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From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

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And I'm Robert Siegel.

Scientists used to think we were born with all the brain cells we'd ever have. But in 1998, the researchers showed that even adults keep making new ones. That discovery launched a massive effort to figure out where these cells come from and where they go.

As NPR's Jon Hamilton reports, a team from New Zealand now seems to have the answer.

JON HAMILTON: The head of that team is Richard Faull, an expert on brain diseases at the University of Auckland. He is not exactly the person you'd expect to make a big discovery about new brain cells. They are, after all, a type of stem cell.

Professor RICHARD FAULL (University of Auckland): I don't have a big background in stem cells at all. And in fact I wasn't interested in stem cells and neurogenesis.

HAMILTON: Faull was more interested in how old brain cells died than in how new ones appeared.

Professor FAULL: But when I read that paper, I said this is the most interesting paper that I had ever read.

HAMILTON: That paper was the 1998 discovery that people continue to make new brain cells throughout their lives. Faull became obsessed with figuring out what was happening to those new brain cells.

Professor FAULL: If we could just talk to them and say where are you going? They would say I'm going off to the basal ganglia, or something. You know what I'm saying?

HAMILTON: Researchers had figured out where new brain cells go in rats, but not in humans. In rats, the cells are born in a part of the brain called the subventricular zone. Then they follow a pathway that leads to an area of the brain involved in smell.

Professor FAULL: And we could see evidence of this pathway in the human, but we lost it.

HAMILTON: Faull's team kept looking. With help from researchers in Sweden, they studied brains from dozens of human cadavers. The teams cut the brains into thin slices, then peered at them through high-powered microscopes. In some brain sections, the pathway seemed to be there. But in others it just disappeared. Meanwhile, researchers elsewhere concluded that this kind of pathway simply didn't exist in people the way it does in animals.

Even Faull began to have his doubts. Then his team got an idea. Maybe they were looking at things from the wrong angle.

Professor FAULL: We like to think kiwis down here in New Zealand, we've got a bit of ingenuity. And so we turned around and we said right, instead of cutting from the front to the back, we cut long sections which go along the length of the forebrain.

HAMILTON: And there it was, the missing pathway. Actually, the biological equivalent of a superhighway for brain cells. And just like in rats, it took new cells from the subventricular zone to the brain's smell center. That raised another question.

Rats need a keen sense of smell to survive. But why would humans send a river of new brain cells to a place that sorts garlic from oregano? Faull thinks the answer is that most of the new cells are diverted before they reach the human smell center.

Professor FAULL: It's like having, you know, the freeway from Boston to Washington, D.C. It's actually got off-ramps going off to New York City and all the rest of it. And we are seeing hints that cells are leaving this pathway well before the end of it.

HAMILTON: They're probably going to places involved in more important functions, like memory or motion. And these new brain cells may be vital to keeping these parts of the brain working. No one has shown that yet. But there are some hints. Story Landis directs the National Institute of Neurological Disorders and Stroke. She says the existence of a common superhighway for new cells could help explain a surprising observation.

Ms. STORY LANDIS (National Institute of Neurological Disorders and Stroke): For some neurodegenerative diseases like Parkinson's Disease, there's some thought that sense of smell is one of the first symptoms.

HAMILTON: You mean, diminishment of -

Ms. LANDIS: Diminished - inability to smell things.

HAMILTON: That would make sense if there was a connection, a pathway between the brain's smell center and the area affected by Parkinson's. Landis says what researchers need to do next is find a way to use this pathway to get new brain cells to places that need repairing.

Ms. LANDIS: Parkinson's, stroke, Alzheimer's disease, all very pressing diseases where nerve cells die. And we would love to be able to replace them.

HAMILTON: And that's the big challenge. The new research appears in the online edition of the journal Science.

Jon Hamilton, NPR News.

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