Fig. 2

Myelinated axons in layer 1 of prefrontal cortex show significant changes in development and in autism. a The percent surface area occupied by axons in children is low (0.89 ± 0.29%), but increases significantly to 5.4 ± 0.44% in adults. b Axon trajectories are less variable in children than in adults in neurotypical development, suggesting increased myelination of diverse pathways in adulthood. c The percent surface area occupied by myelinated axons was similar in autism and neurotypical groups. The percent surface area occupied by axons increased significantly in adults from both groups (p = 0.000), shown on a case-by-case basis (left graph) and average by age group (right graph). d We analyzed images of layer 1 from the atlas of Kaes [62] to cross-validate our findings. Trends of myelin development in layer 1 of the anterior and posterior frontal lobes, which are similar to anterior and lateral prefrontal cortices, and Gyrus Fornicatus, which includes the anterior cingulate cortex, showed increases in myelination with age. In the posterior frontal lobe, layer 1 in children aged 0–3 was less myelinated than older children and adults. The anterior frontal lobe and anterior cingulate cortex had a more prolonged period of myelination and overall had a lower level of myelination compared with the posterior frontal lobe. e In children with autism there was a significant increase in the variability of axon trajectory heterogeneity when compared to neurotypical children (p = 0.033). In adults, this variability was similar between groups. f The relative proportion of thin axons remained approximately stable through neurotypical development. The solid line shows the overall stable trend, the dotted line shows the increase in the relative proportion of thin axons when comparing children and young adults. There was a trend towards a decline in the proportion of thin axons with age in adulthood. g The proportion of thin axons in neurotypical children was on average 64%; in children with autism, there was significant variation in the percentage of axons that were thin (range: 47–75%). Neurotypical adults had a higher average proportion of thin axons than adults with autism (mean control = 73.6 ± 7.7%, mean autism = 57.8 ± 4.1%, p = 0.034). h The relative proportion of medium-caliber axons remained approximately stable through neurotypical development. The solid line shows the overall stable trend, the dotted line shows the decrease in the relative proportion of medium-caliber axons when comparing children and young adults. There was a trend towards an increase in the proportion of medium-caliber axons with age in adulthood. i There was significant variability in the proportion of medium-caliber axons in children with autism. In adulthood, there was a significant increase in the relative proportion of medium-caliber axons in adults with autism compared with neurotypical adults (mean control = 22.5 ± 5.1%, mean autism = 36.2 ± 1.8%, p = 0.011). j There was a trend towards an increase in the relative proportion of thick axons in neurotypical development. k Developmental trends in the proportion of thick axons were not different between autism and control groups. Left panels in e-k present developmental trends on a case-by-case basis, and right panels show mean ± SD for children and adults with and without an autism diagnosis. Autism cases 4021, 4029, AN 03221, 5144, 1182, B-5173, B-6232, B-6677, and control cases 451, 4203, 4337, 3835, B-6004, B-5353, and B-4981 were used for electron microscopy comparisons. Control adults HAW and HAY were included in the study of neurotypical development and aging