Studies using magnetic resonance imaging allow us to measure degree of myelination in the live human brain, but the measures do not have the degree of resolution that post mortem studies do.  Nevertheless, the corpus callosum is a large tract, and it should be possible to chart its development reasonably well.

Most recently, using magnetic resonance imaging, Schneiderman, et al* found that adolescents and adults differ in degree of myelination of several neural tracts of the brain.  They found, however, that adolescents and adults showed no difference in degree of myelination of the corpus callosum.  Apparently, the corpus callosum is fully myelinated by adolescence.  This finding is compatible with Yakovlev and Lecours’ finding.

Further, the corpus callosum being fully myelinated a bit before adolescence fits the experience that hemispherectomy for intractable epilepsy, when a child is 7 or so, results in minimal impairment—that is, as long as communication between the right and left hemisphere is still in flux, one hemisphere may take over for the other.  However, at a later age once the corpus callosum is fully myelinated, hemispherectomy may be expected to have devastating results.

*Diffusion Tensor Anisotropy in Adolescents and Adults. J. S, Schneiderman, et al. in Neuropsychobiology, Vol. 5, No. 2, pages 96-111; 2007.

A. J. Malerstein, M.D.

The editors at Scientific American assured me that they read all letters.  So I concluded that they decided not to publish my letter.

I then got to thinking that I had been hasty in my statement that Dr. Fields was inaccurate when he wrote in his article that, in adults, 30% of the fibers of the corpus callosum remain unmyelinated.   Nonetheless in his letter to me, he erred in his memory of Yakovlev and Lecours’ finding.  It is true that they found a range of time during which the corpus callosum of the individual child becomes fully myelinated.  However, that range extends only till 7 or 10 years of age with no question mark at the end of the range.  When they were uncertain about when myelination of a tract was complete, they added a question mark to the range indicated by dashes on their chart.

Although he was wrong in thinking that Yakovlev and Lecours were uncertain when myelination of the corpus callosum was complete, it must be recognized that their study was a comparative study—that is, they compared the depth of staining of myelin of the corpus callosum in children at different ages to the depth of staining of myelin of the corpus callosum in an adult.  Likewise, the Schneiderman study was also a comparative study—that is, as measured by magnetic resonance imaging, the corpus callosum shows no increase in myelination from adolescence to adulthood, while other neural tracts do show an increase in myelination during that time period.  But neither study rules out the possibility that a significant proportion of fibers of the adult corpus callosum could be unmyelinated, and not be detected by the methods that the two studies used.

At the same time, I realized that, whether a substantial portion of fibers remained unmyelinated was not really the issue.  Both the Yakovlev and Lecours and the Schneiderman studies showed that during adolescence, and possibly as early as age 10, degree of myelination does not change.  Hence, the corpus callosum could be expected to be a stable transmitter of information between the two hemispheres.  Such stable transmission would support stable hemisphere specialization.  Prior to stable transmission, one would expect one hemisphere could take over the other’s function, if necessary.  And this is what we find.  Hemispherectomy, done early enough in childhood, results in very little impairment.  The remaining hemisphere takes over for the missing hemisphere.

Neither Yakovlev and Lecours’ or Schneiderman and his colleagues’ findings determine whether the adult corpus callosum might include a large portion of unmyelinated fibers.  But both findings suggest that, by preadolescence or at least during adolescence and into adulthood, the human has no appreciable change in myelination of the corpus callosum. Hence, the preadolescent or at least the adolescent has stable communication between hemispheres to work with in allocations of right-left brain functions—for example, motor control or language.

So from a functional point of view, the issue is not whether there are unmyelinated fibers in the adult corpus callosum.  The issue is whether those fibers will ordinarily go on to become myelinated—whether transmission of information between hemispheres in adults remains plastic—that is, unstable.  It appears that by two measures—magnetic  resonance imaging and post-mortem myelin staining—myelination of the corpus callosum is stable between preadolescence or so into adulthood.  Hence, transmission of information between hemispheres is stable during that time period.

Nonetheless, I decided to try to find the source of Fields’ assertion in his paper that 30% of the fibers of the adult corpus callosum were unmyelinated.

What I found was: Prior to the studies of Aboitiz et al,* the only report that estimated the proportion of unmyelinated fibers/axons in the human adult corpus callosum was by Tomasch**.  His sample consisted of 4 males.  In his examination of four different areas of the corpus callosum, he found that unmyelinated fibers were as high as 50% and that the average was about 40%.  His method of calculating the number of unmyelinated fibers was to subtract the number of fibers that stained for myelin from all of the fibers, by using a stain that stained for fibers.  On page 129, he cautioned that, because of his method, “It is quite possible…..that a portion of the fibers classified in this group [of unmyelinated fibers] could be very thinly myelinated.”

In contrast to Tomasch’s study, the studies by Aboitiz et al included thinly myelinated fibers.  Aboitiz and his coauthors were particularly interested in myelinated neurons of different sizes, because both the size of fiber and whether it is myelinated determine speed of transmission of impulses by that fiber.  Speed of transmission provides clues regarding the function of a tract.  For example, very fast transmission of visual or auditory information from the one hemisphere to the other is important in determining rapid location of sensory input in order to locate a hazard.  Transmission of information between various association areas of the two hemispheres could be served by small myelinated fibers.  Transmission does not have to be that fast.

In the Aboitiz study of the corpus callosums of 10 men and 10 women, they report on page 148, “Unmyelinated fibers were scarce, except in the genu, where they were found to comprise about 16% of the total fibers.”  These fibers were small—that is, slow-conducting.  “In the rest of the callosal regions, there were almost no unmyelinated fibers (usually less than 5%).”  So, now we have an actual account that shows that almost all areas of the corpus callosum are fully myelinated in adults—that my hasty assertion in the letter to Scientific American was basically correct.

The genu of the corpus callosum (sits below and) connects the two frontal lobes of the brain—parts of the brain that have something to do with executive function.  Is this where plasticity in adults is still helpful?

Similarly, in part of the hippocampus, which has a role in acquisition of new information, new neurons are generated.  Is this where plasticity is helpful?

Determination of which brain structures change and which become stable, and when they do so, is a clue for understanding roles that certain brain structures play in brain function.  I propose that near complete myelination of the corpus callosum helps set right-left brain specialization.

*Aboitiz, F. Scheibel, A. B. Fisher, R. S. and Zaidel, E.. Fiber composition of the human, corpus callosum, dram Researn :W (No2) Elsevier Science Publishers (1992) 143-153.
Aboitiz, F. Zaidel, E. and Sheibel, A.B.. Variability in fiber composition in different regions of the corpus callosum in humans, Anal. Rec. 223 (1989) 6A.
**Tomasch, J. Size, distribution, and number of fibers of the human corpus callosum, Anat. Rec. 119 (1954) 119-135.