So why does our hippocampus shrink? I think we need to know this
simply because it smells like something we can stop or even improve
on. To start with we need comparative studies based on many samples
with documented life ways. We really have to compare Eskimos to
Zulus to discover predictive variation.
It also goes directly to the heart of the race variation question.
It is the one part of the brain that our nurture affects hugely and
whose potential across race and various genotypes can be
investigated. A large number of interesting questions regarding
human variations can be targeted with real potential for a sharp
increase in understanding what make humans both different and
similar.
We now have real confirmation that brain cells are been replaced in
the hippocampus and it now behooves us to encourage this.
Nuclear bomb tests
reveal brain regeneration in humans
Updated 13:16 10 June
2013 by Douglas Heaven
Nuclear bomb tests
carried out during the cold war have had an unexpected benefit.
A radioactive carbon
isotope expelled by the blasts has been used to date the age of adult
human brain cells, providing the first definitive evidence that we
generate new brain cells throughout our lives. The study also
provides the first model of the dynamics of the process, showing that
the regeneration of neurons does not drop off with age as sharply as
expected.
In mammals, most types
of brain cell are created at or soon after birth and are never
renewed. But studies in rodents and monkeys have shown that in two
regions new neurons continue to be created even in adulthood – the
hippocampus, which is involved in learning and the formation of new
memories, and the olfactory bulb, which processes smell.
However, there has
been some controversy over whether the same is true for humans.
Fifteen years ago a study found evidence for such neurogenesis in
adults up to the age of 72 (Nature Medicine, doi.org/b7hjfz), but the
research relied on a chemical called bromodeoxyuridine (BrdU) to
label neurons. BrdU was used at the time to track the spread of
tumours in people with cancer, but it was banned shortly after and so
the study was never repeated, leading some researchers to question
the results.
Another study on
London taxi drivers suggested that the hippocampus grows with
increasing knowledge of the city but this, too, has been
controversial.
Radioactive growth
The new study settles
the debate. "The existence of adult hippocampal neurogenesis in
humans is not arguable this time," says Sandrine Thuret at
King's College London, who was not involved in the work.
Instead of chemical
labelling, Jonas Frisén at the Karolinska Institute in Stockholm,
Sweden, and colleagues used a by-product of the above-ground nuclear
bomb tests carried out by the US, UK and Soviet Union between 1945
and 1963. As a result of these detonations, atmospheric levels of the
radioactive isotope carbon-14 increased dramatically during this
period. It has decreased steadily since.
Carbon-14 enters the
food chain and eventually finds its way into our cells, which
integrate carbon-14 atoms into their DNA when a parent cell splits
into two new daughter cells. The amount of carbon-14 in the
atmosphere is therefore mirrored in the cells at the time they are
born.
By analysing brain
tissue using mass spectrometry equipment, the team was able to
measure the number of carbon-14 atoms trapped in different
populations of cells in different brain regions.
They could then
compare this figure to known data for atmospheric levels of carbon-14
to date the birth of a cell in different people to within about a
year. The level of carbon-14 is higher in older cells grown
closer to the peak of nuclear bomb testing than in cells born more
recently.
Frisén's team
previously used this method to show that humans are the only known
mammal in which neurogenesis does not occur in the adult olfactory
bulb, since the cells in this brain region were the same age.
Forever young
But looking at the
hippocampus in 55 post-mortem brains aged between 19 and 92, the team
has now found that a subset of neurons in an area of the hippocampus
called the dentate gyrus are indeed created throughout adulthood. In
effect, a small population of our brain cells remains permanently
young, renewing itself continually. By modelling the process, the
team estimated that we generate around 1400 new neurons every day.
"Everyone working on adult hippocampal neurogenesis is excited
about this work," says Thuret.
The team was surprised
that the ability to create new neurons appears to stay with humans
longer than it does in mice. "In mice the decline is pretty
dramatic," says Frisén. But instead of the 10-fold decrease
between young and middle-aged mice, the team found only a
four-fold decrease in humans, he says.
But the newborn brain
cells in humans do not appear to live long. In rodents, the
hippocampus gradually gets bigger as new cells are added. "In
mice, there is a net increase," says Thuret. But this is not
seen in humans – in fact, our hippocampus slowly shrinks. "There
are more dying than being born," she says.
The reason for this
turnover remains unknown. Many studies in mice show that young
neurons are necessary for forming new memories. But this has not
been tested in humans.
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