Microtubules and axonal growth.

Twenty years of controversy have not produced a consensus concerning the mechanisms by which the microtubule array of the growing neuronal axon is established. At the heart of the controversy is the issue of whether tubulin is actively transported down the axon as assembled microtubules or as free subunits. This past year has seen the publication of several new studies relevant to this exciting and fundamental issue. Some of these studies failed to reveal evidence for the transport of assembled microtubules. Other studies, however, that used exciting new pharmacological, live-cell and molecular approaches, provide compelling new evidence that assembled microtubules are indeed the form in which tubulin is actively transported down the axon.