A small study that examined brains from children who died found abnormal patterns of cell growth in autistic children. The research bolsters evidence that something before birth might cause autism, at least in some cases.
Clusters of disorganized brain cells were discovered in tissue samples from brain regions important for regulating social functioning, emotions and communication - which can all be troublesome for children with autism.
The abnormalities were found in 10 of 11 children with autism, but in only one of 11 children without the disease. The children's brains were donated to science after death; causes of death included drowning, accidents, asthma and heart problems.
The authors said the clusters, detected with sophisticated lab tests, are likely defects that occurred during the second or third trimesters of pregnancy.
"Because this points to the biological onset in prenatal life, it calls sharply into question other popular notions about autism," including the scientifically debunked theory that childhood vaccines might be involved, said lead author Eric Courchesne, an autism researcher at the University of California, San Diego.
Experts not involved in the latest study called the results preliminary and said larger studies are needed to determine if the unusual brain development found in the study causes problems, and if it is truly common in autism or even in people without the disorder. What causes the unusual structure isn't known, Courchesne said, adding, "It could be gene mutations and environmental factors together."
Scientists have been working for decades to find the cause of autism, and they increasingly believe its origins begin before birth. In addition to genetics, previous research suggests other factors might include infections during pregnancy, preterm birth and fathers' older age at conception.
The study was published in Thursday's New England Journal of Medicine.
Other scientists have suggested that autism may be linked with abnormalities in the brain's frontal region, and that for at least some children, problems begin before birth, said Dr. Janet Lainhart, an autism researcher and psychiatry professor at the University of Wisconsin.
"But this research provides probably some of the most elegant evidence for those two very important biological themes," she said.
The study follows Courchesne-led research suggesting that abnormal gene activity leads to an excessive number of brain cells in the brain's prefrontal cortex, located behind the forehead. The same region and adjacent areas of the brain were implicated in the new study.
His studies suggest that in children later diagnosed with autism, genetic networks that regulate prenatal brain cell growth are faulty. Larger studies are needed to determine how common the abnormalities are and what might be the cause.
"These abnormalities are not trivial," Courchesne said. "These are fundamental to developing a human brain."
The new study involved children aged 2 to 15. Most previous autism brain studies involved samples taken from autopsies of adults.
Dr. Thomas Insel, director of the National Institute of Mental Health, said the authors used advanced methods to examine cellular and molecular markers in more detail than previous research. But he said the study "highlights the critical need" for autopsy brain tissue to gain a better understanding of autism.
"If there really is this disorganized cortical architecture" in autism, it would develop before birth, said Insel. His government agency helped pay for the research.
About 1 in 88 children in the U.S. have one of the autistic spectrum disorders, which include classic autism and a mild form, Asperger syndrome.
Researchers from the Allen Institute for Brain Science in Seattle also participated in the study. In addition to the National Institute of Mental Health, grants from the Allen institute, private foundations and the advocacy group Autism Speaks helped pay for the research.