Opposite development of short- and long-range anterior cingulate pathways in autism

Abstract

Autism has been linked with the changes in brain connectivity that disrupt neural communication, especially involving frontal networks. Pathological changes in white matter are evident in adults with autism, particularly affecting axons below the anterior cingulate cortices (ACC). It is still unknown whether axon pathology appears early or late in development and whether it changes or not from childhood through adulthood. To address these questions, we examined typical and pathological development of about 1 million axons in post-mortem brains of children, adolescents, and adults with and without autism (ages 3–67 years). We used high-resolution microscopy to systematically sample and study quantitatively the fine structure of myelinated axons in the white matter below ACC. We provide novel evidence of changes in the density, size and trajectories of ACC axons in typical postnatal development from childhood through adulthood. Against the normal profile of axon development, our data revealed lower density of myelinated axons that connect ACC with neighboring cortices in children with autism. In the course of development the proportion of thin axons, which form short-range pathways, increased significantly in individuals with autism, but remained flat in controls. In contrast, the relative proportion of thick axons, which form long-range pathways, increased from childhood to adulthood in the control group, but decreased in autism. Our findings provide a timeline for profound changes in axon density and thickness below ACC that affect axon physiology in a direction suggesting bias in short over distant neural communication in autism. Importantly, measures of axon density, myelination, and orientation provide white matter anisotropy/diffusivity estimates at the level of single axons. The structural template established can be used to compare with measures obtained from imaging in living subjects, and guide analysis of functional and structural imaging data from humans for comparison with pathological states.Electronic supplementary materialThe online version of this article (10.1007/s00401-018-1904-1) contains supplementary material, which is available to authorized users.

Imbalance of Laminar-specific Excitatory and Inhibitory Circuits of the Orbito frontal Cortex in Autism

“…Analysis showed that axon orientation variability appeared to be consistent across layers in OFC gray matter, but more heterogeneous than previously reported in the super cial white matter below prefrontal cortices [36]. Taken together with previous studies, our ndings show that axons become thinner and their trajectory becomes more heterogeneous, likely due to increased branching, as they transition from deep to super cial white matter, and nally enter the gray matter [27,36].…”

“…Each sample yielded a large number of data points, because we typically examine a large volume fraction of the areas sampled, and thus increase the number of individual axons and neurons examined. The high sampling fraction used in our studies minimized the variability within each analyzed case, and further increased statistical power, as we have described [34][35][36][37][38]. Our previous work [5,27,[35][36][37] and power analysis showed that the sampling ratios exceed the samples needed to detect differences with a greater than 90% probability, and large effect population size (0.8) with ≤ 10% error, as recommended [39].…”

“…The high sampling fraction used in our studies minimized the variability within each analyzed case, and further increased statistical power, as we have described [34][35][36][37][38]. Our previous work [5,27,[35][36][37] and power analysis showed that the sampling ratios exceed the samples needed to detect differences with a greater than 90% probability, and large effect population size (0.8) with ≤ 10% error, as recommended [39]. We further minimized variability, using only the OFC from right hemispheres of adults (mean age of Control group ± SD: 49.8 ± 15.2; mean age of ASD group ± SD: 36 ± 7.1) with similar PMI (mean PMI of Control group ± SD: 24.2 ± 6.7; mean PMI of ASD group ± SD: 25 ± 8.3).…”

“…EM processing and imaging EM processing and imaging was done as previously described [35,36]. Brie y, OFC sections matching Nissl-and immunostained sections were processed for EM using a high contrast method [35,36]. Sections were rst rinsed in 0.1 M PB and post xed in 6% of glutaraldehyde.…”

“…Measured morphometrics were grouped for comparison, and included axon size, pro le area, and shape indicators (min and max diameters and circularity) that were used to estimate axon density (number of cross-sections per unit), surface area ratio (total surface area/image area), and minor diameter (excluding myelin) as indication of thickness. In addition, axon trajectory was estimated by the angle of the cross-section, as described previously [36]. We estimated the variation of angle value in each sampled image, as a measurement of axon trajectory variability.…”