previous research by Alvarez-Buylla, who reported in 1999 that astrocytes function as adult stem cells in the subventricular zone of mice, churning out fresh, young neurons that then migrate to the olfactory bulb in a unique pattern of movement he called chain migration. The constant replenishment of young neurons, scientists suspect, maintains the animals' keen sense of smell. The finding was one of the first reports identifying adult stem cells in the brain of any mammal as astrocytic (Cell, June 11, 1999). Other scientists have subsequently determined that neural stem cells in the subventricular regions of rats, dogs, sheep, and monkeys also generate neurons that migrate in a chain-like fashion to the olfactory bulb.
Since then, scientists have demonstrated that adult stem cells exist in the subventricular zone of the human brain, but they have never detected the identity, organization or function of these cells. The current study is the first to not only discover these characteristics, but also to isolate human astrocytes that could function as neural stem cells.
At the outset of the study, the researchers discovered that the outer walls of the brain's central cavities, or ventricles, were lined by a ribbon of constantly dividing astrocytes. They also detected fresh, new neurons in the region. Much to their surprise, however, the researchers determined that these young neurons did not form migratory chains to the olfactory bulb. Exactly where the progeny of this ribbon of neural stem cells might go, if anywhere, is still unknown.
The fact that the human brain does not maintain the pattern of chain migration seen in so many other animals, including primates, is surprising, says Sanai, given the evolutionary propensity of animals to conserve anatomical structures. At the same time, he says, it makes sense, given humans' relatively weak sense of smell.
"The fact that we're not seeing a mass exodus of newly-formed neu
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Contact: Jennifer OBrien
415-476-2557
University of California - San Francisco
18-Feb-2004
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