Unmyelinated and myelinated axon membrane from rat corpus callosum: differences in macromolecular structure

The macromolecular structure of unmyelinated and myelinated internodal axon membrane was examined with freeze-fracture electron microscopy. Unmyelinated axons exhibited a gradient of axonal diameters, generally ranging from 0.1 to 0.5 micron, with some unmyelinated axons of up to 0.7 micron diameter. Myelinated fibers also displayed a range of axonal diameters, with axons generally 0.3-1.0 micron. The overlap in diameters, between unmyelinated and myelinated fibers, permitted a comparison of membrane structure in myelinated and unmyelinated axons of the same diameter. Small (less than 0.5 micron) diameter unmyelinated axons exhibited a moderate density (approximately 700/micron2) of P-face intramembranous particles (IMPs), while large (greater than or equal to 0.5 micron) caliber unmyelinated axons displayed a significantly greater P-face IMP density (approximately 1100/micron2). Internodal membrane of both small (less than 0.5 micron) and large (greater than or equal to 0.5 micron) diameter myelinated fibers exhibited densities of P-face particles (approximately 1400/micron2) that were similar to each other, but significantly different from unmyelinated fibers. These results demonstrate that there are differences in membrane structure between unmyelinated and myelinated axons of similar diameter. These findings also demonstrate that membrane structure of unmyelinated axons is not invariant for all unmyelinated fibers within a given CNS tract but, on the contrary, is related to diameter.