by Garrett Dunlap
figures by Rebecca Senft

Limb loss affects nearly 2 million people in the United States alone. While many instances are related to traumatic events like car accidents, the majority of limb loss cases are caused by diseases that affect the body’s blood vessels. One such disease is diabetes, in which gradual declines in blood flow to a patient’s lower extremities can eventually lead to loss of the entire limb. If the incidence of diabetes continues to rise, there will likely be a corresponding increase in the number of people who must confront limb amputation. Unfortunately, the current therapeutic options following amputation are not much changed from centuries ago, with prosthetic limbs remaining the only option for replacement. But while replacement artificial limbs have been able to replace the form of the lost limb, their function remains severely lacking, especially when the lost appendage is an entire arm or leg. So what if instead of relying upon a wooden or metallic impostor, we might one day just regrow a lost limb?

Many animals have the power of regeneration

To begin thinking about how to accomplish human limb regeneration, scientists have taken note of animals that already show this ability. A prime example is the axolotl (Ambystoma mexicanum), a species of aquatic salamander. Unlike humans, it has the “superpower” of regenerating its limbs, spinal cord, heart, and other organs. But the axolotl is not the only member of the animal kingdom that can do this (Figure 1), as many invertebrates (animals without a spine) are masters of regeneration. Flatworms and hydra, for instance, can regrow their entire bodies from only a tiny piece of their original selves. Even among vertebrates (animals that do have spines), the axolotl isn’t the only animal capable of regeneration. Young frogs are known to regrow limbs, though they lose this ability when they change from tadpoles to adult frogs. On the other hand, the axolotl retains it throughout its entire life, making it unique among vertebrates and a great model to study in regeneration research.

Figure 1: Many animals undergo regeneration (at least to some degree). While the axolotl is not the sole master of regeneration in the animal kingdom, it is the only vertebrate that can regenerate many body parts throughout its entire life.

While there are no known mammals that can fully regenerate missing appendages, many harbor hints of regenerative potential—humans included. It has been observed that mice can regenerate the tips of their toes, though loss further up the foot results in the same scarring that humans see after amputation. Humans have also been known to regenerate the tips of the fingers, including the bone and skin. Multiple clinical reports in the past decades have documented such instances following traumatic injury. Unfortunately, this response gets weaker as the site of loss occurs closer to the palm. While this ability has undoubtedly helped some people in the event a traumatic injury, it is a far cry from the axolotl’s ability to regenerate a fully-formed limb with all of its normal muscles, cartilage, and other tissues.

How does regeneration work?

In axolotls, the process that results in regeneration of an entire limb (Figure 2) involves a complex orchestration of the limb’s surviving cells. Following limb loss (B), a clot of blood cells rapidly stops bleeding at the cut site. After this, a layer of cells works to quickly cover the plane of amputation, forming a structure called a wound epidermis (C). During the next few days, the cells of the wound epidermis grow and divide rapidly. Shortly thereafter, the cells underneath the epidermis also begin to rapidly divide, forming a cone-shaped structure known as a blastema (D). The cells that make up the blastema are thought to be bone, cartilage, muscle, or other cells that de-differentiate (lose their identity) to become similar to stem cells, which are cells that can become one of many different kinds of cells. Blastema cells, however, have restrictions on the types of cells that they can become: for instance, a blastema cell that used to be a muscle cell can only re-form different types of muscle cells, not skin or cartilage cells. These de-differentiated cells in the blastema then grow and multiply, eventually regaining their identity as fully-developed bone or skin cells (E). As the blastema and its cells continue to divide, the growing structure flattens and eventually resembles a perfect copy of the lost limb, including nerves and blood vessels that are connected to the rest of the body (F).

Figure 2: Axolotl limbs go through a multi-stage process following injury to regenerate the lost appendageSkin, bone, cartilage, and muscles can be regrown many times with no sign of trauma.

