Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why don't humans regrow lost limbs?

  1. Aug 29, 2010 #1
    Dear readers,

    I have been thinking about this for a while and would appreciate it if anyone could kindly give me their input.

    It is a tragedy when people lose their limbs or are born without them. I have read that some other animals can regrow limbs if they are lost such as the newt or Salamander.would it be possible to use a biological marker such as bioluminescence to identify the processes involved and then use gene therapy or another treatment to engineer a Human to perform the same neat trick?

    thanks for your input.
    Last edited: Aug 30, 2010
  2. jcsd
  3. Aug 29, 2010 #2


    User Avatar

    Staff: Mentor

  4. Aug 30, 2010 #3
    Thankyou for the link Borek, its is an interesting subject I think.
  5. Aug 30, 2010 #4
    I’m not sure whether to take this notion seriously. The only reason why I am taking this on myself is because none of the serious biologists who contribute to this forum have.

    So. Limb development in human beings is something that happens during the embryonic stage. The human embryonic developmental stage lasts about eight weeks, after conception. Once it’s over, your opportunity for any seriously different or new morphology is gone. To the largest extent all that happens after that is that you get bigger. Okay, yes that’s not entirely true. There is some measure of further development that occurs all the way through birth and onward until adulthood. Also the human body has this extraordinary ability to repair itself. And yes, to some extent, that repair process is a tiny little echo of what occurred on a much larger scale during embryonic development. But it’s a long way from being the same thing. And even if technology did find a way to re-trigger the embryonic developmental sequence, if you were an adult who had lost a limb, what you would develop would be an infant limb. To regrow an adult limb would be a completely different prospect than what happens during embryonic development. You’ll gather, to me, the suggestion is ridiculous. And because a particular species of lizard can do it, is no indication whatever that human beings will ever be able to do it.
  6. Aug 30, 2010 #5
    Well ken the issue with your POV is that all lower animals can do this easily, so the challenge isnt that hard. Id barter that the internal muscluature of a crab arm or leg is far more complicated than ours.....rip it off though and he gets a new one. On this one we got the short end of the evolutionary stick.
  7. Aug 31, 2010 #6


    User Avatar

    Staff: Mentor

    I guess infant hand is much better than no hand at all.

    So you suggest that we should not try, because the idea seems ridiculous? Look around - we are surrounded by things that were once just a ridiculous ideas.

    I agree that it is far fetching and may prove impossible at some stage. But to not try would be a mistake.
  8. Aug 31, 2010 #7
    Yes, okay, I was not for one moment disparaging the efforts of biology laboratories. I have seen, on television quite recently, a programme that showed a laboratory somewhere in the USA – I’m sorry I forget where – where they have successfully grown a rat heart – and it was beating! Believe me, no-one needs to convince me of the extent of the achievement this represents. The programme that showed this was all about people with serious injuries, including lost limbs and including serious spinal injuries, and their prospects for a ‘cure’. The four individuals it focussed on were mentally very strong people who were handling their circumstances very well. But it also showed how strong was the need that each of them had to believe in the prospect that their condition might one day be curable – even if not in their own lifetime – and it also reflected the cruelty of giving such people false hope, which some unscrupulous types are not above doing for personal gain.

    But my point was about the embryonic developmental process itself. It is a fantastically complicated sequence of gene expression leading to the making of one protein, which then triggers adjacent cells to express a different gene to make another protein, which in turn triggers other adjacent cells to express another gene… and so on. The developmental process is not even a simple linear growing of the final morphology. The development goes through several stages that are not obvious progressions in the direction of the final form. The point is that in embryonic development, context is everything, and the context of the growing of an embryonic limb is vastly different than the context of growing a new limb for an adult.
  9. Aug 31, 2010 #8
    As far as I remember (I'm not a biologist), limbs don't regrow because some crucial proteins that are responsible for cell regeneration shut down very early on. And the reason they shut down is because letting them remain active exposes you to a higher risk that some cell in your body will go awry and start dividing uncontrollably, a.k.a. cancer.
  10. Aug 31, 2010 #9
    Two companies have come out with some kind of "regenerative matrix" which, when applied to the cut limb, can trigger a regeneration of the limb. However the regeneration process is not like from a sci-fi movie but a slower process.

