The Migration of New Neurons

It's been one of the enduring mysteries of neurogenesis: where do all our new cells go? Do they plug themselves into the cortical network? Do they travel to the olfactory cortex? Or do they wither away and die, a vestigal legacy of a more primitive brain?

Now a big part of the puzzle has been solved, in a groundbreaking paper in the new Science by researchers in New Zealand and Sweden. They located the superhighway that conducts newly born cells across the brain:

The rostral migratory stream (RMS) is the main pathway by which newly born subventricular zone (SVZ) cells reach the olfactory bulb in rodents. However, the RMS in the adult human brain has been elusive. Here we demonstrate the presence of a human RMS, which is unexpectedly organized around a lateral ventricular extension reaching the olfactory bulb (OB), and illustrate the neuroblasts in it. The RMS ensheathing the lateral olfactory ventricular extension, as seen by MRI, cell specific markers and electron microscopy, contains progenitor cells with migratory characteristics and cells which incorporate BrdU and become mature neurons in the OB.

We've known for several years that new neurons in rodents are born in the subventricular zone, and then migrate along a predetermined path to the part of the brain that is involved with the sense of smell. A similar pathway had been identified in humans, but it was hard keeping track of the new cells. (Keep in mind that scientists didn't even believe in neurogenesis until the mid 1990's: these neurons are hard to see.)

So what allowed this group to image the neurogenesis superhighway? The solution, as so often happens, came from a slight correction in their experimental technique. Here's NPR:

Then the research 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.

Of course, many questions remain. The brain produces more new neurons than our olfactory cortex needs, so what happens to the rest of these cells? The scientists hypothesize that most of our new neurons are diverted before they reach the olfactory cortex.

"It's like the freeway from Boston to Washington D.C.," Faull [a senior author] 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.

PS. For more about neurogenesis, check out the new Mind Matters.