Density and distribution of microtubules in the axons of the lizard dorsal roots

The number, density and distribution of microtubules were determined in cross-sections of unmyelinated and myelinated axons of lizard (Lacerta muralis) spinal dorsal roots. In both unmyelinated and myelinated axons the average number and density of microtubules were found to be related to the axonal size. The average number of microtubules rose, while the microtubular density decreased with an increase in the cross sectional area of the axon. More precisely, a linear relationship was observed between the logarithm of the microtubular density and the cross-sectional area of the axon. The average microtubular densities were found to be the same in two samples of unmyelinated and myelinated axons of corresponding size. Therefore, it can be hypothesized that the different microtubular densities usually found in unmyelinated and myelinated axons are correlated with the different size range of the two types of axon rather than with the absence or presence of a myelin sheath. The microtubular densities nearly always proved higher in the sub-axolemmal band and around the mitochondria than in the rest of the axoplasm.