SignificanceWe demonstrate that the number of mature neurons in the human amygdala increases from childhood into adulthood. This trajectory may be due to the incorporation of immature neurons from the paralaminar nucleus in the ventral amygdala. In contrast, individuals with autism spectrum disorder (ASD) show an initial excess of mature neurons followed by a decline into adulthood. Our results suggest a degenerative component in ASD and highlight the need for a more comprehensive understanding of the protracted cellular development of the human amygdala for multiple psychiatric disorders.

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Atypical cortical organization and reduced integrity of the gray–white matter boundary have been reported by postmortem studies in individuals with autism spectrum disorder (ASD). However, there are no in vivo studies that examine these particular features of cortical organization in ASD. Hence, we used structural magnetic resonance imaging to examine differences in tissue contrast between gray and white matter in 98 adults with ASD and 98 typically developing controls, to test the hypothesis that individuals with ASD have significantly reduced tissue contrast. More specifically, we examined contrast as a percentage between gray and white matter tissue signal intensities (GWPC) sampled at the gray–white matter boundary, and across different cortical layers. We found that individuals with ASD had significantly reduced GWPC in several clusters throughout the cortex (cluster, P < 0.05). As expected, these reductions were greatest when tissue intensities were sampled close to gray–white matter interface, which indicates a less distinct gray–white matter boundary in ASD. Our in vivo findings of reduced GWPC in ASD are therefore consistent with prior postmortem findings of a less well-defined gray–white matter boundary in ASD. Taken together, these results indicate that GWPC might be utilized as an in vivo proxy measure of atypical cortical microstructural organization in future studies.

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