Polarity of axoplasmic microtubules in the olfactory nerve of the frog.

Pieces of olfactory nerve of the bullfrog were extracted in a tubulin assembly buffer medium containing detergents. With incubation at 37 degrees C in such medium containing soluble tubulin, ribbons of protofilaments are formed on the surfaces of microtubules, with the ribbons curving in a clockwise or counterclockwise direction. The direction of hooking reflects the polarity of the microtubule. In nerve pieces oriented such that cross sections could be viewed toward the perikarya of the axons, over 90% of the ribbons on microtubules showed a clockwise orientation. When observers were looking toward the axonal terminals, most ribbons on microtubules showed a counterclockwise direction. In single axons in which ribbons appeared on all the contained microtubules, the ribbons showed a single directionality. The evidence suggests that microtubules in axons have a single polarity, probably reflecting their assembly from the perikarya outward through the axoplasm. If bidirectional transport is assumed in these axons, it is not reflected by the polarity of their microtubules, which may mean that the directionality of transport is provided by components other than microtubules.