Complete Agenesis of the Corpus Callosum (ACC)
If the nerve fibers don’t cross between the hemispheres during that critical prenatal time, they never will. Agenesis (or absent) of the corpus callosum 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.