    The two companies that I know of are:

    ACell Inc:

    http://www.lifecell.com/alloderm-regenerative-tissue-matrix/95/ [Broken]

    At least in the case of the ACell product I remember reading that it was based on some found in the stomach of pigs. These substances (proteins, if I do not remember wrong) are what trigger the regeneration process.
    Last edited by a moderator: May 4, 2017
  11. Aug 31, 2010 #10


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Context is always everything, also beyond the embryonic stage.

    So, change the local context around the adult limb, or stump of an adult limb, and odd things might begin to happen.

    The scale of the undertaking would be mind-boggling and totally unprecedented by anything as yet produced, but that in itself doesn't prove impossibility.

    However, I do not think this will ever be done, because the knowledge needed to do it would also open up other technological pathways that will prove a lot more cost-effective:

    The minute understanding required, for example, of the neural system would be knowledge that probably quite easily could be coupled with machinery that can translate and transfer neural signals between the extremity and the central neural system.

    Thus, getting a computerized arm, rather than a new biological one, is a much more likely prospect We might even prefer it to staid, old biology..

    For example, we need not regrow muscular tissue or blood vessels, yet still have a fully functional arm with equal degree of sensitivity as the real thing (or hand), but a lot more durable. Nor do we need to wait a long time for the cells to grow, either, in order to have an arm or hand that does what it is supposed to be doing.

    In the future, we can become cyborgs, rather than lizards.

    That's my speculation, anyways.

    The perfection of prosthetic technology will also open up ethical dilemmas that today are unthinkable:
    Why cauterize a flesh wound (or fix a broken bone) and have the patient on lots of antibiotics and a long reconvalescence period when it becomes easier, and a lot cheaper to chop off the limb and replace it with a new one?

    Sentimentalists will whimper about their rights for bodily integrity, whereas doctors will fume over irrational patients who wont't take their medicine..

    Same old story, but in a new wrapping.
    Last edited: Aug 31, 2010
  12. Aug 31, 2010 #11
    I have to agree here... I'd expect a pretty high-end prosthetic before regrowing a limb, but you know what, there will always be a demand for that. Some people are going to want to be "au natural", and that might be enough to drive research. Personaly, I'd like the full cybernetic package, thanks. :wink:
  13. Aug 31, 2010 #12


    User Avatar

    Staff: Mentor

    This is all pure speculation, but I don't see why we should know how the neural system in the limb works. We have to know how to start regeneration process, but beauty of the solution lies in the fact that if the growing process is started it goes on its own - and it doesn't need detailed instructions from us, as the detailed recipe is already in our DNA.
  14. Aug 31, 2010 #13


    User Avatar
    Science Advisor
    Gold Member

    Surprising, the genomes of animals are more similar than once thought. The differences come not in different coding regions of genes per say, rather in the timing of expression.

    As someone else touched on, during embryonic development the timing of expression along with another important factor: gradients, is crucial to correct development.

    Many of these developmental genes are only expressed in embryonic tissues, not present in adults. Nor are the biochemical gradients present in fetuses, present in adults. These "morphogenetic fields" are what drive cell growth and differentiation toward body structure.

    Also of note is the type of stem cells found in adults and the fetus. The fetus has more "general stem cells" which can differentiate into many different cell lineages (in deed, at one point a fetus was composed of cells that could become any type of cell). We call this pluripotency (totipotency for the part in parenthesis). Adult stem cells are more limited in to what they can become, We have some multipotent cell lines (like hematopoietic cells, which can become any type of blood cell) and many more oligopotent cells, which can only become a few cell lines.

    There are a couple of reasons why those pathways are turned off in humans (and most mammals), that has to do with tissue complexity and differentiation.

