Repulsive and attractive mechanisms for the formation of corticofugal projections

Neocortical neurones project axons to subcortical and cortical structures according to their laminar location. The axon-target interactions which produce lamina-specific efferent connections were studied in an organotypic coculture of rat visual cortex with either superior colliculus (SC) or heat-treated SC. Morphological studies with fluorescent dye demonstrated that axons from upper layer neurones did not invade living SC, which receives axonal projections from layer 5, but did extend into denatured SC. In addition, the treatment considerably reduced the number of layer 5 neurones which project to the SC. These results suggest that SC contains a repulsive factor for axonal extension from the upper cortical layer as well as an attractive factor for axonal growth from layer 5 neurones.