Axon guidance factors in invertebrate development

The contribution that studies in the invertebrates have made to our understanding of the factors responsible for directing axon growth is reviewed. Cellular mechanisms for axon guidance are considered, particularly the question of the accuracy of initial axon growth, and the implications of these observations for models of growth cone turning. The cellular substrates followed by growing axons during embryogenesis are identified, together with the experimental evidence that each is essential for reliable axon navigation. The significance of these studies for investigations into the molecular nature of axon guidance factors is discussed, and the likely cellular roles of putative axon guidance molecules considered.