Yet as the Decade of the Brain proclaimed by President George
Bush draws to a close, neuroscientists are increasingly sanguine
that in George Jr.'s lifetime, brain-cell transplants may
reverse, if not cure, a host of neurological diseases such as
Parkinson's and Alzheimer's, as well as brain damage caused by
strokes and head injuries. Even a year ago, such a sweeping claim
might have been dismissed as nonsense. But that was before last
fall's discovery that the fetal human brain contains master cells
(called neural stem cells) that can grow into any kind of brain
cell. Snyder extracted these cells and "mass-produced" them in
the lab. His hope is that the cells, when injected into a damaged
adult brain, will turn themselves into replacements for cells
that are dead or diseased.
When most physicians got their training, they were taught that
the adult brain is rigid, that its nerve cells, or neurons,
could never regenerate themselves. If you nick your finger with
a knife, the cut will heal in a few days because your skin has
the ability to generate new cells. But when something bad
happens to the brain, it doesn't repair itself. Why's that? "The
brain is not plastic," says Snyder. "It doesn't make new cells.
You are born with more brain cells than you need, and you lose
them progressively and get dumber and dumber as you get
older--or so went the conventional wisdom."
The path to overturning the dogma of the rigid brain was
circuitous. In the early 1960s biologists discovered that new
cells were being made in two areas of the adult rat brain, but
the discovery was regarded as an unimportant peculiarity of the
rodent brain and quickly forgotten. In the mid-1980s, Fernando
Nottebohm of Rockefeller University brought new respect to the
term birdbrain by demonstrating that the brain of an adult canary
has the astonishing ability to regenerate new nerve cells at a
rate of up to 20,000 a day. Other researchers reported similar
regenerative ability in fish and reptiles, but there was still no
evidence that evolution had passed on this ability to the human
brain. Indeed, most neuroscientists wouldn't even entertain the
possibility of new cell growth in the human brain on the grounds
that any additional cells would disrupt the brain's complex
wiring.
Snyder was not so sure. "I'm an optimist. Why would evolution
have been parsimonious in depriving the human brain of the power
of self-healing? I was a pediatrician before I became a
neuroscientist. As a pediatrician, I was impressed by how much
plasticity there really must be in the human brain. Pediatricians
know that damage to the infant brain doesn't have the same
outcome as damage to the adult brain. If a newborn has a stroke,
even in the cortex [an area important to higher intellectual
functions], he or she may sustain it and develop quite normally.
The exact same injury would put an adult in a wheelchair. I
wondered if the source of the brain's apparent plasticity was at
the level of the single cell."
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mystery
