CHANGE IN CHEMOATTRACTANT RESPONSIVENESS OF DEVELOPING AXONS AT AN INTERMEDIATE TARGET

Developing axons reach their final targets as a result of a series of axonal projections to successive intermediate targets. Long-range chem oattraction by intermediate targets plays a key role in this process. Growing axons, however, do not stall at the intermediate targets, wher e the chemoattractant concentration is expected to be maximal. Commiss ural axons in the metencephalon, initially attracted by a chemoattract ant released from the floor plate, were shown to lose responsiveness t o the chemoattractant when they crossed the floor plate in vitro. Such changes in axon responsiveness to chemoattractants may enable develop ing axons to continue to navigate toward their final destinations.