Brains from seven boys with autism had a mean of 67% more neurons in the dorsolateral and mesial prefrontal cortex than did those of six age-matched control subjects in a postmortem study reported in the Nov. 9 issue of JAMA.
"To our knowledge, this study is the first direct quantitative test and confirmation of the theory that a pathological overabundance of neurons in critical brain regions is present at a young age in autism," said Eric Courchesne, Ph.D., of the department of neuroscience, National Institutes of Health–University of California, San Diego, Autism Center of Excellence, University of California San Diego, La Jolla, and his associates.
"Because cortical neurons are generated in prenatal, not postnatal life, pathological overabundance of neurons indicates early developmental disturbances in molecular and genetic mechanisms that govern proliferation, cell cycle regulation, and apoptosis," they noted (JAMA 2011;306:2001-10).
Among autistic children, brain overgrowth (excess weight and size) and neuronal dysfunction have long been observed in many regions involved in higher-order social, emotional, communication, and cognitive development. But the neural basis of these abnormalities was unknown because it could only be explored in "direct quantitative studies of the young postmortem autistic brain" – specimens that are very rarely available.
Dr. Courchesne and his colleagues were able to perform such a study when they received access to entire dorsolateral and mesial sections of the prefrontal cortex in seven autistic and six control male subjects who had been aged 2-16 years when they died. Valid cell counts cannot be obtained using density estimates from small blocks of cortical tissue, the researchers explained.
They used stereological methods to count neurons, sampling 100 or more locations within 8-12 sections per reference space, and they calculated the total number of neurons using the optical fractionator method. Neurons were distinguished from microglia and satellite oligodendrocytes by doing detailed morphological examination, not just by estimating cell size.
The control cases were free of mental illness, intellectual disability, and neurological disorders. The autistic cases displayed a wide range of intellectual abilities, social skills, communication abilities, and functional impairments. All had been born at term after unremarkable perinatal courses.
Most of the study subjects had died of acute global ischemic hypoxia from accidental trauma (drowning, hanging, or electrocution); 1 died in a car accident, 1 of rhabdomyosarcoma, and 1 of possible cardiac arrest.
There had been no abnormalities on gross examination of most of the brains, according to medical examiner or neuropathology reports. The mean weight of autistic brains was 1,484 g, and was 17.6% greater than the normative mean weight for same-aged subjects reported in the literature. The control brains were significantly lighter and deviated from the norm by only 0.2%.
There were 79% more neurons in the dorsolateral prefrontal cortex among autistic brains than control brains, and 29% more neurons in the mesial prefrontal cortex. "Together, these 2 subdivisions gave a total combined prefrontal neuron count that was 67% greater in the autistic children (mean, 1.94 billion) compared with controls (mean, 1.16 billion)," Dr. Courchesne and his associates said.
This significant difference remained robust when the data were adjusted to account for subject age and postmortem interval.
The greater excess of neurons in the dorsolateral than in the mesial prefrontal cortex "parallels MRI volumetric data showing greater deviance in the dorsolateral prefrontal cortex than the mesial prefrontal cortex in living autistic toddlers," the investigators said.
Moreover, "these global increases in prefrontal neuron numbers were not apparent either at low or high magnification, and thus [were] undetectable by neuropathology visual inspection and neuron density measurement without formal quantitative stereological procedures," they noted.
There were no differences between autistic and control brains in the size of neurons, nor in counts of glial cells in the same brain regions.
It remains to be determined whether excess numbers of neurons occur in other regions of the brain besides "these two major prefrontal subdivisions," they added.
As has been noted in previous studies, this study found that the average weight of the autistic brains was heaver than that of the control brains. This indicates that the pathological increase in neuron numbers may be an important contributor to the brain overgrowth that has long been known to occur in autism.
However, the autistic brains did not show the linear correlation between number of neurons and brain weight that has been described in normal brains and was found in the control brains in this study. "Neuron counts in the autistic children should have been accompanied by brain weights considerably larger than was observed, reaching 29.4% enlargement rather than the observed 17.6% enlargement. Thus, the size of the autistic brain, overlarge as it is, might actually underestimate the pathology of excess neuron numbers," Dr. Courchesne and his colleagues noted.