Does the human brain continue creating new neurons throughout adult life? The idea that neurogenesis exists in the adult human hippocampus has generated a huge amount of excitement and stimulated much research. It’s been proposed that disruptions to neurogenesis could help to explain stress, depression, and other disorders.
But a new study, published in Nature, has just poured cold water on the whole idea. Researchers Shawn F. Sorrells and colleagues report that neurogenesis ends in humans some time in childhood.
Sorrells et al. studied tissue samples from two sources: post mortem brains from deceased donors, and tissue taken from the brains of living epilepsy patients. In both sets of samples, they found that the rate of newborn hippocampal neurons (DCX+PSA-NCAM+ cells) rapidly declined after birth, and was undetectable in people aged over about 13.
Here’s the key result:
What’s more, the supposed neurogenesis “hub” in the human hippocampus simply doesn’t exist, according to the new study. In rodents, adult neurogenesis takes place in a region called the subgranular zone (SGZ), located in the dentate gyrus of the hippocampus. Strikingly, Sorrells et al. were unable to detect the SGZ in humans at any age: unlike it rodents, it never forms as a distinct structure in humans, they say.
This result challenges previous studies, using other methods, that did report adult neurogenesis in the human hippocampus. However, the new study is consistent with another no-neurogenesis finding from a couple of years ago. For more on the methodology of these neurogenesis studies, see this excellent post by Jason Snyder. Overall, it seems to me that the evidence against adult neurogenesis is becoming pretty convincing.
So what does this all mean? Sorrells et al. conclude by speculating, provocatively, that our lack of adult hippocampal neurogenesis may actually be part of what makes us human:
Interestingly, a lack of neurogenesis in the hippocampus has been suggested for aquatic mammals (dolphins, porpoises and whales)(5), species known for their large brains, longevity and complex behaviour.
This hypothesis seems pretty wild to me. But it’s no wilder than some of the other theories that have long surrounded adult neurogenesis, the function of which remains unclear. From memory to depression, hundreds of papers have been published on the possible function of the phenomenon. Most of the claimed functions are based on rodent studies, and while it may be that in rodents, neurogenesis helps to (say) prevent depression, it looks like the same is not true of humans.
Still, I don’t think we should worry about our lack of neurogenesis. Our brains are still plastic, thanks to the fact that even old neurons are able to change and develop new connections. The brain doesn’t need new cells – our existing ones are adaptable enough.