Extra brain cells may be key to autism
Chicago - Children with autism appear to have too many cells in a key area of the brain needed for communication and emotional development, helping to explain why young children with autism often develop brains that are larger than normal, US researchers said on Tuesday.
Their study suggests the condition starts in the womb because brain cells in this area known as the prefrontal cortex typically develop during the second trimester of pregnancy.
The findings could help narrow the search for a cause of autism, which affects one in every 150 children born today in the US, or about 1% of the population.
"We found a really remarkable 67% increase in the total number of brain cells in the prefrontal cortex," said Dr Eric Courchesne of the University of California San Diego Autism Centre of Excellence, whose study appears in the Journal of the American Medical Association.
Courchesne and colleagues carefully counted the number of brain cells in tissue from seven boys with autism who had died and six boys who did not have autism at the time of their deaths.
They focused on the prefrontal cortex, a part of the brain which is thought to grow too large and too fast in children with autism.
"It's a part of the brain that's important for social, emotional and communication functions, and it composes about 25% to 30% of the cerebral cortex," Courchesne said in a telephone interview.
His team was first in 2003 to link rapid growth in head circumference in the first year of birth with autism.
He said the finding of excess brain cells in the prefrontal cortex explains brain overgrowth in autism, and hints at why brain function in this area is disrupted.
"This isn't just a simple increase in neurons. It means a huge increase in potential connections and, therefore, a potential for mis-wiring which would lead to abnormal function," Courchesne said.
Autism is a spectrum of disorders ranging from a profound inability to communicate and mental retardation to relatively mild symptoms such as with Asperger's syndrome.
Scientists have found dozens of genes that may raise the risk of autism. But genetic causes only explain 10% to 20% of cases, and recent studies have pointed to environmental factors, possibly in the womb, as a potential trigger.
"For years, it's been a big puzzle from the standpoint of evidence. Where is the evidence that autism has a prenatal origin?" Courchesne said.
"For the first time, we have something really solid," he said.
The team found excess brain cells in each child with autism they studied, Courchesne said. And the brains of the autistic children also weighed more than those of typically developing children of the same age.
Lizabeth Romanski of the University of Rochester Medical Centre said the findings show that the origins of autism occur very early.
"The generation of new neurons, what we call proliferation, occurs prenatally during the second trimester," said Romanski, who was not involved in the study. "That is when these neurons are being born."
She said the finding of a large number of these neurons in children with autism suggests something occurred during this period to change the way the brain develops.
The researchers acknowledge that their study is small.
Courchesne said it is difficult to find brain samples from young people with autism, and his study included some from very young children, ranging from ages 2 to 16.
"This really says prenatal life is a very important time to study and mechanisms there will eventually lead to our understanding of how autism comes about," he said.