Dear Cecil, Straight Dope Science Advisory Board, or Teeming Millionth:
I’ve heard that every cell in my body eventually dies and is replaced by another cell. This “serve, die, and get replaced” cycle will continue until, eep, my death. Supposedly I could say that my entire body is comprised of 100% different material after “x” months. (“x” being a number that I don’t remember - but could be, for argument’s sake, eleven) If that’s true, then why do: my joints stiffen, my scars stay forever, and other yucky aging things happen as I get older? It seems to me that all these vibrant, springy new cells have no right making me saggy and sluggish.
I don’t recall the exact scientific reasons, but basically as we get older our bodies get less effective at replacing cells, and the replicas aren’t exact copies of the dying cells. They’ve figured out that there’s a part of the cell that controls this, it gets smaller and smaller with each cell replication. I’m sure some of the teeming millions can find links and/or explain this better than I can.
Where did you get the idea that cell replacement goes on until death? Just because most body cells are replaced on a regular basis, and not all of them are, doesn’t mean the replacement never stops. One of the things learned from recent cloning experiments is that DNA may have a finite life span.
The problem is that cells do not replicate exact copies, except for cancer cells which are pretty much “immortal”. Check out this site.
The problem is in an enzyme called telomerase. This enzyme keeps the genes from unravelling, which is what causes aging. Scientists are at work on at finding an anti-tolemrase substance that can be used to fight cancer. Cancer cells, it seems, produce telomerase which is what allows them to continue growing instead of dying out like regular cells.
When scientists know more about how it works, they may be able to help a body produce more of the stuff, in order to prevent aging. That’s probably a good ways down the road, yet.
BTW, not all of your cells get replaced. Loss of bone mass is a pretty typical problem in older folks. This is because bone cells do not get replaced (IIRC). Cecil covered the topic of the body replacing its materials, but so far I haven’t found it. It seems to me that the brain is also excluded.
Now with 1000 posts of pure wisdom!
[Note: This message has been edited by manhattan]
And remember, we’re made of more than just cells. Much of us is actually acellular, eg. connective tissue proteins, intercellular matrix,…, but it, and we, ‘AGE’ nevertheless. The acronym AGE refers to advanced glycation endproducts. These are formed when glucose molecules serve to crosslink or bridge together previously solitary protein molecules . (Imagine an ‘H’ with the two vertical lines being proteins and the horizontal line a glucose molcule). When linked in such a manner, the proteins’ function is invariably compromised.
The chemical reaction described above occurs in direct proportion to both the time available for it to occur and to the ambient glucose concentration. So if you’re old or diabetic, you AGE.
I remember hearing on the Learning Channel that no cell in the human body at any given time is more than ten years old. Other reasons for aging to occur, which go along the lines of the telomeres, are that cell replication is not exact sometimes due to mutations in the DNA. These mutations may not be harmful at first, but a mistake here and there can add up over hundreds of replications.
I also remember hearing about things that cause aging, and one of them was oxygen. Oxygen is actually damaging to the cells somehow. I definitely remember hearing this, but I don’t remember where I heard it, or to what extent damage is done. Maybe someone who remembers the program better than me can explain further.
Bone cells do continue to regenerate throughout life. But what gives bones their structural strength is NOT the cells, but rather the calcium salts which are deposited in the protein matrix.
As other posters have mentioned, nerve cells in the brain & spinal cord divide & grow into early childhood, but what we have by age 6 is what we have for life. After that, nerve still grow new extensions, or dendrites that add to the complexity of their connections to other cells, but do not regenerate. This is why spinal cord injuries lead to permanant paralysis; the nerve cells cannot regrow & reestablish their connection to parts of the body on the other side of the injury. Hopefully, in the future, this will change as medical researchers learn how to isolate primitive neaural stem cells which retain the ability to grow & divide and have them replace nerve cells which were lost in the injury.
Sue from El Paso
Experience is what you get when you didn’t get what you wanted.
How does the DNA effectively “reset” itself when an animal reproduces? I recall that “Dolly” (the sheep cloned in Britain) is showing signs of advanced age, because her DNA is degrading-it is as if she is as old as the sheep who was cloned. So, how does the biological clock reset itself, seing as the new DNA is a conbination of the DNAs of the parents?