Region specific micromodularity in the uppermost layers in primate cerebral cortex

Abstract

A micromodularity specific to the uppermost cortical layers, layers 1 and 2, is demonstrated in primates. This is most pronounced as patches of zinc-positive (Zn+) terminations, preferentially in the pre-Rolandic and limbic areas. The upper layer modularity can frequently be demonstrated by parvalbumin-immunoreactive (PV-ir) GABAergic terminations and by bundles of apical dendrites. Double-labeling or alternate section analysis shows that PV-ir and Zn+ terminations co-mingle at the layer 1,2 border and appear to coincide with dendritic bundles, proposed to originate from layer 2 pyramidal neurons. This model is basically similar to the prominent wall-and-hollow honeycomb organization in rat visual cortex. The organization of the PV-ir and dendritic components, however, is more difficult to define in primate than in rat. Moreover, the micromodularity is not uniform across areas. In some areas (motor and limbic), the modularity can be visualized by both zinc and PV. In other areas (i.e. primary sensory, sensory associational and prefrontal areas 46 and 8), although PV immunohistochemisty shows a periodic distribution, there is no detectable Zn+ modularity. These results add to the evidence for the complexity of layers 1 and 2 and raise the possibility that patches of Zn+ terminations correspond to zones of area-specific zinc-related plasticity. This might figure in the context of top-down or feedback influences, as often associated with layer 1.