The finding a few years ago that adult primates, including humans, could produce new neurons garnered much excitement. Ever since, scientists have been searching for further evidence, in various parts of the brain, that this is indeed the case. According to a study published in the current issue of the journal Science though, adults primates can't create new neurons in the most sophisticated part of the brain, the neocortex.

David Kornack of the University of Rochester and Pasko Rakic of Yale University studied the brains of adult macaque monkeys injected with two types of markers: radioactive material that tags new cell growth and molecules that mark neurons. The scientists then examined more than a thousand cells from the neocortex that contained both types of markers, suggesting that these cells were new neurons. They found that many seemingly new neurons were instead two separate cells such as an old neuron and a newly generated glial cell. "The present findings," the authors write, "lead to the conclusion that neocortical neurons are not normally renewed during the life-span of macaque monkeyssimilar limits may exist in the human forebrain."

The scientists did discover new neurons in two other parts of the brain, the hippocampus and the olfactory bulb. According to Kornack, "if we can find out what allows stem cells in those few restricted brain regions to continue producing neurons into adulthood, perhaps we can mimic that magic in other areas of the brainsuch as the neocortexthat can suffer neuronal loss but don't normally make neurons."