Updated March 8 at 1:53 p.m. ET
A major study is challenging the widely held view that adult human brains make new neurons.
The study of 59 samples from 29 brains of people of various ages found no immature neurons in anyone older than 13, scientists report online Wednesday in the journal Nature.
"In all of the adult samples we looked at, we couldn't find any evidence of a young neuron," says Shawn Sorrells, the study's lead author and a senior researcher in the lab of Arturo Alvarez-Buylla at the University of California, San Francisco.
The finding challenges decades of research suggesting that new neurons continue to appear in the hippocampus, an area of the brain involved in memory and emotion. It also calls into question claims that both exercise and some anti-depressant drugs could boost the production of new neurons in the hippocampus.
The finding is bound to be controversial. "I'm sure that there are going to be people who challenge [our results]" says Alvarez-Buylla, a professor of neurological surgery at UCSF.
One of those who questions the new study's conclusion is Rusty Gage, interim director of the Salk Institute in La Jolla, Calif., and one of the scientists whose research in the 1990s helped establish the idea that new cells do appear in the hippocampus of adult humans.
"I feel confident in my lab's results," Gage says in a written statement. And results from many labs, he says,"repeatedly confirm that neurogenesis can occur in the adult brain."
The new study relied on measures of neurogenesis that aren't always reliable when used on brain tissue taken from someone who has died, Gage says. (The study used also tissue from patients undergoing brain surgery.)
But knocking down the latest findings won't be easy, says Jason Snyder, an assistant professor at the University of British Columbia who wrote a commentary accompanying the study.
The research is "as convincing as you can be," Snyder says. "This is certainly one of the best or perhaps the best of study of neurogenesis in humans."
Even so, he says it is unlikely to settle the issue.
One reason is that it is very difficult to study the birth of new neurons, a process called neurogenesis, in people. High-quality samples of brain tissue are in short supply. And each sample tells you only what's happening at a single point in time at one place in the brain.
Also, several previous studies have found indirect evidence that neurogenesis does occur in the adult hippocampus.
Alvarez-Buylla himself thought he might find new brain cells in people when his lab began investigating the question.
He had trained in the Rockefeller University lab of Fernando Nottebohm, who discovered new cells forming in adult canaries.
And Alvarez-Buylla knew that many studies had found strong evidence of neurogenesis in adult mice.
So when his lab began studying human brains, "we were expecting to find evidence of young neurons" in the hippocampus, he says. But that's not what happened.
The first hint came several years ago when Alvarez-Buylla visited a colleague's lab in China.
"We were looking through some brains that he had collected from humans," he says, "and when we looked at the hippocampus we couldn't find the young neurons there."
So Alvarez-Buylla assembled an international team and launched the new study of 59 brains. The team used both classic techniques and the latest technology to search each sample for immature brain cells.
And they found lots of these cells in babies and young children. "But then by 7 years of age, these cells were much more sparse," Sorrells says. And the numbers dwindled even further in older brains.
If the results hold up, scientists will have to figure out how the adult human brain can continue to change throughout life without adding new cells, the way a rodent brain does.
One clue is that it can take many years for a new human brain cell to mature, Snyder says. So even a grown-up's brain may contain many cells whose functions aren't yet fully defined.
"Cells that were born in childhood could play a big role in learning, in memory and in emotional disorders," Snyder says. "So their impact on adult brain life could still be big."
It also may be possible to use drugs or other therapies to get an old brain to start making new cells, Snyder says. That might help repair the damage caused by a brain injury or Alzheimer's disease, he says.
"I think the possibility is real that someday we'll be able to rejuvenate the aging brain," Snyder says.
MARY LOUISE KELLY, HOST:
Now some news that's a bit of a bummer for those of us with adult brains. A new study finds that mature brains don't make new neurons. NPR's Jon Hamilton has more.
JON HAMILTON, BYLINE: In our bodies, cells keep on dividing until we die. And for a couple of decades now, many scientists have thought we also keep making new cells in our brains. Arturo Alvarez-Buylla at the University of California, San Francisco, says that made sense to him. He knew adult birds and mice made new neurons, so when his team decided to study human brains...
ARTURO ALVAREZ-BUYLLA: We were expecting to find evidence of young neurons.
HAMILTON: He thought he'd find them in the hippocampus, an area that's involved in memory and emotion. But that's not what happened. The first hint came when Alvarez-Buylla visited a colleague's lab in China.
ALVAREZ-BUYLLA: We were looking through some brains that he had collected from humans. And when we looked at the hippocampus, we couldn't find the young neurons there.
HAMILTON: So Alvarez-Buylla launched a much larger and more rigorous study. He brought together an international team. They got bits of hippocampus from the brains of 59 people of different ages, and they used the latest technology to search each sample for immature brain cells. Shawn Sorrells, a senior researcher in Alvarez-Buylla's lab, says the team found lots of these cells in babies and young children.
SHAWN SORRELLS: But then by 7 years of age, these cells were much more sparse.
HAMILTON: And Sorrells says the numbers continued to dwindle in older brains.
SORRELLS: Thirteen years of age was the oldest sample where we were able to find cells that had immature features. And then in all of the adult samples we looked at, we couldn't find any evidence of a young neuron.
HAMILTON: The finding is a big deal because many scientists have suspected that everything from memory to mental disorders can be influenced by new neurons. And studies have offered tantalizing hints that both exercise and antidepressant drugs may help our brains by boosting the production of new neurons in the hippocampus. So Arturo Alvarez-Buylla expects his new study to be controversial.
ALVAREZ-BUYLLA: I'm sure that there's going to be people that are going to challenge it, and that's the process of scientific discovery, right?
HAMILTON: Jason Snyder of the University of British Columbia finds the research pretty convincing.
JASON SNYDER: It's as convincing as you can be with humans.
HAMILTON: Snyder wrote a commentary that accompanies the new study in the journal Nature. He says it's really hard to see new neurons appear in people, a process known as neurogenesis. Samples of brain tissue are in short supply, and each sample tells you only what's happening at a single point in time at one place in the brain.
SNYDER: Given all of these challenges, this is certainly one of the best or perhaps the best study of neurogenesis in humans.
HAMILTON: Snyder says if the results hold up, scientists will have to figure out how the adult human brain can continue to change without adding new cells.
He says one clue is that it can take many years for a baby brain cell to mature. So even a grown-up's brain may contain many cells whose functions aren't yet fully defined.
SNYDER: Cells that were born in childhood could play a big role, I think, in learning, in memory, in emotional disorders. So their impact on adult brain life could still be big.
HAMILTON: And Snyder says it may still be possible to use drugs or other therapies to get an old brain to start making new cells. He says that might help repair the damage caused by a brain injury or Alzheimer's disease.
SNYDER: I think the hope is there, and the possibility is real that someday we'll be able to rejuvenate the aging brain.
HAMILTON: Perhaps by reactivating the process that allows young children to produce so many new brain cells. Jon Hamilton, NPR News.
[POST-BROADCAST CORRECTION: In the audio of this report, it was said that 59 brains were studied. In fact, 59 samples from 29 brains were used.] Transcript provided by NPR, Copyright NPR.