En passant neurotrophic action of an intermediate axonal target in the developing mammalian CNS.

During development, neurons extend axons to their targets, then become dependent for their survival on trophic substances secreted by their target cells. Competition for limiting amounts of these substances is thought to account for much of the extensive naturally-occurring cell death that is seen throughout the nervous system. Here we show that spinal commissural neurons, a group of long projection neurons in the central nervous system (CNS), are also dependent for their survival on trophic support from one of their intermediate targets, the floor plate of the spinal cord. This dependence occurs during a several-day-long period when their axons extend along the floor plate, following which they develop additional trophic requirements. A dependence of neurons on trophic support derived en passant from their intermediate axonal targets provides a mechanism for rapidly eliminating misprojecting neurons, which may help to prevent the formation of aberrant neuronal circuits during the development of the nervous system.