The cytoskeleton in myelinated axons: Serial section study

Abstract

The three-dimensional arrangement and continuity of microtubules and neurofilaments in myelinated axons of the mouse saphenous nerve were analyzed using the serial section method. The course of individual microtubules was followed in two sets of transversely cut serial sections for the internodal region and in one set for the nodal region, and approximately 97%, 96%, and 94% of all microtubules, respectively, were continuous within the examined 10 μm-long segments. Assuming that all microtubules are homogeneous in length, their average length was calculated to be about 370-760 μm. Furthermore, the number of microtubules was not constant but fluctuated slightly along the 185 μm-long segment of one myelinated axon. The results favor the hypothesis that microtubules in the myelinated axon are long, but do not extend the entire length of the axon. On the other hand, neurofilaments decreased remarkably in number at the node of Ranvier, indicating that most of them were discontinuous at the nodal region.