The laminar organization of efferent neuronal cell bodies in the retrosplenial granular cortex

The fluorescent dye technique for retrograde axonal tracing was used to characterize the laminar organization of neurons in the rat retrosplenial granular cortex (Rg) that project to cortical or thalamic targets. The data from 40 experiments demonstrate that each of the projections studied arises from a slightly different layer of cells. The commissural projection from Rg to Rg can be divided into two categories. A point to point projection originates in the superficial layer II neurons and a somewhat more widespread projection originates from neurons in layers III, V b and V c. The intrinsic projection from Rg to Rg arises from cell bodies that are in the same laminar position as the cells of origin of the commissural projection, but there is an additional input from neurons in layer VI b. In contrast to the homotopic projections, the commissural projection from Rg b to retrosplenial agranular cortex (Rag) originates in layer V a, while the ipsilateral projection from Rg b to Rag originates in layer V a, V c and VI b. All of the input to the anterior thalamic nuclei originates in layer VI a, but while the ipsilateral projection originates in both superficial and deep layer VI a neurons, contralateral projections do not arise from cells in the deepest part of layer VI a.