Ontogenetic changes in the projections of neocortical neurons

Development changes in the origins of contra- and ipsilateral projections from the rat parietal cortex were examined using the retrograde transport of horseradish peroxidase and of the fluorescent dyes fast blue and nuclear yellow. The aim of the study was to determine the fate of the neurons which send axons through the corpus callosum in the neonatal, but not the young adult, rat. 1. Neurons which project across the corpus callosum at postnatal day 8 (PND 8), but not at PND 20 or later, do not die; they merely lose their contralateral processes. 2. At PND 8, many of these neurons--in particular, those in lamina Va of the barrel field area--project simultaneously across the corpus callosum and to the ipsilateral motor cortex. 3. By PND 20 and later, many neurons in lamina Va of the barrel field area still project to the ipsilateral motor cortex but not across the corpus callosum. We conclude that at least some neocortical neurons initially elaborate more axonal processes than they will maintain and that they selectively eliminate major projections during development.