Can we humans grow healthy new brain cells as adults and during later aging?
The answer to this question is a big deal because it could have major impact on aging. Those hoped-for new brain cells could keep our brains functionally younger. Who doesn’t want to slow down the aging of their brain, right? Slowing down aging of the brain via new cell growth more broadly would have major impacts on society.
Researchers have gone back and forth on this over years.
Fairly long ago, it was thought the answer was definitely “No, we humans can’t grow new brain cells”, but in the last few decades a loose consensus has emerged that in fact the answer is, “Yes, to some degree.”
A recent Nature paper, however, said “No” again. It challenged that “Yes” by directly arguing we don’t make new brain cells. Neuroscientists who study this stuff generally reacted skeptically. The paper’s title was, “Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults.”
Yet an even newer paper,this time in Cell Stem Cell that is Boldrini, et al. argues “Yes” and goes further to say, in fact, that we adults and even older adults can grow brain cells just about as well as the younger folks. This paper’s title pretty much directly contradicts the above one as it says, “Human Hippocampal Neurogenesis Persists throughout Aging.”
So who’s right and who’s wrong?
My gut feeling is that there is adult human neurogenesis and there are stem cell populations in adults and even fairly aged humans.
It’s really hard to conclusively prove a negative thing in science like the claim that there’s no neurogenesis in adult brains. I bet the team recently arguing no detectable new neurogenesis used methods that simply didn’t detect it. It’s much harder to dismiss the findings in the newer paper (and in many past ones) showing adult human hippocampal neurogenesis and the presence of stem/precursor cells. See part of Boldrini Figure 1 above showing evidence of neural progenitor/stem cells in the adult brain. The data in that paper are clear.
Still, this new paper arguing for sizable levels of stem cell activity in the adult human brain found some effects of aging on the brain and that neural precursor numbers in older adults are somewhat lower than say teenagers.
People are more generally really wanting their brains to stay robust as they age. I’ve covered brain aging in the past on this blog and efforts to counteract it, mainly via stem cell-related efforts such as exosomes, which were the focus of a paper I was fairly skeptical about some months back. Some people also think that stem cell transplants or infusions of young blood might fight brain aging, but those are somewhat wilder ideas.
I’m hopeful that there are real ways eventually found to keep our brains healthy and even producing more new, healthy cells and stem cells as we add on the decades. The debate over adult human brain stem cells might continue in years to come, but most people I know think there is some neurogenesis going on even as we age, which provides hope for the aging human brain.
Hi, Julius.
Of course it is worth pressing on to answer questions relating to the potential of neuronal replacement from endogenous neural stem cells. I am merely saying that neuronal replacement may not be the only mechanism that endogenous neural stem cells can use to help patients.
Dear Andreas,
“In the adult, development is long over.” I think the “hard-wired in infancy” dogma has been countered many times by evidence of neuronal rewiring in adults with brain injury or disease, and also by the unequivocal evidence of hippocampal neurogenesis in the adult.
Taking up your analogy of throwing in some cables into the computer (transplanting stem cell derived neurons into the brain), the specific joining of the cables you speak of is driven by a hierarchy of neurotransmitters, and these signals will be there when the dysfunctional neuron is unable to respond. Hence (and probably within a window of opportunity), there is reason to believe that exogenous neurons, such as dopaminergic neurons, can be steered by the local environment and wired up appropriately.
Whether these application applies only to “basic” functions, such as motor responses, or if it may also be useful for higher functions, such as facial recognition, speech, etc. is an unanswered question, But I think it´s worth pressing on to answer these questions, if only for patients with ALS, for example, that may benefit from the renewal of basic input-output circuits.
Is neuronal replacement the only way to slow down brain aging?
Is it possible that (1) we, mammals, still have a population of adult neural stem cells in or brains, but (2) the environment within our advanced brains has evolutionarily changed over time in such a way that our stem cells are not able to generate a significant amount of neurons but, still, (3) with some help – e.g., through pharmacological manipulations – we might be able to coax these cells to be activated?
Activation could conceivably see them replace some injured neurons, especially local ones that are also physically short. But, generally, expecting cell replacement to work well would be like taking a broken computer, opening it up, throwing in some bits of cable, closing it, and then hoping it will work again. Cables join very specific points together. This happens during development. In the adult, development is long over.
Might it be that, as the mammalian brain function gradually shifted away from its original regenerative ability (seen in more primitive animals where it regularly saves their lives) towards grreater rigidity (animals whose survival depends on acquired knowledge need rigidity to store valuable information in the long-term), endogenous neural stem cells have adopted a new role that combines their inherent plasticity and a less complex but still highly consequential role: to respond to injury and activate functions that protect injured neurons from stress and damage? There is corroborating precedent in the scientific literature for this.
Well, if this study
http://www.sciencemag.org/news/2012/09/bearing-sons-can-alter-your-mind
is accurate, then perhaps there is some neurogenesis. And it might be good, or bad.
While there is a controversy, it is heartening that there atleast is a debate on this controversy. Comparing CNS neural stem cell biologists, adult enteric neurogenesis while even more robust and controversial lacks any kind of debate, whether within or outside of the community