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The natural world is a treasure trove of wonders, and among its most impressive marvels is the ability of certain animals to regenerate lost body parts. It may sound like something out of science fiction, but these creatures possess astonishing self-healing capabilities, allowing them to regrow lost limbs and even reproduce by creating new ones.
In this article, we’ll delve into the fascinating science behind their regenerative powers, shedding light on the incredible abilities of the animals equipped with this extraordinary trait.
How Can Some Animals Regenerate Limbs?
According to science, every single species on the planet is capable of regeneration to some degree. For example, humans can grow new skin after a wound by regenerating cells. But in some animals this ability is taken to a new level.
Primitive vertebrates and invertebrates have much greater regenerative abilities than mammals, but these abilities vary according to several different factors. For example, the age of the organism, its overall health, and even the conditions of its environment can affect how well it’s able to regenerate limbs and other body parts.
There are some invertebrates that are able to regenerate their entire bodies, and it’s reported that younger animals have a greater capacity to regenerate than their older counterparts.
Certain genetic factors are responsible for this phenomenon, which causes cells to begin reforming even after they have sustained damage. This typically starts from a bunch of stem cells called a blastema, which then has the capacity to form into new, replacement body parts.
Animals that Can Regrow Limbs
A lot of animals that can regrow limbs are found in an aquatic environment, but there are some that may surprise you. Let’s meet some of these incredible creatures.
1. Axolotl (Family: Ambystomatidae)
Ambystoma mexicanum, more commonly known as the axolotl, is an aquatic species of salamander found in high-altitude bodies of water in the Valley of Mexico. They’re a popular aquarium pet, but they have a very unique ability; they can regenerate almost every part of their body! From reproductive organs to limbs and even parts of the brain, the axolotl certainly is a miracle of nature.
These animals use a process known as dedifferentiation, which involves creating a blastema of young cells that will regrow into the new body part. While these are embryonic cells, they do know what they’re supposed to become, and this is a process called epimorphic regeneration. What’s even more interesting is that axolotls are able to do this regardless of their age.
Scientists have been looking at the regeneration of the axolotl brain in order to research the potential for regenerative medical treatments for humans in the distant future. During studies, it was noted that the regeneration process occurs in three stages over the course of around 12 weeks. What’s more, in other studies, researchers have been looking at how axolotls might inspire human limb regeneration by making tweaks to the immune and central nervous systems.
2. Planarian (Genus: Planaria)
The genus Planaria is made up of several species of flatworms, and what’s so unique about them is their ability to regrow their entire bodies just from a tiny piece. This is all thanks to their stem cells, which have the ability to reform into any body part using a process known as stem cell mediated regeneration.
But this process isn’t something that’s only saved for a severe injury (although they can regrow their heads when necessary), planarian, including Dugesia flatworms (Schmidtea mediterranea) and Girardia flatworms (Dugesia tigrina), have stem cells that are constantly repairing damaged cells. It’s kind of like a never ending process of regeneration. Then, when a more serious injury occurs, those cells leap into action and can regrow the entire body of the worm, even where it has lost 90% of itself.
When looking at flatworm regeneration over the course of the last 200 years, scientists have noticed that the process can take anywhere between a few days and several weeks. Through genome sequencing of planarian and other species, researchers hope to one day come up with a way for humans to be able to self-repair wounds.
3. Sea Star (Class: Asteroidea)
Most sea stars have the ability to regenerate lost limbs. In the majority of cases, the process requires that the central disc of the animal remains intact but there are a handful of species, it is possible for the animal to regrow an entire new body from just one limb. Even more interesting is that scientists have discovered that these creatures are even able to regenerate their entire central nervous systems!
The process for most starfish is known as unidirectional regeneration, and the crown of thorns sea star is very well known for this ability. That’s why it’s considered such an invasive species because it’ll just keep growing and quickly take over an area.
This process occurs when signals are sent from the damaged area of the sea star’s body to the stem cells. These are essentially immature cells that have no initial purpose to become something specific. But when those signals are sent, the stem cells start growing into a particular body part. Although, we aren’t entirely sure how and where these signals come from without further research.
4. Sea Squirt (Class: Ascidiacea)
The sea squirt is able to regenerate body parts using the same method as planarian; stem cell mediated regeneration. Just like planarians, sea squirts also have the incredible ability to regenerate an entire new body from a tiny fragment; in some cases just a single blood vessel is needed!
I also find it truly fascinating that there are records of some species of sea squirts, such as the Polycarpa mytiligera being able to regenerate into multiple new individuals. And this can take as little as a month! This is pretty handy since this particular species is also known to throw up its own organs in self-defense, so regenerating is a must for survival.
While sea squirts are often seen as a nuisance, they’re actually our closest living invertebrate relative, so scientists are looking at how their regenerative properties could be used in human limb regrowth.
