Ultrastructural Analysis of Axons in Temporal Lobe White Matter in Autism

Background:   From a physiological perspective, autism is often characterized as a disorder of aberrant cortical connectivity, which has been demonstrated through a number of gross functional and structural imaging methods. For example, functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) studies demonstrate desynchronized and disrupted cortical communication in autism. Additionally, diffusion tensor imaging (DTI) and structural MRI scans show abnormal growth trajectory and loss of white matter fiber integrity in autism particularly within the temporal lobes. While these studies demonstrate aberrant cortical connectivity in autism, the microanatomical mechanisms that underlie this large-scale deficit are relatively unknown and limited. Because white matter tracts serve as the conduits for which short and long distance cortical communication occurs, the ultrastructural characteristics of these axons may provide such a microanatomical basis in autism. 

Objectives:   The goal of the present study was to assess the ultrastructural characteristics of axons within the superficial and deep white matter in subregions of the temporal lobe in autism relative to their age-matched neurotypical counterparts. 

Methods:   White matter from the superior temporal gyrus and fusiform gyrus was dissected from autistic and age-matched neurotypical subjects from 50μm frozen sections. Using a novel method for tissue processing (Schumann & Liu, 2014), these samples were prepared for ultrathin sectioning and electron microscopic analysis. Myelinated axons were randomly selected for g-ratio analysis and imaged at high magnification (8,400x) whereby the myelin sheath thickness is calculated relative to the axon diameter. Data regarding the overall density and size distribution of axons are acquired at low (1,600x) and medium (4,800x) magnification, respectively, and analyzed in age-matched autistic and neurotypical subjects across a wide age range. 

Results:   Consistent with previous reports, preliminary data from the g-ratio analysis indicate decreasing proportional myelin thickness with increasing axon diameter in both autistic and neurotypical subjects. However, these data show thinner myelination patterns across all axon sizes in the autistic subjects relative to their neurotypical counterparts. Data regarding axon density and size distribution in autistic and neurotypical subjects will also be presented. 

Conclusions:   While preliminary, our data show a trend of reduced myelination across all axon sizes in the temporal lobe white matter of autistic subjects. These data are consistent with similar reports of alterations to axon ultrastructure in areas of the frontal lobe in autism (Zikopoulos & Barbas, 2010) and may provide a basis for the observed altered neuronal communication in the disorder. The results may also provide a substrate for increased diffusivity of white matter tracts in autism.