Formation of crossed and uncrossed projections in the central nervous system

Cellular mechanisms for decision of laterality of projections are discussed based on the observation of corticorubral and interpositorubral projections in the kitten. In both systems unilateral projections in adults appear to emerge from bilateral projections which occur early in development. Early unilateral lesions of the cerebral cortex or the interpositus nucleus cause persistent bilateral projections. Thus the neurons which eventually make a unilateral projection have the capability of forming synaptic connections with their target cells on both sides of the brain. Aberrant crossed (or uncrossed) projections in neonatal animals retract without forming complex axonal arbors, while those in lesioned animals form complex axonal arbors. Thus, success or failure in forming axonal arbors may be crucial for the maintenance of the aberrant crossed (or uncrossed) projections.