Microtubules released from the neuronal centrosome are transported into the axon

There is controversy concerning the source of new microtubules required for the development of neuronal axons. We have proposed that microtubules are released from the centrosome within the cell body of the neuron and are then translocated into the axon to support its growth. To investigate this possibility, we have developed an experimental regime that permits us to determine the fate of a small population of microtubules nucleated at the neuronal centrosome. Microtubules within cultured sympathetic neurons were depolymerized with the anti-microtubule drug nocodazole, after which the drug was removed. Microtubules rapidly and specifically reassembled from the centrosome within three minutes of nocodazole removal. At this point, low levels of vinblastine, another anti-microtubule drug, were added to the culture to inhibit further microtubule assembly while not substantially depolymerizing the small population of microtubules that had already assembled at the centrosome. Within minutes, released microtubules were apparent in the cytoplasm, and many of these had already translocated to the cell periphery by ten minutes. By one hour, virtually all of the microtubules had been released from the centrosome and were concentrated at the cell periphery. With increasing time, these microtubules appeared within and progressively farther down developing axons. Nonneuronal cells within the culture also reassembled microtubules at the centrosome, but only a small portion of these microtubules were released. These observations indicate that microtubules were released from the neuronal centrosome and transported into growing axons, and that microtubule release and relocation from the centrosome are especially active in neurons compared to nonneuronal cells.