Learning from the axolotl

To even begin to think about how we can one day be able to regrow lost human limbs, scientists must become intimately familiar with the changes that axolotl cells undergo during regeneration. One approach that has been successful thus far is discovering molecular tweaks that cause an axolotl to lose its regenerative ability, which can reveal regeneration’s most important components and contributors. For instance, the immune system was found to be an important player the limb regeneration process. Macrophages, which are cells that serve a critical role in the inflammation response after injury, were previously connected to regeneration. In fact, injecting a drug to get rid of macrophages in an axolotl’s limb before amputation leads to the accumulation of scar tissue instead of regrowth. This scarring, which happens when a protein called collagen becomes disordered, is a normal part of wound healing in humans, but it is unusual in axolotls. This result suggests that macrophages may be essential for regeneration. Tweaking the nervous system has also been shown to interfere with regeneration. Scientists have observed that surgically removing a limb’s nerves prior to amputation can hinder regeneration, though work is still being completed to better understand why this happens.

All of these previous methods, though, rely on needing to remove an otherwise crucial part of a healthy body (e.g. immune cells and parts of the nervous system). But scientists are now diving down to the level of genes to search for new insights. To accomplish this, researchers first attempted to answer the question of how many times an axolotl limb can successfully regenerate. By repeatedly amputating limbs, it was seen that by the fifth time, few limbs could regrow to their previous potential. Further, when the limbs that could not regenerate were studied further, researchers again found extensive scar tissue build-up, paralleling what is often seen in human injuries. By comparing the genes that were turned on or off when the axolotl’s limb wasn’t able to regrow, scientists have found more molecules and processes to study that hold promise for kick-starting regeneration in humans. Perhaps one day, drugs can be made to modulate these genes, causing them to turn on and help a human limb to regrow after amputation.

Looking to the future

While we are still a long way from regrowing a human limb, we place ourselves at a disadvantage if we lack an understanding of how regeneration occurs in the lucky animals that already hold this “superpower.” Aided by tools that allow scientists to see the fine genetic details of the regeneration process, we are slowly inching closer to understanding what makes regeneration tick. To test this, scientists are working diligently to develop new tools that will allow them to identify other targets and begin transferring these insights to mammals like mice, meaning that perhaps one day, the millions living with lost limbs will have a new avenue for treatment: regeneration.

Garrett Dunlap is a student in the Biological and Biomedical Sciences Ph.D. program at Harvard University.

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75 thoughts on “Regeneration: What the axolotl can teach us about regrowing human limbs

  1. Looking to the future:
    What we are is a long way from ethical code of conduct in the scientific animal studies. (Repeat amputation?) I understand how this particular study has benefited a potential to future human medical procedures, but have we absolutely no conscience at all? What kind of a person could design a study that inflicts repeated harm on an animal? There must be a smarter way. I would never be so naive to think that we know everything about how animals experience pain.
    Forgive me. For as interesting the discovery, and well-written the article, I found myself reacting pitifully to the barbaric treatment of a living creature that I know is widespread in the scientific world. May we open & evolve to have the wisdom to contain our urges; seeking to know everything at the cost of the innocent reveals just how irresponsible and foolish we really are.

    1. You rightly point out that we probably know very little about how animals experience pain, and that repeated amputation must be a horrible experience to be subjected to (the word “torture” comes to my mind, even though the scientists does not do it for fun). You also say that “there must be a smarter way”, but here I have to disagree with you, currently there is no smarter way (as far as I am aware). There are many people that are critical about the use of animals in experiments, but at the same time they enjoy the advantages that came from such experiments. What is more, aside from voicing their oppinins about the use of animals in experiments, they do not come up with alternatives. Indeed we must “evolve” , and perhaps if you can come up with an alternative to animal models, we can proceed on that trajectory.

      1. Very sensible and well-put comment; I totally agree! Knowledge and discovery come at a price; and new discoveries deserve proper respect and responsible use.

    2. You can easily monitor it among a population of them or from breeders. The shop I purchased mine from has tons of babies and since they do not have their sense of smell yet many of the babies do not have arms, legs or tails because the alpha babies will eat the the smaller ones limbs if it is in front of their mouths . There will be one or two that start to grow a lot faster than the others because they eat the food and the smaller axolotl’s limbs over and over again until you separate them but then one or two will take their place and grow larger and so on and so on.

        1. What do you meanby that, they need energy (kcal) in order to reproduce the new limb so if they eat themselves what is the profit?

