What are subcortical U fibres?

These are myelinated fibers just at the junction of the gray matter and the white matter which travel in a tangential, rather than radial, fashion connecting areas of cortex to other areas of cortex. What's special about these U-fibers, and why they are either spared or not in leukodystrophies, is that they comprise the slowest myelinating fibers within the nervous system. These U-fibers begin myelination early in gestation and often aren't completely myelinated until the third or fourth decade of life. Therefore, leukodystrophies in which the pathology is dependent on myelin turnover will demonstrate relative sparing of these fibers as the turnover is extremely slow; while in leukodystrophies which depend on toxic damage to the oligodendroglial cell, subcortical U-fibers are as vulnerable as other myelinated fibers within the nervous system.