I’ve always heard that brain cells do not reproduce and that once you lose brain cells, they dont grow back. You have a set amount of brain cells and you’ll eventually start losing them, but you wont gain any back.
I’ve always kinda accepted that. But is that true? I was just reading something that said ALL cells in the body are completely replaced within 7 years. It was talking about memory and conciousness and how this is unaffected by the cells dieing and reproducing.
I also know that the cells have to reproduce at least in the begining… The brain starts off pretty small in a child and then grows to adult hood. So the cells divide and reproduce at least through adolescence.
So what’s the straight dope? Are brain cells just like other cells in the body? Should people really be concerned about “losing braincells” because you can’t get them back?
I’m not a registered member at sciam.com, but according to their archive page, the article “New Nerve Cells for the Adult Brain” in the April 2002 special issue “The Hidden Mind” starts with:
Looking over some of the articles that came up through google, it looks like it was believed until very recently that neurons couldn’t reproduce, but then researchers discovered that they can. The brain doesn’t seem to be able to repair damage to itself the way skin and bones can, though.
And it seems they don’t replace themselves far more often than they do. And many which do replace themselves are the glial, or support cells, rather than the more crucial neurons carrying signals back and forth.
I’ve never done this before, but I’m afraid I’ll have to ask for a cite for this.
Or at least a little bit of background info. Are you a doctor? Was this just something your friend’s cousin told you? It’s not that I don’t believe you, but I’d like to be convinced before I repeat it. I’m staring at my 6 month old nephew right now and he has such a tiny little head with such a tiny little brain.
It sounds absolutely plausible… I’ve seen the effects of a vacuum on a marshmallow… but I’ve held many a human brain and it doesn’t feel very airy or fluffy…
Damn - and my hopes are dashed yet again!!! When the evil zombies ate my brain they only left me a few dozen stray braincells - but I’d hoped that they would eventually reproduce and grow me a new brain. Curses!!!
Sorry about being facetious in GQ, but, considering my name, I felt I just HAD to comment.
Oh… and Wanderer, at what point do you claim the cells actually stop and the brain has all the cells it ever will? The brain has to start as a couple cells and divide at some point. When does it decide, “That’s enough - Im a brain now”? 8 months in utero? 9 months? Birth? The rest of the body gets to keep growing after birth…
Damnit!!! You know, I read that post a couple times, then reread it on preview just to make sure… I thought “you’re being woooshed”, but I was convinced you were serious.
Damn… I’ve never been woooshed before.
It really would make some sense though. IF people claim that brain cells are not produced, then something would have to account for the larger volume from birth to adulthood…
If I understand the current literature correctly, terminally-differentiated neurons in the brain do not proliferate, and have extremely limited regenerative capacity. However, there does appear to be, within the tissue of the central nervous system, and originating primarily from a strip of tissue lining the lateral ventricles and the hippocampus, a population of stem cells. These stem cells are pluripotent, and can give rise to both neurons and certain glial cells. Last I checked, these cells seem to migrate to the olfactory bulb or stick around in the hippocampus, and indeed, that is where virtually all of the neuronal progeny of these stem cells appear to wind up. So, while this discovery that the brain does have the capacity to regenerate is certainly a paradigm shift of sorts, the brain is still nothing like the liver or the skin. Its regenerative capabilities are still very small, and apparently quite localized to certain regions. Exploiting the pluripotency of this stem cell population to repair injuries in a variety of brain regions is an area of intense research, and a very exciting prospect. I’m not sure if my money is on the adult or the embryonal stem cells, though. Time will tell.
loopydude, ok so the brain can’t heal regions that have been damaged. But as for the average brain cell… does it just live as long as a person lives? It doesn’t go through mitosis and then die? New brain cells are only created by this small group of stem cells? I know blood cells don’t reproduce themselves, but enough of them are created in the marrow, we dont mind that they only live a short period.
Do our brain cells live “forever”? If people did not die by other means, how long could his brain cells live?
It would appear that the great majority of the brain cells we have shortly after birth are all we ever get, and that some percentage of these cells do literally live as long as we live.
What the new research is telling us is that the brain may well have some very low-level capacity for repair. Especially in the hippocampus, there seems to be a natural rate of turnover that is entirely normal. In fact, this low rate of repair may be essential for forming memories and maintaining them. What the exact connexion is remains a mystery (last I checked), but it’s now pretty well accepted that the hippocampus regenerates a little, and that if this natural rate of regeneration decreases, memory is affected. Some have gone so far as to suggest that mood is also connected to this, and that reduced hippocampal regeneration could be connected directly to the etiology of depression. I’m not sure if I buy that, personally (cause or effect?), but it is still a pretty amazing finding.
However, I must stress again this is really a piddly amount of cell turnover compared to other parts of your body, like blood, liver, or skin. You can hack off half of somebody’s liver, and it will grow back in a few weeks. We all know this is NOT true of the brain. So what limited regenerative capacity the brain has is certainly not enough to help us cope with any kind of injury very well. Nor does it keep pace with the depradations of age or the sundry neurodegenerative diseases that afflict the CNS.
What biomedical science is hoping to do, now, is harness and augment this modest capacity for self-repair, and thus cure some of our neurological frailties.