    3. You are insane to say the least..
      Research like this helps the human race to have a better future.
      May if you lose a limb
      you will realise how much this ” barbaric act ” will help people in future
      As for alternative way , doing research is extremely difficult and pretty often there is dead end to any procedure.
      So whatever options we have we should cling onto it..

      1. we will all die, doesn’t matter how many limbs we have. the problem is everyone wants to live forever and our money goes to funding these stupid projects.

    4. It is probably the only reason I left a whole career in medicine. My specific interest was to regrow human limbs. I have dreamt of this ever since I was a child. But I couldn’t go through with the cutting of frogs and birds etc to save humans and lost out on following through with a formal medical degree to pursue this. Today, I’ve taken to healing through alternative therapies and the likes of energy healing as a passion. I would suggest using electromagnetic fields in conjunction with gene manipulation, in our method for re-growing human limbs.

    5. Yes, you’re right. We need to find a more “humane” way of discovering new things, especially, about axolotl’s regenerating capabilities. Although it could be a fascinating feat for humans to learn to regrow limbs too, there must be a way to do this without losing our humanity. Just look at how cute these lotls are!

    6. I once grew a dead frog’s front right leg from the elbow to it’s claws. I’m a citizen scientist that had a theory that proved true. The frog had gotten smashed under stairs to a mobile home. I regenerated not only his dehydrated body, but grue back his leg to normal nail grow. If you would like to correspond here’s my email- and phone 520 248-5456

    7. Mr Humphrey I agree with you. There must be better ways to conduct this study without inflicting continuous harm on animals.

  2. Crippled children before these little critters till we learn or relearn how to do it on our own. Let the children thrive

    1. They’re critically endangered now because of our acts, do you want our main subject of getting them to thrive in the first place to go extinct?

    1. You can buy them online but you pay for express shipping. Many local pet store can get their hands on them they are a handful but easy to take care for if you just make sure your axolotl is in brackish water. Also make sure its an axolotl and not a mud puppy they look exactly the same but axolotls never mature and stay in a juvenile state while a mud puppy will mature into a salamander.

      1. Do NOT put the axolotl in brackish water. It comes from a freshwater lake.

        They require cold water. Around 65 degrees. Thats about it. Good filtration and a cycled aquarium.

      1. Does it make you feel better to correct people’s typos and mistakes. I always wondered what kind of people do that. What a waste.

  3. As an owner of an Axolotl the regeneration ability of them are amazing. My axolotl lost five of his six gills due to a fungi infection and they have fully regrown in 5 days. They will also eat their own arms, legs and tails to keep themselves fed with their limbs growing back quickly. When my axolotl was growing he ate his tail and it regrew in a week once I found the amount of food he required. Chimera axolotls are amazing because they will be split right down the middle two different colors and can be both male and female depending on which embryos merge.

    1. I have a few and would like to try and breed them. I was told that I need to gradually drop the temperature and eventually raise the temperature back to 18, thereby mimicking what happens in nature. this prompts the axolotls to reproduce. How can this be achieved practically? Thanks

      1. I was under the impression that once they reached maturity they did it naturally and you didn’t have to do anything? How old are your axolotls? They usually reach sexual maturity at around 12-14 months

    2. I completely agree! I would love a Chimera axolotl, especially if it was partially leucistic, partially golden albino.

    3. Mines ate his hand like 3 times . and this time its taking longer . i havent seen the hand come out ever since. Its been like 2 months or longer now . what should i do .

      1. Remember what it said in the article? Axolotls can only regenerate a limb so many times before the ability is reduced. Maybe if you feed him more he won’t eat his hand?

  4. For as interesting the discovery, and well-written the article, I found myself reacting pitifully to the barbaric treatment of a living creature that I know is widespread in the scientific world. May we open & evolve to have the wisdom to contain our urges; seeking to know everything at the cost of the innocent reveals just how irresponsible and foolish we really are.

    1. Hurting without cause is barbaric. Your children’s children may benefit from this research. If you take a step back you’ll see that it’s a small sacrifice for something with great potential.

