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IT’S SETTLED: HOW ADULT BRAINS MAKE NEW NEURONS
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Nora Bradford
July 3, 2025
Scientific American
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_ Adult brains grow new neurons, and scientists have finally
pinpointed where they come from. These new findings settle a major
dispute that neuroscientists have been arguing about for more than 60
years. _
Neural precursor cells (green) are have been difficult to identify in
human brians, Carol N. Ibe and Eugene O. Major/National Institutes of
Health/Science Source
For at least six decades, neuroscientists have been arguing over a
big, foundational question: Do adult brains make new neurons? This
process of “neurogenesis
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had been shown in other adult animals, but its evidence in humans was
circumstantial—until now. Using a new technique, scientists have
found newly formed neurons in the brains of adults as old as age
78—and, for the first time, have identified the other brain cells
that birthed them.
The results, published on Thursday in _Science_
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cells with the capacity to turn into neurons, called neural precursor
cells, exist in adult human brains. “Now we have very strong
evidence that the whole process is there in humans, from the precursor
cells to the immature neurons,” says Gerd Kempermann, a
neurobiologist at the Dresden University of Technology, who was not
involved in the study.
Throughout gestation, our brain churns out new neurons
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it reaches the 100 billion we start life with, and that count declines
as we age. As early as 1962, studies in rats had shown that
neurogenesis continued throughout the animals’ life. Others found
that young neurons existed in adult human brains. But it was unclear
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these “immature” neurons were truly new—or whether humans just
start life with a collection of them, after which they slowly develop
during adulthood.
One thing was clear from these studies: if adult neurogenesis happened
anywhere, it was in the hippocampus, a deep-brain structure known for
its role in memory processing and storage. But even in the human
hippocampus, neuroscientists had not yet found the precursor cells
that divide and develop to turn into new neurons.
Researchers at the Karolinska Institute in Sweden had previously found
immature neurons in the human brain. Marta Paterlini, a neuroscientist
at the institute, and her colleagues, set out to pin down how those
neurons came to be. Paterlini and her team took advantage of a new
combination of techniques to examine immature neurons and neural
precursor cells in the hippocampi of six young children, whose brain
had been donated to science upon their death. From more than 100,000
cells, the researchers sequenced RNA—bits of genetic information
used to carry out actions within each cell. These markers come
together to form a sort of molecular fingerprint that can be used to
predict a cell’s stage of life. “It’s not a matter of one marker
defining active neurogenesis; it’s the combination of many
markers,” says Paterlini, who is co-lead author of the new study.
After identifying these markers in young brains, the team then
searched for those same signatures in 19 postmortem brains ranging
from 13 to 78 years old. All of the brains contained immature neurons
except one. The researchers also found neural precursor cells in each
of the child brains and in 12 of the 19 adolescent and adult brains.
Two adults stood out for having many more neural precursor cells and
immature neurons than the rest. The younger of these two people had
lived with epilepsy, which could potentially connect to the apparent
abundance of neurogenesis. In mice, higher levels of neurogenesis can
cause seizures, though the connection to epilepsy in humans is still
unclear.
The team suspects that neurogenesis happens in other parts of the
adult brain, too. In mice, new neurons are regularly made in the
olfactory bulb (a structure that processes smells) as well—but the
same hasn’t been shown in humans. Paterlini plans to investigate
whether adult neurogenesis might happen there or elsewhere in the
brain.
Some research in mice suggests that disrupted neurogenesis is linked
to Alzheimer’s disease and depression. Learning more about how
neurogenesis happens—and whether the process can be altered—could
prove helpful for understanding a range of disorders and diseases,
says the study’s co-lead author Ionut Dumitru, a neuroscientist at
the Karolinska Institute.
With the question of adult neurogenesis resolved, scientists can begin
learning more about what neurogenesis does in the brain and how it
affects various disorders. “This is an important paper because it
should finally put this all to rest,” Kempermann says. “And we can
now concentrate on the question: How do these cells in the human
contribute to brain function?”
_NORA BRADFORD
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current news intern at Scientific American, a freelance science
writer and a Ph.D. student in cognitive science. Follow Bradford on
Blusky @norabradford.bsky.social
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_Founded 1845, Scientific American
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has published articles by more than 200 Nobel Prize winners._
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