Some of the molecules important for neuronal outgrowth and pathfinding were first identified in in grasshopper, Drosophila , and C. elegans . The growth cone continually expands and contracts. In general, they move towards the most stable contacts. Different types of neurons adhere to different ECM components and, therefore, take different paths. Surrounding cells secrete paracrine signals that can bind to growth cone receptors and influence migration patterns.
Some ligands promote migration.
Other ligands inhibit migration. The competence of the neuron to respond to these signals is cell-type dependent. Ephrins bind Eph receptors, which have tyrosine kinase domains. Activating the Eph receptor in NC cells causes them to stop migrating. See figure 13.4 Recall that neural crest cells leave the neural tube and migrate through the anterior portion of the somites.