of the mammalian forebrain can be divided into 2 compartments, the patches and the matrix. We have investigated embryonic events involved in the formation of these compartments in rats. Early in development, dopamine fibers from the substantia nigra selectively innervate the patches. In the perinatal striatum, we observed a close match between the distributions of striatal cell bodies with axonal projections to the substantia nigra and patches of afferent dopamine fibers. Striatal cells projecting to the nigra are first seen in the ventrolateral striatum at embryonic day (E) 17. Striatonigral cell bodies are distributed homogeneously through the striatum from E18 to 19. At E20 and until postnatal day 4, these cell bodies are organized into discrete patches. After this time, striatonigral cell bodies assume the dense and homogeneous distribution characteristic of the adult striatum. A retrograde tracer injection in the nigra at E18 (during the early period of homogeneous striatonigral distribution) produces a patchy striatonigral distribution if the embryo is not sacrificed until E21. The number of retrogradely labeled striatonigral cell bodies in a midstriatal section, at times immediately before and after the early homogeneous to patchy changeover did not differ significantly. We suggest that the neurons of the patch compartment of the striatum are born first and project to the substantia nigra first. The patch neurons only become restricted to “patchy” areas as the later-born matrix neurons migrate out into the striatum.