FrontalCortex.com features lots of neuropathology, including podcasts from one of our favorite neuropathologists, Dr. Mark Cohen. In a podcast on demyelinating diseases, Dr. Cohen clarifies questions I've long harbored about the so-called subcortical U-fibers. The photomicrograph above (from the textbook Neuropathology by Ellison and Love) shows a blue myelin stain of an adrenoleukodystrophy case where the subcortical U-fibers are spared. The following is a transcription of Dr. Cohen's insightful comments on this topic. I should first clarify that when Dr. Cohen talks about the subcortical U-fibers being the "slowest myelinating fibers within the nervous system", he is not talking about conduction velocity, but rather about how long they take during one's lifetime to get completely myelinated. OK, here's Cohen:
"As you read through either your textbooks or the literature on leukodystrophies, you'll inevitably comes across a statement which describes either preservation, or lack of preservation, of the subcortical U-fibers. The subcortical U-fibers are, as the name implies, 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."