A Golgi study on the neuronal organization of the interhemispheric cortex in the mouse. I. Projection neurons.

Projection neurons in the interhemispheric cortex (IHC) of the mouse were studied by the rapid Golgi method. Five layers were discerned in the IHC. Projection neurons in layer I had stellate or piriform cell bodies with dendrites which were distributed in layers I and II. The cell bodies of projection neurons in layer II were fusiform, piriform, triangular or stellate in shape. Fine axons of these neurons sent collaterals mainly to layer IV. Projection neurons in layer III were medium-sized pyramidal, and small spindle cells. Basal dendrites of the former neurons were distributed mainly in layer III, while those of the latter neurons extended into layer IV. Projection neurons in layer IV were largely pyramidal, medium-sized pyramidal, medium-sized fusiform, and small cells. In the large pyramidal cells, the basal dendrites were distributed mainly in layer IV, and the apical dendrites extended into layer I. The axons of these neurons sent collaterals to all cortical layers. In layer V, spindle and small stellate projection neurons were observed. All apical dendrites of projection neurons in layers I-III extended into layer I, whereas some apical dendrites of projection neurons in layers IV and V did not reach layer I.