  5. hey the axolotl can regrante ANYTHING in there body not just there spine and limbs the can regrante heart brain, anything! so you were incorrect. sorry

  6. Solve Diabetes before taking on changing Human DNA. Get back to me when you can cure that disease and we will see if that helps this problem. I bet if you tried to get a group of diabetics to give up meat and animal products they would start to get better and their limbs would be fine. And do you really need to know how many times they can regrow a limb? Are we going to war with machetes? How many times do you think a person will need to regrow a limb? Do you really think that the government will support tampering with DNA in humans? Or that trying to give a medicine that will do what that animals DNA is telling it to do will honestly be Possible? How do you plan to achieve making such a change? Is making that change going to cause other problems such as cancer? How would the human body handle such a thing? This is delicate, complex, and far ahead of where we are at the moment. Focus on things like Cancer, Diabetes, different genetic diseases like Muscular Dystrophy. Less people would need amputations if you cured the disease instead of treating the symptoms.
    Stop torturing these animals because you can and you think you can play god and change our genetics. It’s not going to happen, and if it does you have no idea what else it could do to us. Use your time on something that’s actually going to have results and help people. Oh, and don’t bother trying to help any of those diseases if you plan to get funding from the dairy, egg, or meat industry, Just an FYI.

    1. Well, I agree with the premise of addressing other widespread diseases… but that brings up another issue, your solution of giving up meat and animal products is missinformation at its best, which is the reason diabetes is and so many other so called diseases are so epidimic. The cure for diabetes in most cases is reducing carb intake in order to allow the pancreas to heal from years of over stimulation, fatiuge and failure. Once the insulin is kept low and inflamation in the body goes down, the body will generally being to heal immediately from years of diet induced inflamation and diseases.

      1. Yes, I’ve learned a lot since I made that comment. Carbohydrates (a lot of them not all of them) tend to turn to sugars faster than other foods and it’s can lead to problems with creating insulin. I’m not an expert but that’s what I’ve heard and the way it was explained made sense. Overwhelming the body with glucose and cause insulin problems.

    2. The knowledge is there to be gleaned. It may help with a vital jigsaw piece of any one of the high-priority areas you mention. Especially cancer. You just can’t predict where any discrete piece of information may lead so your argument falls in a heap whe you say to ‘concentrate on cancer, diabetes, etc’ because you can’t say definitively that those pursuits won’t be helped by this information.

  7. So you plan to solve a symptom and not the disease? And with a medicine? That’s like giving the whole body something that could make a paper cut sprout some sort of form. If you manage to make it so it’s only stem cells specific to what they were before then it could only be rather crazy skin growth. Say you can’t control the stem cells part and it sprouts a whole finger on your finger? Say you start getting random growth? Can’t remove that with surgery until you get off the drug because damage would cause growth. Say it gives you cancer, is that worth it? Wouldn’t you prefer a prosthetic leg to having cancer? Even though I don’t agree with the things being done to this animal there are more problems with this then just that. Say you try to change our DNA, then what could happen? No one knows how the body would react to the DNA being altered. There are things that shouldn’t be done. Solve the problem not the symptom. And for those who lose limbs in a trauma? Try coming up with a way to keep a limb alive while not on a body and do the repairs that need to be done without the danger of loosing someone. If you think that’s crazy, fine. Maybe you’ll say that an infection can’t be treated on a limb without a body or maybe it’s already killed it. Maybe that’s true, but are there medications we can work on that can fight infection better then existing ones? Say it’s a bad break, a surgeon can’t fix everything, and they have to cut it off: technology is advancing and for the few who have this problem there will be more functional options. Say they make a prosthetic arm that listens to voice commands or can sense the position of objects like a car can when backing up. The arm can tell were objects are and can interact with them when given direction. Think harder. There are far more intelligent ways to deal with this. You can’t tell me that this way is the right way because it’s not. You are doing this to an animal for no reason and it won’t help anyone. If you can figure out how to grow a heart with heart tissue in a lab then maybe I’d understand the research. Growing that in the body wouldn’t be possible though with the research you are doing with this animal. It’d be too dangerous to be accepted without some serious ground breaking ideas on how to control what is growing and what the side effects are. Stop torturing an animal for no good reason. You have to come up with something better then this to have this be acceptable. Your reasoning is weak and pitiful.

