Guidance and induction of branch formation in developing axons by target-derived diffusible factors.

Developing axons appear to navigate to their targets partly in response to long-range diffusible chemoattractants secreted by target cells. The past year has seen the identification of the first target-derived attractants for developing axons, the netrins. Target-derived diffusible factors have also been implicated in controlling the elaboration of axon collateral branches, and a recent study provides insight into the mechanism by which this is achieved. Evidence has also suggested neurotrophins and neurotransmitters as candidate branching and chemotropic factors.