Axonal microtubules: comparative anatomy in vertebrates, including man.

The microtubular density was assessed with the electron microscope in 3 microns myelinated fibers, myelin excluded, of 11 species from the following classes: Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia. The average for all species was 20.6 microtubules/microns 2. Dispersion of values was restricted as shown by a coefficient of variation of 15.8. The microtubular content of nonmedullated axons was assessed in trout, lizard, finch, and man. In the four species, the number of microtubules increased with the cross sectional area of the axon. In trout, lizard and finch, the microtubular density decreased from over 100 microtubules/microns 2 in fibers smaller than 0.1 micron 2 to about 30 in 1 micron 2 fibers; in axons of equal size, the packing of microtubules of nonmedullated was similar between them, and with reported values for peripheral axons of cat and rat. In man, the microtubular density of nonmedullated fibers exhibited only a mild decrease with the axonal size. In the finch, myelinated and nonmedullated axons overlapped in the range 0.23-0.60 micron 2 and both groups exhibited similar microtubular densities. We conclude that the packing of microtubules of the vertebrate peripheral axon is a feature largely conserved during evolution.