Intrinsic programmes of growth and survival in developing vertebrate neurons.

Neurons become dependent on a supply of target-derived neurotrophins for survival when their axons reach their targets in development. Because the distance axons have to grow varies from one population of neurons to another, the timing of dependence on neurotrophins likewise varies. Although it would be expected that the simplest way of getting the timing right would be for the target to provide a suitable signal to the arriving axons, detailed studies on developing cranial sensory neurons suggest that these neurons are programmed, before differentiation, to acquire dependence at the correct time independently of external signals. Neurons not only partly compensate for different target distances by extending axons more rapidly the further they have to grow, but possess an intrinsic clock that switches on dependence at the right time in accordance with the time it normally takes their axons to reach their targets. Here the experimental evidence for these intrinsic programmes of growth and survival is reviewed, the rationale that might have favoured their evolution is discussed, and parallels are drawn with other developing neuronal systems.