Cytoarchitectonic heterogeneity of the primate neostriatum: subdivision into Island and Matrix cellular compartments.

The cytoarchitecture of the caudate nucleus was examined in Nissl-stained sections from rhesus monkeys, in some of which the corticostriatal terminals had also been labeled by anterograde transport of tritiated amino acids injected into prefrontal cortex. The cytoarchitectonic analysis revealed the existence of two cellular compartments that could be distinguished on the basis of cell size, density, orientation, and tinctorial properties: (1) cell islands consisting of approximately 1,500 to 15,000 densely packed neurons that form aggregates of variable shapes and sizes embedded in (2) a matrix compartment of slightly larger and more loosely packed neurons that comprise the remaining and greater part of the caudate nucleus. In coronal sections, cellular islands appear mostly as round or elliptically shaped areas, 300-600 micrometer in diameter, but can assume more elongated and complex forms particularly in the sagittal and horizontal planes. They are encircled by fibers arranged in a thin, cell-sparse capsule that sets them apart from the matrix compartment. Analysis of cellular organization and corticostriatal connections in counterstained autoradiograms indicates that the prefrontal cortex projects only to the matrix zone and not to the territory occupied by island cells. Therefore, according to present observations the neostriatum in primates should be viewed as a cytoarchitectonically heterogeneous structure composed of at least two distinct cellular compartments with specific connectivity. These compartments may be related to the histochemical and functional diversity of the neostriatum.