5. Octopus (Class: Cephalopoda)
We all know that octopuses have eight arms, but what happens if one is lost or damaged? Well, the animal simply regrows it. I say simply, it’s still a pretty complex biological process involving the regeneration of important components like nerves. This is important for octopuses since their central nervous system spreads across each leg and is connected to a brain in the center of the body. In order to create a new, fully functional leg, the octopus has to regenerate its nervous system, at least in part.
In male octopus, an arm called the hectocotylus is used to impregnate the female, and this breaks off during the act. But fear not Mr Octopus, this arm will regrow, and you’ll be able to continue to reproduce once more.
What’s interesting about octopus limb regeneration is that scientists have discovered a protein responsible for cell differentiation called AChE which is also present in the human central nervous system. Could this be the key to unlocking regenerative abilities in humans? There’s still a lot of work to be done.
6. Deer (Family: Cervidae)
We’re moving out of the water and back onto land for this one; the deer. These land mammals are able to regrow their antlers which are used for self-defense, establishing dominance, and attracting a mate.
It’s only male deers that have antlers, but they’re not like fingernails made up from dead cells as most people imagine. No, deer antlers are actually considered to be organs and are made up from millions of living cells. What’s even more incredible is that these beautiful animals go through this process every year.
There are stem cells at the base of the antlers, and they grow very rapidly over the course of just 120 days. Throughout its life, a deer will shed and regrow its antlers several times, with each set having its own unique shape, size, and cell make up.
Knowing what we know about deer, scientists have been looking at how the process of antler regeneration could be used in human treatments for nerve damage. What’s more, since deer are the only mammal known to have this ability, there’s greater hope involving the potential to use the same processes for humans to grow new bones.
7. Sea Cucumber (Class: Holothuroidea)
Sea cucumbers have an impressive ability to regenerate most of their internal organs. It’s worth noting that they don’t have the same organs as us; they lack a heart and a brain, for example. However, they are able to regrow their reproductive and digestive organs.
This is a pretty useful ability for an animal that eviscerates itself when threatened. Not all species of sea cucumber do this, but many do, including the Bohadschia argus, which will eject its internal organs to alarm and distract a predator. Amazingly, this regeneration process happens very quickly; in most cases within just a few weeks.
The genome sequencing of the sea cucumber has given scientists hope that we may be able to apply the same regeneration processes in human medical treatments. While there’s still quite a way to go with this, it’s a promising concept.
8. Lizard (Order: Squamata)
Before I get into the nitty gritty of lizard regeneration, it’s important to keep in mind that out of the 7000 lizard species on the planet, only a handful have this amazing ability.
Some examples of this include the leopard gecko (Eublepharis macularius), which can regrow a lost tail in as little as 30 days. The green anole (Anolis carolinensis) and the bearded dragon (Pogona spp.) are other great examples that display a unique ability called tail autotomy.
When these species of lizards are threatened, or perhaps grabbed by the tail by a predator, they’ll simply drop their tails to make an escape. Interestingly, the process involved in regrowing the tail requires both blastema-based regrowth as well as fracture healing where the tail has broken off.
After research, scientists have also determined that some species of lizard are able to regenerate skin and organs, which could be used to study how these same processes could benefit humans. For example, the chameleon (Chamaeleonidae) is not only able to regrow its tail but also its nerves and skin.
9. Jellyfish (Phylum: Cnidaria)
There are thousands of jellyfish species in our oceans, and many of them, including the moon jellyfish, are able to heal themselves after an injury. But unlike other animals I have discussed in this article, jellyfish have a rather unique way of regenerating themselves.
They aren’t actually able to grow new limbs or other body parts. Instead, they go through a process known as symmetrization, which involves the animal rearranging its body parts to restore its symmetry once again. Research has shown that this process happens as the muscles of the jellyfish relax and contract, slowly reforming its bodily structure for full functionality to return as a result of being more symmetrical.
As if that wasn’t enough, the way that jellyfish reproduce is just as amazing. Adults are called medusas and they’re able to produce polyps, which are essentially baby jellies that attach to rocks. But in one study in China, it was noted that when an individual reached adulthood, it started aging backwards and returned to its polyp state.
10. Hydra (Genus: Hydra)
Hydras are small freshwater organisms that are related to jellyfish. They’re pretty primitive creatures with a tube-like body, a foot-like appendage at one end, and a mouth at the other. However, while it may appear simple, the hydra has an impressive secret; it can regrow its own head!
According to research, the same process occurs when a new hydra is formed that happens when an older one regenerates its head (or other body parts.) Even more mindblowing is the fact that these creatures are constantly regenerating cells so they could be seen as immortal. They even use their ability to generate new cells to reproduce asexually and create a bud on their bodies, which develops and eventually detaches.
Hydras use a process called morphallaxis, which is essentially the ability to regeneration by transforming existing body tissues. The cells are taken and shuffled around to create something new, which can happen in as little as two days!
11. Spiny Mice (Genus: Acomys)
It was long believed that mammals didn’t have the ability to regenerate tissue in a complex way. Yes, we can regrow skin over a wound, but some species of spiny mice have been observed to have the ability to regenerate damaged tissues and cells after a predatory attack.
