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.]
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