Axonal transport of tubulin in Ti1 pioneer neurons in situ.

In neurons, tubulin is synthesized only in the cell body or dendrites, yet the growing axon requires a steady supply of this protein at the growth cone. Hence, some mechanism must exist to move tubulin from the cell body to the growth cone. Transport could conceivably occur by simple diffusion, translocation of polymer, or some form of monomer or oligomer transport. Evidence for all these has been presented in a variety of experimental systems. We have directly studied the movement of microtubules in 12 growing axons in live grasshopper Ti1 neurons in their natural environment by labeling the polymer with a caged fluorophore, biscaged fluorescein. No evidence of polymer transport was found. Hence, tubulin movement in these neurons must occur by movement of monomeric tubulin, either by transport or diffusion. To resolve these conflicting views, we discuss the conditions under which diffusion is feasible as a transport mechanism.