Axon guidance by diffusible repellants and attractants.

Diffusible guidance cues are now understood to play an important role in axon guidance. The past year saw the first evidence for long-range chemorepulsion of axons, and the identification of a diffusible growth cone collapse-inducing protein, collapsin. Evidence was obtained against a chemoattractant role for nerve growth factor in vivo. A neurotransmitter, acetylcholine, was shown to be capable of acting as a diffusible attractant, but, as with nerve growth factor, whether it has this function in vivo is unknown.