Organization of callosal connections in rats with experimentally induced microgyria

The anatomical organization of projections from the microgyric cortex to th e contralateral hemisphere through the corpus callosum has been studied in the rat. Microgyria was induced on the first postnatal day by a freezing in jury. Once adult, animals received injections of horseradish peroxidase in the cortex contralateral to the lesion. Callosally projecting neurons locat ed in the normally layered cortex adjoining the focal lesion showed a diffe rent laminar distribution from that observed in control animals. The highes t percentage of callosal neurons was found in layer 6 in lesioned animals, whereas layers 2/3 and 5 gave rise to most of interhemispheric projections in controls. Possible mechanisms lending to the establishment of aberrant c allosal connections and potential implications for the human pathology are discussed.