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."
Thank you, Dr. Cohen!
8 comments:
Marc Cohen may be right about the relative preservation of subcortical U fibers (also known as the fibers in the "arcuate zone" because these are fibers making cortical-cortical local connections through the superficial white matter) but the phenomenon is seen in other conditions where this explanation is unlikely. For example in old contusions with extensive underlying white matter degeneration the U fibers just beyond the zone of actual destruction are typically spared whereas deeper axons and their myelin are degenerate. Irwin Feigin wrote a lot about this phenomenon in the 1960s and 1970s, and he believed that this was due to some toxic effect of "edema". I don't believe Irwin was correct, but his observations are accurate.
-Douglas C Miller MD,PhD
So there may be some resistance to damage that these fibers possess that deeper fibers in the white matter do not possess. Hmmm.
As further follow-up: Irwin Feigin measured mucopolysaccharid content in brain sections separately for cortex, arcuate zone white matter, and deep white matter, and the content differed significantly in each area. This may have something to do with the relative resistance to some kinds of damage exhibited by arcuate zone white matter.--Doug Miller
This is fascinating! :-)
I wish I had the time to investigate these issue more closely. :-(
Any idea why PML likes to involve the subcortical U fibers?
These seem to conflict with disorders which u-fiber involvement is an early imaging finding- e.g. 3-methylglutaconyl-CoA hydratase related leukoencephalopathy?
Dylan
Thanks for the discussion! Learned something new today!
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