Demyelination induces the decline of the myelinated fiber length in aged rat white matter.

To determine the exact reason for the age-related decline of the myelinated fiber length in white matter, we performed this study. In middle-aged rats, there was age-related loss of the unmyelinated fibers with large diameters. The demyelination of the myelinated fibers with small diameters in middle-aged rat white matter might make the age-related decrease of the unmyelinated fibers with small diameters in the white matter unnoticeable. However, in old-aged female rats, the unmyelinated fibers with large and small diameters significantly degenerated together and that the unmyelinated fibers formed from the demyelination of the myelinated fibers could not replenish the age-related loss of the unmyelinated fibers in the white matter. In conclusion, this study suggested that demyelination of myelinated fibers with small diameters in aged white matter might be the key mechanism of the significant decline of the myelinated fiber length in aged white matter.