Control of cell number in the developing neocortex. II. Effects of corpus callosum section.

To determine if cell death participates in the regulation of cell number between interconnecting populations of the neocortex, we sectioned the corpus callosum of neonatal hamsters, thus depriving callosally projecting cells of their normal targets and callosally-recipient cells of their normal afference. The numbers of neurons per unit column in two areas of the cortex which have heavy callosal projections (the 17-18a border and area 6) and one area that is relatively acallosal (area 3) were compared in animals with early corpus callosum sections and controls. No differences were found, either for a 'unit cortical column,' or for the callosally-projecting layers (II-III and V). Mean soma sizes in layers II-III and V of all three areas were likewise unchanged. In area 6 and part of area 3, however, the distribution of soma sizes in callosally projecting and recipient laminae was significantly altered. The change in size distribution without change in mean soma area suggests that the cortex responds to the elimination of the callosal pathway in more than one way. Since no role for cell death in removal of diffuse connectivity or in target regulation of neuron number has yet been found, a new hypothesis for the function of cell death in local cytoarchitectural differentiation of the cortex is proposed.