But you have to wonder: The relative lack of plasticity in the CNS probably is dictated, from an evolutionary and physiological standpoint, by the very complexity of the system. Humans do have certain innate capabilities, but much of what makes us who and what we are is learned. It takes many years for our brains to mature, and all the skills we learn, and the memories we form, are dependant to a great respect on the brain’s relative lack of plasticity. Think about it: If the cells in your cerebral cortex were dividing and dying at a rate they do in, say, the skin, you’d literally forget everything in a few weeks or months. You need to make synapses between those neurons to form a memory, and those synapses have to last literally for years or decades to keep one. Since synapses are coposed of extensions of neurons, those neurons have to stick around too.
Our ambitious goals for stem cells in the brain may smack into one harsh reality: When we lose parts of our brains, we often lose parts of “ourselves”, that collection of memories and experience that makes us us… Replacing cells won’t replace such things. One wonders if we can save patients some day from the ravages of diseases like Alzheimers; however, I doubt very much we could ever save the person. Maybe diseases like Parkisons will prove good candidates. However, for Alzheimers (just to give one example), prevention may well be the key, and the only ethically viable intervention.
Oh, to answer the other question about “forever”: Since we don’t live forever, it’s pretty hard to say anything with certainty. However, I do know that some people die of old age even though mentally they remained pretty sharp. Some of us become demented long before we die. The rate we age mentally seems to have definite genetic and environmental determinants that are not necessarily connected to other parts of of our bodies. One of my great grandmothers was as sharp as a tack until she died at 97 of kidney failure. One of my granfathers, however, had to be put in a home at age 80 due to Alzheimers, and by age 87, when he died, he was completely demented. However, as many of the nurses at the home could tell you, he was physically stronger than many younger men at that age, which made him quite a handful to deal with. The arc of human life can be cruelly ironic can’t it?
“Forever” is a dream some scientists are pursuing. Whether they will have any success…well, who can say? A memory, theoretically, should last as long as the substrate it resides in (connections between neurons, so we think). What is the theoretical maximum of human neuronal life? Of the macromolecular structures of a synapse? 150 years? Could this span be significantly lengthened? I don’t think anyone really knows.
Thank you for the loads of information, loopydude!
What accounts for the change in mass (or is it just size) of the brain from neonate to adult? Do they come from the residual stem cells near the hypocampus as well??
OK, so the “gaining more air” comment was a joke. But maybe it swells with CSF and never really gains cellular mass, just size.
Well, you might note that babies and toddlers have disproportionally large heads, compared to adults. So our heads don’t grow as much as the rest of our bodies as we reach adulthood, proportionally speaking. I don’t know the exact dates, but from what I remember, most neuronal cell proliferation is over by age two. I guess there could be a trickle after that, but probably not much. Also, there are other cells in the brain besides neurons, like astrocytes and oligodendrocytes. There are also Schwann cells that cover the axons and make up a good-sized chunk of the brain’s white matter. Those cells keep dividing for your whole life, and its possible the ratio of glial cells to neurons might change as we mature, such that they make up a greater proportion of the brain’s mass as we age. Neurons themselves can also simply get physically bigger, via a process know as hypertrophy. Most references I’ve seen to neuronal hypertrophy are (like cardiac myocyte hypertrophy) related to pathological processes, but just like in the heart, I believe a certain amount of cell enlargement is a physiologically normal part of maturation and adaptation.
I’m no neuroscientist, so I can’t attest to the accuracy of all the above statements; I am using conjecture to a certain degree, informed by what I know in general about cell biology and what I remember from learning about the CNS years ago. I offer the above just to point out what is possible, as alternatives to neuronal proliferation, to explain real and/or perceived changes in brain size as a human being ages.
Oh! I can answer some of that.
Mylenation of the neuron sheath.
There is a fatty layer of umm… fat, that covers each neuron that increases neuron speed in sending impulses but slightly decreases the chance of new neuron connections being made. The mylenation process begins at about 22-24 years of age depending on nutrition. That increases the brain mass over time.
Also some of the prefrontal cortex (that brain right behind your eyes and forehead) is growing, expanding and sorting itself out at around ages 10-16 depending on nutrition and genes.
Should we expect then, in animals for whom memories and learned behavior aren’t as important, that their neurons would reproduce more readily than ours? For example, I doubt that mice have much of a sense of self, or personal history, that natural selection would bother to protect. (I’m prepared to be proven wrong here.) So perhaps a mouse’s brain would replace itself over a course of months, since it can better afford to “throw away” old neurons in favor of new ones.
Of course, mice don’t live that long either, so there might not be much selective advantage in having a brain that can regenerate like a liver, even for them. They’re going to die soon enough anyway, probably inside the belly of a predator. Perhaps I didn’t pick the best example species to consider.
I’ll rephrase my question. In contrast to us, are there any mammals whose neurons do regenerate — if not as rapidly as skin cells do, then at least more so than ours? Are there any birds or reptiles with this trait? Any vertebrates?
As a side opinion: if modern medicine were able to keep me alive and healthy to the age of 150 or more, I’m not sure I’d want to keep all my old memories. To be sure, they’re part of what makes me who I am. On the other hand, I would find it boring to be the same person for all that time. My real concern (and I guess this is part of what makes Alzheimer’s so cruel a disease) is that I wouldn’t be able to form new memories, and learn new skills.