    As someone pointed out, cancer is the "haywire division of cells" and certain genes (when broken) are more prone to cause a cell line to quit it's day job and live life selfishly. We call those genes, oncogenes --Or genes which help turn a cell cancerous.

    The life of the fetus or embryo is one big race of division, which means many of those checks and balances on potential oncogenes are "off". Of course, this is fine for a fetus or embryo, from who's standpoint rapid division is a necessity of survival. However, in an adult it would make little sense (evolutionary speaking) to leave these genes on and run the risk of a cancer killing off the organism before successful reproduction (ever wonder why cancer is mostly a disease of age?).

    The upside then is a lower risk of cancerous growths claiming you before reproduction; at the cost however, of regenerative ability.

    You'll note that animals with "regenerative prowess" have some common features: Small, simple (from a histological stand point), mostly short lived, mostly lacking in (mammalian level) nervous system development, etc

    As to whether we'll be able to do it one day? With the advances in gene therapy and our understanding of genetics-I'd venture yes. I think its more likely we'd see grafts grown in a lab then transplanted to our body, before we see some kind of magic tape you wrap around a stump. As someone else mentioned though, this kind of therapy may only be a novel approach to fixing limbs, as our technological abilities (particularly in the field of brain-computer interfaces and cybernetics) may simply beat biology there (and maybe an improvement on evolution's design to boot).
  15. Sep 1, 2010 #14


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Totally WRONG!

    Embryonal development is NOT simply a local unfolding of what is contained in the genes; rather, as the embryo grows, it receives, in all its parts, signals from the rest of the body on how, and at what pace that particular part should develop further.

    However, as our bodies are already fully developed, we would need to constantly mimic the signal flow that the regrowing limb will need, we can't go out having a cigarette while waiting.
    (as a matter of fact, I'll do that just now).
  16. Sep 1, 2010 #15
    Sorry arildno but you CANT know that. Scientificly we know next to nothing about how it would work. And even if you are right about the interbody signaling, its just as possible that the new growing limb will put out signals that will cause the adult body to respond with the right signals. Might be totaly self sustaining.

    On the other hand I honestly think we'll have replacement bodys and brain transplant technologies first so we might never get to the end of this research chain.
  17. Sep 1, 2010 #16


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    There is such a thing called rational expectation..
  18. Sep 1, 2010 #17
    do any animals with a size limit regrow lost limbs? (i.e. any mammals or birds?)
  19. Sep 1, 2010 #18
    Yeah, people assumed that about stem cells, and the result is that unless you pre-program them, you end up with a body full of potential neoplastic cells! That bit of research didn't end well at all, and people still seem to think that you can just shove stem cells into a given region with no scaffolding or programming and all will be well.

    Remember, a gecko has the genetic instructions to regrow a tail, and a starfish a leg. Humans don't have those instructions for an adult to regrow an adult limb, so you might end with a baby hand and a body full of potentially cancerous cells. Arildno is going with the body of research as it's been shown so far.

    granpa: AFAIK, birds do not have the capacity to regrow limbs, nor do ANY mammals, large or small.

    Note in this bit of wikipedia, the factors that go into the regrowth of limbs, such as pre-existing bundles of stem cells, and special "programming". http://en.wikipedia.org/wiki/Regeneration_(biology [Broken])
    Last edited by a moderator: May 4, 2017
  20. Sep 1, 2010 #19


    User Avatar

    Staff: Mentor

    I stand corrected. Partially. What I stated looked like if I think the process goes on its own once started - that's not the case, it needs correct conditions/signals all the way up. My mistake of not being clear.

    Still, you need to know conditions needed for the limb to develop, not details about how the regenerated tissues work - they will self organize during growth. That's what I meant.
  21. Sep 1, 2010 #20
    Ok, now THAT makes sense, and I can easily believe the earlier statement was a miscommunication.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Why don't humans regrow lost limbs?
  1. Why grapes don't mold (Replies: 13)