Scientists have taken another step toward understanding what makes the human brain unique.
An international team has identified a type of brain cell that exists in humans but not mice, the team reported Monday in the journal Nature Neuroscience.
"This particular cell type had properties that had never been described in any other species," says Ed Lein, one of the authors of the study and researcher at Allen Institute for Brain Science ] in Seattle ,
The result could help explain why many experimental treatments for brain disease in mice have worked but have failed in humans. It could also provide new insights for scientists exploring human brain disorders ranging from autism to Alzheimer's disease to schizophrenia.
"To fully understand mental disorders, we need to gain access to these special types of neurons. They exist only in humans," says Joshua Gordon, director of the National Institute of Mental Health, which funded the research.
Researchers have proposed several other brain cells that may be unique to humans. But these cells were either found in other species, or the evidence for them was less convincing.
It is still possible that these newly identified neurons are also found in the brains of monkeys such as monkeys or chimpanzees, says Lein. 1
A scientist named Gábor Tamás and members of his lab studied brain cells called inhibitory neurons that act like the brakes in a car. They tell other brain cells when to slow down.
Tamas recorded electrical signals from inhibitory neurons from the cerebral cortex of two deceased men.
"In the course of these shots, he noticed a very distinctive type of cell, which for him had the shape of a rose after the petals dropped off," says Lein. "So he called her the rosehip cell."
By chance, Allen Institute scientists had also identified these cells with a completely different approach, a new technique that allowed them to recognize the genes that are activated in the human brain.
So the researchers combined what they had learned and confirmed that rose hip cells were a specific subtype of inhibitory neuron.
The discovery calls for earlier evidence that the human brain is merely larger and more sophisticated than a mouse brain. At some point, humans acquired at least one type of brain cell that a mouse does not have.
Scientists are not sure what exactly rosehip cells do, although they appear to be involved in controlling the flow of information specific to the brain.
But regardless of their exact function, the discovery of rosehip neurons has an impact on brain research. First, "it raises doubts about the ability to use the mouse to study specific elements of human function and disease," says Lein. And because the rosehip neurons are a kind of inhibitory neuron, they could play a role in mental illness.
"These types of cells [inhibitory neurons] are extremely important," he says. And if there is dysfunction in them, he says, it may be "directly linked to various types of neuropsychiatric disorders, such as schizophrenia."
The identification of rosehip neurons is part of a much larger effort by the National Institutes of Health to identify every type of cell found in the brains of mice, monkeys and humans. It is also part of the Federal BRAIN initiative announced by President Barack Obama in 2013.
These initiatives use new technologies that are likely to reveal other brain cells that exist in humans but not in animals, says Gordon.
"I think it's very, very likely that this is the tip of the iceberg," he says.