Scientists used to think that we were born with all the brain cells we'd ever have. But in 1998, 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. Now a team from New Zealand seems to have found the answer.
The head of the team is Richard Faull, an expert on brain diseases at the University of Auckland. He's 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.
"I don't have a big background in stem cells at all," Faull says. "In fact, I wasn't interested in stem cells and neurogenesis."
Faull says he was more interested in how old brain cells died than in how new ones appeared. "But when I read that paper [on adults producing new brain cells], I said, 'This is the most interesting paper that I had ever read.'"
Faull says he became obsessed with figuring out what was happening to those new brain cells.
"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," Faull says.
Researchers knew 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.
"And we could see evidence of this pathway in the human," Faull says. But they couldn't trace the path to its end.
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.
Elsewhere, some researchers had 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.
"We like to think 'kiwis' down here in New Zealand; we got a bit of ingenuity," Faull says. "So we turned around and we said, 'Right, instead of cutting from the front to the back, we cut sections which go along the length of the forebrain.'"
And there it was: the missing pathway. Actually, what they'd found was the biological equivalent of a superhighway for brain cells. And just as it did in rats, the pathway took new cells from the subventricular zone to the brain's smell center.
The research was published in the online edition of the journal Science.
Faull says the discovery raised an important 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.
"It's like the freeway from Boston to Washington D.C.," he says. "It's actually got off-ramps going off to New York and all the rest of it. And we are seeing hints that cells are leaving this pathway well before the end of it. "
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, who directs the National Institute of Neurological Disorders and Stroke, says the existence of a common superhighway for new cells could help explain a surprising observation.
"For some neurodegenerative diseases like Parkinson's Disease," she says, "there's some thought that an inability to smell is one of the first symptoms."
That supports the idea that there is a connection — a pathway — between the brain's smell center and the area affected by Parkinson's.
Landis says that next, researchers need to find a way to use this pathway to get new brain cells to places that need repair.
"Parkinson's, stroke and Alzheimer's disease are all very pressing diseases where nerve cells die," Landis says. "We would love to be able to replace them."