Microtubules of frog olfactory axons: their length and number/axon.

Counts of the number of microtubule profiles in cross-sections of axons of bullfrog olfactory nerves indicates that about 80% of them have 2 or 3 microtubules, regardless of whether one samples axons in the proximal or distal ends of the nerve. In the olfactory lobe of the brain, however, only 50% of the axon profiles showed 2 or 3 microtubules. In the brain, 30% of the olfactory axons showed 4 or more microtubule profiles, compared with only 4% in axons sampled from the nerve proper. By following microtubule profiles in sets of serial cross-sections of frog olfactory axons, data were obtained indicating that, in the olfactory nerve proper, the average length of the microtubules is about 400 microns. In olfactory axons sampled in the brain, however, the average microtubule length is about 5-fold less than that found for microtubules in the nerve proper. The evidence suggests that as the olfactory axons approach their synaptic terminations in the brain, the microtubules become fragmented into shorter segments which may reflect their depolymerization.