Autism Spectrum Disorder (ASD), characterized by deficits in complex processing such as social communication and interaction as well as repetitive behaviors, is considered to be related to the connectivity between neurons in the brain. The corpus callosum is a bundle of nerve fibers that connects the left and right hemispheres of the brain, facilitating the integration of information, such as those related to sensation and movement, between the two.
In an electron microscopic study, researchers analyzed the corpus callosum of 9 pairs of subjects with autism and control subjects. An electron microscope has the ability to visualize tissues on a molecular level which makes it the perfect candidate for exploring neuronal connections. The corpus callosum was divided into 5 segments, and in each segment, axons of neurons were measured for their diameter, area, and myelin thickness*. These factors are associated with the speed and volume of signal transmission between neurons.
*Axons are the part of the neuron that transmits electrical impulses to other neurons. Myelin is a layer of phospholipids and proteins that insulate the neuron to allow for the electrical impulses to travel faster.
In ASD patients, a thinner corpus callosum was observed. In certain segments of the corpus callosum especially, there was an increase in the percentage of axons with smaller diameter and a decrease in percentage of axons with larger diameter in ASD patients compared to the control as well. No significant difference in myelin thickness was found. These differences between the brains of ASD patients and control subjects can be traced to abnormalities in either the mechanism controlling the number of neuron connections between the hemispheres, or the mechanism that directly controls the structure of the neuron, including axon diameter, area, and myelin thickness.
Corpus callosum agenesis (when an organ fails to develop properly during the embryonic stage) and hypoplasia (the underdevelopment or incomplete development of an organ) both lead to a reduced number of axons, and therefore a loss of connectivity between the right and left hemispheres of the brain compared to the average brain. Other studies have linked ASD with other developmental issues, such as abnormal neurogenesis during a critical embryonic stage. These developmental brain anomalies may explain the loss of connectivity necessary for higher-order cognitive functions seen in ASD patients.