Evidence that the pharmacological strategy reveals microtubule
transport, not microtubule assembly.
a and b show a neuron treated with nocodazole, rinsed free of the drug, immediately injected with fluorescent tubulin, and then prepared for immunofluorescence visualization of microtubules 30min later. The rhodamine image (depicting fluorescent tubulin that had incorporated into microtubule polymer [a]) and the fluorescein image (immunofluorescence image of total microtubule mass [b]) are similar in appearance, indicating complete or near-complete incorporation of fluorescent tubulin into polymer. c and d show a neuron treated with nocodazole, rinsed free of the drug, simultaneously exposed to 50nM vinblastine, injected with fluorescent tubulin (see Materials and Methods), and then prepared for immunofluorescence visualization of microtubules 30min later. The immunofluorescence image shows the expected peripheral concentration of microtubules (d). The rhodamine image shows only faint background fluorescence and a fluorescent "scar" made by the injection (c). No fluorescent microtubule polymer was detected. Bar, 5Ám.