It has long been a dogma of neuroscience that the human brain is born with all the neurons it will ever have, and that those neurons must endure for a lifetime. But evidence has been accumulating that this dogma may not strictly be true. Neurons in the hippocampus and cerebellum, for example, differentiate through the first several years of childhood. In other species, such as mice and marmoset monkeys, new neuron growth has been found in adults. And neuronal progenitor cells have been discovered in adult human brains, although it was not known whether these stem cells normally divide into neurons. It now appears that they do, according to a study appearing in the November 1 issue of Nature Medicine by Peter S. Eriksson et al. The study involved terminal cancer patients who had been administered a diagnostic agent, BrdU (bromodeoxyuridine), that labels dividing cells. Upon the patients' death, their brains were examined for the presence of BrdU. "All of the patients showed evidence of recent cell division," said Salk Professor Fred H. Gage, senior author on the study. "It's interesting to note that this was not a particularly young or healthy group of people, so new cell growth may usually be even more prominent than we observed." The new growth was observed in the hippocampus, which plays a crucial role in learning and memory. "At this point, it's premature to say that the new cells are being used for learning and memory, but given their location in the brain, it seems reasonable to suggest that they likely do," said coauthor Daniel A. Peterson, a postdoctoral fellow in Gage's group.—June Kinoshita

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  1. . Neurogenesis in the adult human hippocampus. Nat Med. 1998 Nov;4(11):1313-7. PubMed.