The following sections clarify the differences between complete agenesis of the corpus callosum (ACC), partial ACC, hypoplasia of the corpus callosum, and dysgenesis of the corpus callosum.
Complete Agenesis of the Corpus Callosum
If the nerve fibers don't cross between the hemispheres during that
critical prenatal time, they never will. ACC becomes a permanent
feature of the individual's brain. The callosal fibers may have started
to grow, but when unable to cross between the hemispheres, they grow
toward the back of the same hemisphere where they began. These fibers
form what are called Bundles of Probst.
Some smaller connections between the hemispheres develop in most individuals with ACC. These are the anterior commissure, posterior commissure, and hippocampal commissure.
However, each of these is at least 40,000 times smaller than the corpus
callosum. Thus, they cannot compensate completely for the absence of
the corpus callosum.
Partial Agenesis of the Corpus Callosum
In partial ACC, the corpus callosum began to develop, but something
stopped it from continuing. Since the corpus callosum develops from
front to back, the part of the corpus callosum that is present in
partial ACC usually will be toward the front of the brain, with the
back portion missing. Partial ACC includes the entire
range of partial absence, from absence of only a small portion of
callosal fibers to absence of most of the corpus callosum. In partial
ACC, the other smaller commissures usually are present.
Hypoplasia of the Corpus Callosum
Hypoplasia refers to a thin corpus callosum. On a mid-line view of
the brain, the structure may extend through the entire area
front-to-back as would a typical corpus callosum, but it looks notably
thinner. It is unclear in this case if the callosal nerve fibers are
fully functional and just limited in number, or if they are both less
plentiful and more dysfunctional.
Dysgenesis of the corpus callosum
Dysgenesis means that the corpus callosum developed, but developed
in some incomplete or malformed way. Thus, partial ACC and hypoplasia
of the corpus callosum would be forms of dysgenesis, as would any other
form of inadequate callosal development. Dysgenesis is a broad term for
any malformation of the corpus callosum that is not a complete absence
(agenesis).
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