Axon substitution in the reorganization of developing neural connections.

Insights into the mechanisms of normal and pathological neural development may be gained by studying the reorganization of developing neural connections, caused experimentally or by disease. Many reorganized connections are assumed to arise by the anomalous stabilization of transient connections that occur during normal development. We report that, although the retina projects transiently to the somatosensory system in normal developing hamsters, the permanent retinal projections to the somatosensory system that arise as a consequence of early brain lesions are not formed by the stabilization of the normally transient projection. Instead, the transient retinal axons are replaced by retinal axons that do not normally project to the somatosensory system. The distinction between anomalous stabilization and substitution is significant for determining the cellular mechanisms underlying the development of neural connectivity.