Is Human Adult Neurogenesis Dead? And Does It Matter?

By Neuroskeptic | March 14, 2018 3:14 pm

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:

sorrells_neurogenesisWhat’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.

CATEGORIZED UNDER: neurogenesis, papers, select, Top Posts
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  • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

    The brain is fluidly partitioned into evanescent recursive topological insulator domains. Any attempt to partition functionality creates its own partitions, which vanish when the observer withdraws. May I have my massive grant funding now?

  • rthorat

    All those studies that say antidepressants increase BDNF, and therefore neurogenesis…this sure puts them in a new light. Since one main criticism was that it was more likely BDNF increases were a sign of neuron damage. Now that the neurogenesis claim doesn’t seem to be real…

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  • Dane Parker

    This is hardly yet sufficient to displace, in my opinion, the idea that human adult neurogenesis is real. But by all means, let’s get some follow-up studies going; then we can perhaps better understand where things stand. The studies thus far — many, admittedly, taking an indirect approach of various happenings downstream, which hypothetically promote neurogenesis — exploring psychopharmacological models of antidepressant efficacy (as another commenter already pointed out) must mean something. At the same time, direct provision of BDNF (a presumably important agent vis-a-vis antidepressants) to the CNS has not itself been effective in resolving conditions for which these BDNF-provoking agents are utilized to treat. Perhaps neither view is completely wrong — even if both are, when construed in their strongest forms, incompatible. If the effect of antidepressants, not so much as promoting neurogenesis, but rather prove to protect neurons from destructive stress while promoting ameliorating neuronal rewiring, is it any less true that, as we say now, antdepressants can indeed help patients who are depressed?

  • reasonsformoving

    Do these results implicate myelinogenesis? It’s often said that the brain doesn’t finish myelinogenesis until one’s mid 20’s.

    • http://blogs.discovermagazine.com/neuroskeptic/ Neuroskeptic

      No, this study doesn’t look at myelin, that’s a separate process.

  • Neuro_Seance

    I for one am relieved. I thought it was horrible. There was no proof that any putative drug-induced neurogenesis was helpful. I hold a low opinion of any scientist who would proclaim such an effect beneficial without knowing whether it was or was not.

    It’s a grave matter, adjusting the physical brain. Neurogenesis seems like a great first step on the way to neoplasm.

    Less drastically: pretend you’re my hippocampus, just idly hippocampusing around in your part of my brain. One day, 300,000 new neurons appear and say they want to help you. They can’t all be sent out for tea and biscuits, or bagels and coffee. Whatever are you going to do with them? You have your connections all connected up and humming along. Will the new nerves form a suburb on your outskirts? We know from immigration studies that if they do that, they’ll never integrate. Or will they be more like urban infill, nestling into gaps between existing neurons, glibly gumming up the works while doing nothing more useful than sending headshots to Lundbeck, hoping to star in its next misleading Vortioxetine marketing campaign?

    I don’t know what we are supposed to think about the many studies that claimed to show certain drugs causing neurogenesis. Weren’t they reporting on MRI findings? Will RetractionWatch have to double their rack space?

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  • Marc

    This study leaves a lot to be desired as well.

    First of all a very small sample.

    Secondly, this leaves open the question of “stimulated” neurogenesis in response to exogenously induced growth factors.

    Thirdly, human hippocampal volume has been shown to increase in response to exercise, SSRI’s, and other promoters.

    Increased human hippocampal measured volume correlates to improved hippocampal-related symptoms in individual subjects.

    We know very little about the history of these cadaver specimens, their life-state, or state at death.

    Using this study to close the door on hippocampal neurogenesis seems premature and much too definitive. I remain open to multiple possibilities for adult human hippocampal neurogenesis — none, a little, a notable amount, etc.

    The article also included rank speculation on “why” humans might be different from similar high-order mammals. That didn’t provide a lot of confidence in the humility of the researcher.

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About Neuroskeptic

Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.

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