Now this, doesn’t apply to all species but some, including Kemp’s spiny mouse (Acomys kempi), Percival’s spiny mouse (A. percivali), Cairo spiny mouse (A. cahirinus), and the eastern spiny mouse (A. dimidiatus) were noted to regrow cartilage, sweat glands, fur, and hair follicles.
It’s not uncommon for African spiny mice to lose their tails in fights or attacks, and the ability to regrow them was previously thought to be only something that reptiles could do. However, after researching, scientists now believe that the mammalian ability to regenerate could be much more widespread than we thought.
12. Sea Anemone (Class: Hexacorallia)
Sea anemones have a very interesting way of regenerating damaged or lost body parts, and there’s no limit to what their cells are able to do.
In humans, each cell has its own job. For example, a brain cell is a brain cell, and it can’t evolve into anything else. But with the sea anemone, all of the cells within its body have the potential to turn into different cells.
So, if a tentacle is lost, any of the other cells within the body of the sea anemone could change their job role and help to regenerate what has been lost. Not only this, but these amazing creatures are even able to regenerate their nerve cells, which has caused scientists to believe that there may be the potential to inspire the regrowth of human brain tissue.
13. Crab (Order: Decapoda)
There are 4500 different species of crabs, and while not all of them have the ability to regenerate lost limbs, many do. One example of this is the fiddler crab (Uca rapax), which is able to regrow new limbs after it has molted. All crabs molt, but if a fiddler crab loses a limb, a new one will appear after its next molt.
This is a similar process to the lizards I discussed earlier called autotomy and occurs usually when a crab has been the victim of a predatory attack. Males will even drop their important major claw as a way of escaping, safe in the knowledge that a new one will grow.
The major claw is essential for male fiddler crabs since it is used as a way of attracting a female. While female fiddler crabs have two smaller claws, they’re still able to regenerate between molt cycles.
14. Fish (Order: Actinopterygii)
As is the case with crabs, there are thousands of species of fish (more than 34,000), and some of them are able to regenerate various body parts. For example, the zebrafish (Danio rerio) has the ability to regrow lost scales and fins, but perhaps more impressively, it can also regenerate parts of its spinal cord and heart! Unlike humans, the muscles in the zebrafish’s heart are much more easily able to recover after an injury. Because of this, researchers are able to assess the process and potentially come up with ways to repair human heart muscles in a similar way.
If that wasn’t enough, these fish, found in the eastern Ganges river in India, are also able to regenerate their retinas, which has allowed scientists to look at potential methods for treating human blindness and eye damage.
Another species of great interest in terms of fish regeneration is the African lungfish (Protopterus spp.) which is also able to regenerate various body parts like the fins but also internal organs like its lungs.
But what’s really caught the attention of researchers is the lungfish’s ability to regenerate its tail in a very similar manner to the salamander. The process is very similar, using a blastema mound at the base of the tail, but what’s different is that this species is able to go one step further and even regrow and repair parts of the spinal cord.
The Mexican tetra (Astyanax mexicanus) is, like the zebrafish, able to regenerate parts of its heart muscle. However, this isn’t something that’s seen in all Mexican tetra species and some, just like humans, repair an injury via the formation of scar tissue. According to Oxford University, the ability to regenerate heart tissue could pave the way to similar manmade treatments for humans.
Can Humans Regenerate Body Parts?
The loss of an arm or leg in humans can be life-altering and is classed as a serious disability. Wouldn’t it be amazing if humans had the ability to regrow lost body parts or regenerate them after being injured? Sadly, this isn’t possible. While humans are able to regenerate skin to heal a wound, we just don’t have the correct genetic make up to regrow lost limbs.
So, why is this?
Well, the truth be told, scientists aren’t 100% sure as to why this is, but they do offer some viable theories. For example, some scientists believe that it has to do with the way our immune systems have developed to create scar tissue in place of regeneration. When regeneration occurs, there is also the possibility of tumors as they are formed in the same way. This could be our body’s way of preventing something potentially more serious than a lost limb.
With all that said, scientists are looking at how we may advance medical research and potentially come up with a way of allowing humans to regrow limbs. Of course, this is something that wouldn’t happen until many years from now, if it ever did, but tests with frogs show some serious promise.
It’s also worth mentioning about stem cells. These cells have the unique ability to turn into any type of tissue and can regenerate over and over. They are the cause of regeneration in many animals but in humans, they are present at conception but do nothing beyond the development of the fetus. Still, researchers are wondering whether there is a way to ‘reawaken’ dormant stem cells with the potential of regrowing limbs and lost body parts.
What’s more, even if we cannot master the medical technology and science to actually grow new limbs, there are always many other ways that these animals can inspire scientists. As I have discussed throughout this article, researchers are already using animal regeneration processes to look at possible ways of manufacturing regeneration of human tissues.