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Stem Cell Source Found in Human Brain

Could mark a step toward repairing brain damage

THURSDAY, Feb. 19, 2004 (HealthDayNews) -- Researchers have discovered a cell-producing region in the human brain that resembles one found in rodent brains -- but with a basic difference that has profound implications for research aimed at repairing damage by growing new cells.

In rodents, a region called the subventricular zone (SVZ), roughly in the middle of the brain, constantly produces cells that migrate to the olfactory bulb, which governs smell. The rodent SVZ produces stem cells, which have the ability to be transformed into many different kinds of cells. Most end up as nerve cells.

Until now, no such cell-producing region has been identified in the human brain. But researchers at the University of California at San Francisco, studying tissue samples taken from patients undergoing brain surgery and post-mortem brain donors, report in the Feb. 19 issue of the journal Nature that they have found one.

It is a ribbon of astrocytes, which make up the great majority of brain cells, the report says. It is also something that has not been seen in any animal brain and that has its own unusual characteristics.

"Unexpectedly, we find no evidence of chains of migrating neuroblasts [young nerve cells] in the SVZ or in the pathway to the olfactory bulb," the researchers write. "Our work identifies SVZ astrocytes as neural stem cells in a niche of unique organization in the human brain."

So what is happening to those cells? One important fact is that something is preventing them from becoming nerve cells, as happens in the rodent brain, says Dr. Pasko Rakic, professor of neuroscience at Yale University School of Medicine and author of an accompanying editorial.

The study shows that the failure of human brains to repair damage is not due to a lack of stem cells, Rakic says. "In humans, there is the same potential pool of cells, but they do not migrate as they do in the mouse," he says. "It is important to see what prevents it."

The human nervous system differs from that of animals in important ways, he notes, using the salamander as an example. "You can cut the spinal cord of salamanders and they grow back," Rakic says. "In humans, this cannot be done."

One obvious difference between humans and salamanders -- and mice -- is that human brains are much more highly organized, he says. The result is a complex structure built to resist changes, such as the introduction of the new cells produced in the SVZ, Rakic says.

But the whole idea of stem cells research is to implant new cells into damaged areas, where they can replace dead nerve cells. Now the challenge is "not just how you implant the cells, but how they make the right connections," he says.

Successful implantation remains a challenge, one that this discovery may help overcome, says Dr. Nader Sanai, a neuroscientist at UCSF and lead author of the journal report.

"The most interesting aspect of the work to me is identification of this structure in the human brain," he says. "In the past it has been observed that stem cells are created in the brain. Now we know where."

But almost as interesting is the indication that the brain suppresses the ability of these stem cells to become functioning nerve cells, Sanai says.

"Understanding the mechanism behind that repressive activity could be a very powerful tool," he says. "We could manipulate that area and other areas in the opposite direction to harness the potential of stem cells."

More information

The basics of stem cells and their possible use in medicine can be found at the National Institutes of Health or Northwestern University.

SOURCES: Pasko Rakic, M.D., Ph.D., professor of neuroscience, Yale University School of Medicine, New Haven, Conn.; Nader Sanai, M.D., neuroscientist, University of California at San Francisco; Feb. 19, 2004 Nature
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