Autism is a neurodevelopmental disorder of social cognition that is marked by frontally pronounced early brain overgrowth. Functional magnetic resonance imaging studies have shown that the regions of the brain that display the greatest overgrowth early in life, such as dorsolateral prefrontal cortex, also demonstrate significant deficits in older children and adults with the disorder. However, there is relatively little information about the cellular abnormalities that underlie these observed macrostructural and functional alterations. A single study has directly observed altered glial morphology in the postmortem autistic frontal cortex, while a handful of studies have observed neuron loss in older subjects with autism. Therefore, we conducted a series of experiments aimed at elucidating both population and organizational abnormalities in the microglial and neuronal populations in the autistic dorsolateral prefrontal cortex. In chapter II of this thesis, we use optical disector and isotropic nucleator to describe Iba-1 positive microglial density and volume in an unprecedentedly large sample of postmortem autism tissue. Our findings indicate that increased density and somal volume are present in a sizeable fraction of subjects with autism from an early age, including during the period of early brain overgrowth, and that these alterations are not attributable to seizure. In chapter III, we turn to an examination of average horizontal neuron distance in the same set of subjects. We describe an increase in horizontal neuronal spacing early in life that likely reflects an early overproduction of neuropil, while confirming previous findings of reduced horizontal distance in adults, suggesting a gradual degenerative decline. In chapter IV, we turn to spatial pattern analysis to examine whether there are alterations in neuronal and microglial organization. We discover emergent alterations in local neuron-neuron and microglia-microglia organization across the autistic lifespan, along with group differences in local microglia-neuron clustering that do not show a developmental trend. These experiments describe several novel microstructural features of the developing autistic brain, and suggest that microglial activation and neuropil overproduction are present during early brain overgrowth and are followed by gradual neuropil degeneration and loss of cellular organization in adolescents and adults that may be related at least in part to ongoing microglial activation. Further investigation of cellular and microstructural abnormalities in the autistic brain may hold the key to understanding the complex set of alterations that underlie this disorder, and point the way towards diagnostic strategies and therapeutic instruments

The amygdala plays a critical role in emotional processing and has been implicated in the etiology of numerous psychiatric disorders. It is an evolutionarily ancient structure that is enlarged in primates relative to rodents. Certain amygdala nuclei, such as the lateral nucleus, show relatively greater phylogenetic expansion than other nuclei. However, it is unknown whether there is also differential alteration in neuronal features. To address this question, we examined the dendritic arbors of principal neurons, visualized by using the Golgi method, in the lateral and central nuclei of young adult rhesus macaques and rats. Total dendritic length is greater in the macaque than in the rat. Dendritic trees are increased by 250% in length in the lateral nucleus of the monkey compared with the rat (6,009 μm vs. 2,473 μm); dendritic tree length in the central nucleus is increased by 50% (1,786 μm vs. 1,232 μm). Somal volume is increased 62% between species in the lateral nucleus and 48% in the central nucleus. Spine density is lower on macaque lateral nucleus dendrites compared with rat (-22%) but equivalent in the central nucleus. Spines are equally long in the lateral nucleus of rat and macaque, but spines are longer by about 20% in the central nucleus of the macaque. The alterations in dendritic structure that we observed between the two species suggest differences in the number and spacing of inputs into these nuclei that undoubtedly influence amygdala function.

The amygdala undergoes aberrant development in autism spectrum disorder (ASD). We previously found that there are reduced neuron numbers in the adult postmortem amygdala from individuals with ASD compared to typically developing controls. The current study is a comprehensive stereological examination of four non-neuronal cell populations: microglia, oligodendrocytes, astrocytes, and endothelial cells, in the same brains studied previously. We provide a detailed neuroanatomical protocol for defining each cell type that may be applied to other studies of the amygdala in neurodevelopmental and psychiatric disorders. We then assess whether cell numbers and average volumes differ between ASD and typically developing brains. We hypothesized that a reduction in neuron numbers in ASD might relate to altered immune function and/or aberrant microglial activation, as indicated by increased microglial number and cell body volume. Overall, average non-neuronal cell numbers and volumes did not differ between ASD and typically developing brains. However, there was evident heterogeneity within the ASD cohort. Two of the eight ASD brains displayed strong microglial activation. Contrary to our original hypothesis, there was a trend toward a positive correlation between neuronal and microglial numbers in both ASD and control cases. There were fewer oligodendrocytes in the amygdala of adult individuals with ASD ages 20 and older compared to typically developing controls. This finding may provide a possible sign of altered connectivity or impaired neuronal communication that may change across the lifespan in ASD.