Reorganization and movement of microtubules in axonal growth cones and developing interstitial branches

Local changes in microtubule organization and distribution are required for the axon to grow and navigate appropriately; however, little is known abou t how microtubules (MTs) reorganize during directed axon outgrowth. We have used time-lapse digital imaging of developing cortical neurons microinject ed with fluorescently labeled tubulin to follow the movements of individual MTs in two regions of the axon where directed growth occurs: the terminal growth cone and the developing interstitial branch. In both regions, transi tions from quiescent to growth states were accompanied by reorganization of MTs from looped or bundled arrays to dispersed arrays and fragmentation of long MTs into short MTs. We also found that long-term redistribution of MT s accompanied the withdrawal of some axonal processes and the growth and st abilization of others. Individual MTs moved independently in both anterogra de and retrograde directions to explore developing processes. Their velocit ies were inversely proportional to their lengths. Our results demonstrate d irectly that MTs move within axonal growth cones and developing interstitia l branches. Our findings also provide the first direct evidence that simila r reorganization and movement of individual MTs occur in the two regions of the axon where directed outgrowth occurs. These results suggest a model wh ereby short exploratory MTs could direct axonal growth cones and interstiti al branches toward appropriate locations.