Jake over at Pure Pedantry links to a recently published article which shows that the adult neocortex has roughly the same number of neurons (but more glial cells) than the neocortex of a newborn. This is an interesting study and deserves a brief comment. As I wrote in Seed earlier this year, neurogenesis – the idea that neurons in our brain are constantly dividing – is no longer controversial. Thanks to the persistence of Elizabeth Gould, Fernando Nottebohm and others, the paradigm has finally shifted.
But the details of neurogenesis remain hotly debated, especially when it comes to the neocortex. In 1999, Gould announced that she saw new neurons migrating to neocortical cell layers. Pasko Rakic disagreed. Other scientists weighed in, but no one offered definitive proof. (Apparently, it’s not easy detecting new cells…) This latest study seems to support the idea that neurogenesis has little, if any, impact on our neocortex. This is a potentially deflating idea. If our cortex is suffused with newborn neurons, then it might be possible to repair structural damage caused by stroke, tumors, etc. (Of course, even if we don’t naturally produce new cortical neurons, several labs are trying to create new neurons anyways, both by stimulating endogenous neurogenesis and transplanting stem cells.)
But this study also leaves some important questions unanswered. As Jake notes, “There is still the possibility of a certain level of turnover — cells being produced at the rate that old cells are dying.” I think the definitive answer to the question of whether or not new neurons actually migrate to the neocortex will come from Dr. Jonas Frisén, of the Karolinska Institute. His lab has pioneered a method that uses the decay of nuclear fallout in order to precisely date the age of our cells. This strikes me as the only way to determine whether or not there is a high amount of cell turnover throughout the brain, and not only in the hippocampus and olfactory cortex. When I talked to Frisen last year, his tentative dating results, he said, made him “more excited than ever to be part of the neurogenesis field.” So stay tuned. This story is far from over.