Author Topic: BRAIN MAY GROW NEW CELLS DAILY  (Read 805 times)

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BRAIN MAY GROW NEW CELLS DAILY
« on: December 12, 2007, 02:13:31 PM »
BRAIN MAY GROW NEW CELLS DAILY:

Princeton Study on Monkeys Challenges Long-Held View

by Nicholas Wade


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In a new challenge to the long-standing belief that adults never generate new brain cells, biologists at Princeton University have found that thousands of freshly born neurons arrive each day in the cerebral cortex, the outer rind of the brain where higher intellectual functions and personality are centered.

Though based on research in monkeys, the finding is likely to prove true of people, too.  If so, several experts said, it may overturn ideas about how the human brain works and open new possibilities for treating degenerative brain diseases.

If the new brain cells, or neurons, are involved in memory and learning -- perhaps with each day's batch of new cells recording that day's experiences -- scientists will have to make major revisions in the long-time view that the adult brain's neurons are static in number and that memory is stored only in the way they interconnect.

In addition, if the brain's cells are in constant turnover, as the new finding suggests, physicians may discover ways to use the brain's natural regeneration system for replacing cells that are lost in diseases of ageing.

The discovery, by Dr Elizabeth Gould and Dr Charles G Gross, is reported in today's issue of the journal Science.

The belief that the adult brain does not make new cells rested on careful, well-known studies by Dr Pasko Rakic of Yale University, who looked for the formation of new neurons in the monkey brain and found none.

But the Princeton work is likely to be convincing, because it builds on previous reports of brain cell turnover, notably by Dr Fernando Nottebohm of Rockefeller University, who showed that canaries grow new neurons to learn new songs, and recent studies showing that new cells are formed in the hippocampus, a brain region where initial memories of faces and places are formed.

"The scientific community can easily believe something it is 50% ready to absorb, but not something that comes out of left field," said Dr Eric R Kandel, a leading neuroscientist at Columbia University.   "But here, we are prepared for it."

Dr Kandel compared the likely change in view to the paradigm shifts described by the historian of science Thomas Kuhn as occurring when one major scientific theory is replaced by another.

Although the new study was done in macaque monkeys and has yet to be confirmed in humans, as fellow primates monkeys are usually quite predictive of what occurs in people.

Dr Gould, who has studied new cell formation in the hippocampus, and Dr Gross, an expert on the cerebral cortex, injected macaques with a chemical that is incorporated in the new DNA formed when a cell divides.

They found that a stream of new neurons were generated in the monkey's brains in a zone just above the brain's fluid-filled central chambers.  This zone was recently identified by other scientists as the home of the brain's stem cells, the source cells from which an organ is replenished.

The new neurons migrated toward the cortex, matured and sent out axons to make connections with other brain cells, the Princeton biologists found.

The researchers looked for new neurons in four areas of the cortex, and found them in three areas where memories are known to be stored: the frontal cortex, used for decision-making, and two areas on the side of the brain used for visual recognition.  No new neurons were detected in the fourth area, the striate cortex, a region at the back of the head that simply processes visual information from the eyes and passes it on to other parts of the cortex.

Whatever the new cells are doing in the cortex, they affect regions of the brain that are central to human thought and identity.   The Princeton work, said Ronald D G McKay, an expert on brain stem cells at the National Institute of Health, "places new neurons in the region of the brain involved in the highest level of personality: it's the frontal cortex that is important in determining who you are in a very human way."

Dr Gould said that it was possible that the new neurons arriving in the cortex would be particularly sensitive to recording information for a certain period while they matured.

"They would become integrated in the circuitry and represent the information being learned at that particular time," she said, after which they would not record anything more.

In other words, the conveyor belt of new neurons might record successive days' experiences almost like a moving tape.

"We know the characteristic of memory is that events are tagged with times," Dr Gross said.  "We have no idea how that is done.  But since we have now shown there are new cells added every day, which cover a spectrum of ages, these cells could possibly provide the substrate for the temporal dimension of memory."

Dr Kandel, of Columbia University, said the idea was perfectly possible, given how little was now known about the brain's system for ultimate long-term memory storage.

"How do you distinguish the memory of 20 years ago from the memory of 30 years ago?  You would have to mark the birthday of the cell in some way," Dr Kandel said, suggesting that the train of new neurons offered a plausible mechanism whereby the brain might somehow be able to do this.

The notion that new memories are stored in a train of new nerve cells was advocated in the 1960's by Dr Joseph Altman, then of the Massachusetts Institute of Technology.  But his proposal was not widely accepted.   And when Dr Rakic, an authority on neuronic formation in the embryonic monkey brain, reported in 1965 that no new neurons were formed in the adult monkey's brain, this became the accepted view.

Even when Dr Gould and others showed recently that new cells were formed in the hippocampus, Dr Rakic argued that this was a primitive area of the brain -- even reptiles have a hippocampus -- and that brain organs acquired more recently in evolution, like the primates' cerebral cortex, would not be expected to behave the same way.

Dr Gould said it was this argument that had made her determined to look for new cells being formed in the cerebral cortex, despite the expense of doing work on monkeys and the risk in "redoing an experiment that a very well respected person," Dr Rakic, had already performed.

If indeed the brain is constantly renewing the cells in its cortex, hippocampus and maybe other areas, the prospects for learning how to repair the aged or damaged brain begin to look much more hopeful.

"Degenerative diseases of the brain are really defined by loss of nerve cells." Dr Kandel said.  Though diseases like Parkinson's affect specific areas of the brain, it might become possible to channel young new neurons into the areas of disease.  "This is pie in the sky," he said, "but at least there is now the possibility of thinking about it."

Dr William T Greenough, a neuroscientist at the University of Illinois, said the Princeton work created a "whole new ball game" for addressing brain diseases, by harnessing the brain's own restorative potential.

The Princeton biologists plan to follow up their discovery by blocking the formation of new neurons in monkeys' brains and seeing what happens.  If the new neurons are essential for memory and learning, then serious deficits should appear in the monkeys' performance.  The researchers as yet have no idea whether the loss of brain cells and the generation of new ones are separate events or part of the same cycle.

"Our discovery," Dr Gross said, "suggests more questions than answers."

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