How the growth cone recognizes and responds to its environment

Four interactive processes—adhesion, guidance, migration and growth—combine to direct the axonal growth cone to its targets. It is becoming clear that the sensors of the external environment, the axonal receptors and adhesion molecules, activate second messenger systems in the growth cone. This allows a cytoplasmic integration of guidance signals acting upon the growth cone, that feeds back upon the adhesion molecules and the cytoskeleton to select the direction of growth. Movement is primarily generated by the actin microfilaments, growth is dependent upon the microtubules. This review examines the interdependence of these processes during the initial phase of axon elongation, using examples from insects to mammals.