Evidence from pathfinding studies in both vertebrates and invertebrates indicates that growth cones are not guided by simple stop or go signals. Rather, the navigation of growth cones through the preexisting tissue is controlled by a continuous integration of both positive and negative cues. The path taken by an axon is determined by the continuously changing situation encountered by the growth cone at any given site along the trajectory of the axon to the target. The signals derived from interactions of surface molecules with these cues provided by the environment of the growth cone are constantly changing both temporally and spatially. Therefore, each growth cone encounters a unique set of guidance cues directing it to its specific target, thus allowing for the tremendous complexity required for the guidance of millions of axons in the developing nervous system.
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