Brain cell regeneration has been discovered in a new
location in human brains. The finding raises hopes that these cells
could be used to help people recover after a stroke, or to treat other
brain diseases.
For years it was unclear whether or not we
could generate new brain cells during our lifetime, as the process –
neurogenesis – had only been seen in animals. Instead, it was thought
that humans, with our large and complex brains, are born with all the
required neurons.
Then last year Jonas Frisén
of the Karolinska Institute in Stockholm, Sweden, and his colleagues
found that neurogenesis occurs in the hippocampi of the human brain.
These structures are crucial for memory formation
Now they have found more new brain cells
in a second location – golf-ball-sized structures called the striata.
These seem to be involved in many different functions, including in
learning and memory. These particular aspects, related as they are to
the hippocampi, lead Frisén to speculate that these new brain cells may
also be involved with learning. "New neurons may convey some sort of
plasticity," he says, which might help people learn and adapt to new
situations.
Radioactive clue
To reveal the new brain cells, the team
exploited the fact that there have been varying levels of a radioactive
isotope of carbon – carbon-14 – in the atmosphere since nuclear bomb
tests during the cold war. This means that the year of creation of many cells in the body can be found by measuring the ratio of carbon-14 to carbon-12 in its DNA. Analysis of 30 donated brains revealed which brain cells had been born during the lifetimes of the donors.
The finding of new brain cells in the
striata solves a long-standing mystery. In rodents, neurogenesis is seen
in the hippocampi, as well as another area called the lateral ventricle
wall. After they are created, the cells made in this second location
migrate to the part of the brain that controls the sense of smell. Hints
of neurogenesis had already been seen in the lateral ventricle walls of
human brains. But when Frisén looked for new brain cells in human
smell centres, he couldn't find any. Now it looks like we know where
they ended up.
Arnold Kriegstein
at the University of California, San Francisco, agrees that the latest
carbon-14 work is confirmation that in humans, the striatum is their
destination. "It's nicely demonstrated," he says.
Fresh hope
It is too early to know what these new brain
cells are doing in the striata, but any evidence of neurogenesis in the
human brain provides fresh hope for the development of treatments for
neurodegenerative brain diseases.
Frisén's team also found that the donor
brains of 11 people who had had Huntington's disease – a rare,
degenerative brain disorder – had fewer new neurons in their striata
than the donor brains of formerly healthy people. This lack of new
neurons may have contributed to the characteristic problems of
Huntington's disease, which include movement issues and cognitive
deficits.
And immature neurons have in the past been
spotted in the striata of people who had had a stroke. "It's very
tempting to think that it would be possible to promote the generation of
more striatal neurons," says Frisén.
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