  8. For as interesting the discovery, and well-written the article, I found myself reacting pitifully to the barbaric treatment of a living creature that I know is widespread in the scientific world. May we open & evolve to have the wisdom to contain our urges; seeking to know everything at the cost of the innocent reveals just how irresponsible and foolish we really are.

    1. You are very right, but I would like to add, they may go extinct because of this act. Support of this is the very thing that will ruin it.

  9. After reading several of these articles. I’m not surprised that people react as they do.
    Whoever said that life was fair, just, and equally divided or dispersed among everyone through life or anyone’s life. Should have learned the answer to that as far back as 2008 in this country. That’s another topic but not for now.

    Without such science where would be today? I would think many years in the past still today wondering, what do we do now?
    Maybe if you were an amputee as I am. Maybe the use of science would be a little more understanding.
    How does one put such empathy on an animal and still drive a vehicle? How many people have been cripples or killed by an automobile each day? Yet we drive them anyway. No one stands on a street corner with a sign saying, vehicles are bad for you!

    Don’t think I agree with cruelty at all. There is a big difference between that and science.
    We just have to say to ourselves that is another necessity in life and just say thank you to those that do the world a service.
    I would hope to think they don’t go home and feel happy about an animal losing its life for no reason. Now think about war. Put your empathy out there for those special people that gave all for you so you could feel sorry for an animal.
    They would love to be today what they were before they left to war for you. Hence, I hope science develops a way to grow my limb back. I could shake your hand again.

    1. Amen, so glad someone sees the practical side of the article and isn’t just trying to turn all of us down a guilt trip path…of course, that’s how a lot of people argue these days, instead of reasoning rationally and considering the feelings of those who are actually amputees, they just talk about (correct me if I misunderstood) going vegetarian and avoiding large sources of protein to cure basically anything.(Ahem, Ariana Boes) Huh, right. let me know when you do a little actual research on diseases like diabetes and also when you lose an arm like Tom C over here…you might feel different then.

    2. A veteran, Tom C, correct? if so, thank you for your service and the same to all the heroes who do the same so we can ‘feel sorry for an animal’. Very well put. I hope you do get your limb back, because I would like to shake your hand.

    3. I understand your pain, but if we destroy the ecosystem, we will all be equally extinct. You are crippled, but also lucky. Don’t throw away your chance, next time you won’t live.

  10. I hate to sound like I know it all ..but I actually do know why humans can can’t grow limbs back…. It’s because none of us humans believe that we can… Because we have the ability and the freedom to think positively or negatively… On this subject we’ve been taught through generations that we cannot grow limbs… It’s in our DNA and will be overcome in future generations… Our DNA contains the sum total of thoughts wisdom or evolution however you would like to put it. … , We humans can Manifest anything that we believe we can… Including limbs of course… Sadly, most scientists don’t understand that a belief is only a thought we have over and over again. And that we can change our beliefs eventually so that we can manifest anything….

    1. Please do a LITTLE research on mRNA and tell me you still believe it’s possible to just: ‘think hard enough and regenerate limbs’ …Correct me if I misunderstood but I’m pretty sure that’s what you are saying.

  11. I came here to learn about Axolotls as pets, but this was far more interesting than I expected!

  12. When I was young and working in our garden by our pond I happened to see a monster green bull frog along the bank. Thinking it would be good in our frying pan I snuck up on it and hit it with a garden hoe nearly severing one of it’s back legs above the knee joint. It somehow managed to get back in the water and get away. About a month later I saw this same frog along the bank of the pond with the same damaged leg that was rotted and just hanging on by some skin with a new shorter leg growing where it should be.

  13. I don’t understand why people eat axolotls. Totally acting like they’re critically endangered… Yeah no

  14. I used some of this info to do my project. Thank you! This was super helpful and I would love to see the names for Axolotls! I love axolotls!!! They are so super cute & I don’t know how you guys do it but you know so much about them and it also helps others to know about them too! Thanks again for having this website so that I can do my research project so much more easily.

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