AP Science Writer
6:41 PM EST; October 14, 1999; Washington, D.C. (AP) -- Contradicting a long-held belief, researchers say the brain is constantly churning out new neurons for its learning and memory center, according to a study that could lead to novel ways to treat brains damaged by injury or diseases such as Alzheimer's. In studies using macaque monkeys, the researchers at Princeton University have for the first time traced the path followed by neurons that are created in one part of the brain and then migrate to the neocortex, the center of the mind's ability to reason and think.
Once the new neurons arrive in the neocortex, they "plug in" and become a new part of the brain's central circuitry, said Elizabeth Gould, head of a Princeton brain research team. "This shows that there is a naturally regenerative mechanism" in the mature brain, said Gould, the first author of a study appearing Friday in the journal Science. "If we can understand better how it works, maybe we could use that to direct the regeneration and repopulation of neurons in damaged areas of the brain. For decades, brain researchers have thought that once neurons were lost in the neocortex, they were not replaced and that their function was forever gone. It was believed the brain contains so many cells that this natural attrition had little effect until late in life.
Now the new Princeton study, if confirmed by other researchers, shows that the mature primate brain does produce new neurons that incorporate themselves into the thinking center. "This study could be very significant in terms of understanding the range of mechanisms that the cortex has available to it for the storage of information and for repair," said William T. Greenough, Director of Neuroscience at the University of Illinois' Beckman Institute. "These data scream for a re-analysis of human brain development."
Earlier studies had given strong hints that the brain had an unsuspected regenerative power. Some researchers had shown in rats and monkeys that new cells were regenerated in the hippocampus, a very primitive part of the brain that controls basic functions and is the same in many animals. But it was generally believed that the neocortex, with its storehouse of memories and advanced reasoning power, was unique, stable, and unchanging.
The neocortex reaches its most sophisticated level of development in primates, including humans. Most experts believe the structure and function of the neocortex is very similar in all primates and that basic processes in the brains of monkeys are like those in humans. But Greenough and others cautioned that Gould's study needs to be confirmed in humans. In the Princeton study, Gould and her team injected monkeys with a compound called BRomoDeoxyUridine (BRDU) that is taken up by cells in the process of making new cells. An examination just hours after the injection showed that the cells in one area of the brain took up the BRDU, proving that the cells were dividing and making immature neurons. An examination a week after injection showed that the new neurons had migrated, matured, and, in effect, had plugged themselves into the neocortex -- the thinking center of the brain.
Just how these new neurons function is not known, Gould said. They could take up existing memories from the old neurons in the neocortex, or they could be like a new "floppy disk in a computer" -- clean and ready to start storing new data. Gould said more research is needed to understand exactly how the brain uses these new cells. Gould said it is not known if Alzheimer's or other dementias are linked to a failure or a decline of this natural neuron regeneration process. But at least now, she said, "it is possible to consider that and to ask the question. This issue had not even been considered previously."