There are countless natural wonders around the globe. They are too numerous to count, and we humans never get tired of the surprises nature has in store for us.
In addition to all the marvels, it is worthwhile to consider if we would be able to regrow a lost limb or damaged organ. These skills may seem like something out of a sci-fi film, but they are really found in the animal kingdom. Although the majority of animals lack these skills, there are some animals that regenerate limbs, organs, and other body parts with ease. These few organisms could help us understand how regeneration works in their species and possibly even ours. Who knows? Maybe science could use these fascinating creatures to one day make regeneration possible for humans. In fact, many of the animals on this list are currently being studied for their potential use in regenerative medicine for humans.
Pseudoscience aside, let’s take a look at some real, living animals that regenerate! But first, let’s ask a very important question: Why can’t humans regenerate?
Why Human Beings Cannot Regenerate Major Body Parts
So how exactly can the species listed in this article regenerate? And why are the majority of creatures, including humans and other mammals, so terrible at regeneration? According to science, that’s still a confounding topic today. There are a few competing theories, and the scientific community is still undecided.
One theory relates to how our immune system has evolved. Mammals and birds, which have very high immunity levels, cannot regrow their legs, fingers, and other body parts. This could be a result of the immune system’s desire to avoid cancerous tumor growth and the fact that the molecular pathways of regeneration and tumor growth are identical, including the usage of stem cells. Therefore, evolution guarantees that these animals won’t have as many cancers, but they also won’t regenerate.
Research on the African spiny mouse, a species of mouse that can regrow its skin and hair after an injury, lends credence to this notion. According to a few studies, the skin these animals regenerate doesn’t appear to include any immune cells called macrophages. Macrophages are white blood cells stimulated by the body’s immune system. Because of this, a large portion of the scientific community believes that immunity and regeneration are somehow related.
The Future of Human Regeneration
If and when humans are able to use any of these regeneration abilities will depend on advancements in our understanding of how and why certain animals can regenerate while others cannot. Doctors, scientists, and other professionals involved in the field of regenerative medicine should pay particular attention to this. Humans, for instance, cannot create new fingers or legs, but during fetal development, these genes all help the formation of our fingers or legs, and they are also present in starfish and hydra, which are regenerative animals mentioned later on in this guide.
Perhaps a method to activate these genes during postnatal development to restore limbs will emerge. Maybe mankind will find some way to make human regeneration possible. For now, though, it’s all still a pretty big mystery.
Now that we understand a bit more about regeneration in animals, let’s take a look at a few animals that regenerate!
1. Sea Stars (a.k.a. Starfish)
Classification: Asteroidea class
Sea stars have the capacity to regenerate their tube feet and ray arms after accidents. Most sea stars, also known as starfish, have five limbs, but some have as many as 40. Because the majority of their essential organs are located in their arms, certain sea stars can regenerate complete bodies or a new sea star merely from a section of a severed limb. When predators catch them, they can also release or drop one arm.
Starfish are capable of developing a new body from a lost limb in addition to a new limb. The original starfish can be broken apart into several new ones. Fission is a term used to describe this type of asexual reproduction. Fission occurs when the starfish loses one or more of its limbs and its central disc splits into two parts. From there, another sea star is created that is genetically identical to the parent plant.
Classification: Ambystoma mexicanum
Axolotls are a type of aquatic salamander that have remarkable regeneration powers. They can grow new skin, limbs, organs, or just about any other part of the body. Axolotls live permanently underwater because they never develop lungs and instead retain their gills. Axolotls can regenerate limbs and organs flawlessly and without leaving any scarring, which is even more astounding. In as short as three weeks, they can repeat this as often as required.
The Axolotl is the only vertebrate, regardless of age, that can regenerate a number of its body parts. However, it does not use its stem cell population to do this. Instead, it takes advantage of a process called dedifferentiation. When their bodies are damaged, neighboring undifferentiated cells help them form a stub known as a blastema.
These animals basically turn back the clock on their body’s aged cells so they may begin to behave like embryonic or stem cells, despite the fact that they are not stem cells. They haven’t undergone differentiation since they fall midway between stem cells and adult cells, but they are already pre-programmed for what they will become. Many other creatures with the potential to regenerate prefer this method of regeneration, which is known as epimorphic regeneration. Salamanders and terrestrial lizards also employ this strategy. The starfish does as well, and occasionally it can develop a completely new body from just one arm.
Classification: Selachimorpha superorder
Now this is a surprising entry! Sharks can renew their dental structures, but they cannot restore their organs or other bodily components like other animals on this list can. Over the course of a lifetime, they lose at least 30,000 teeth. However, each one may regenerate in a matter of days or months. Over the course of its lifetime, a shark can regenerate missing teeth up to 50 times.
A shark’s ability to regenerate teeth might take anywhere from a few days and several months. Dentistry could undergo a real revolution if scientists can figure out how this regeneration process works!
Classification: Planaria genus
Flatworms known as planarians have a remarkable capacity for self-regeneration. In just a few weeks, one might create two planarians by slicing one in half; each half would quickly fill in the gaps in a very short amount of time.
One of the most remarkable regeneration techniques in the animal kingdom is used by these flatworms. These aquatic worms are invertebrates, and even after losing up to 90% of their bodies to damage, they can completely rebuild their whole bodies. They can even grow their head back if they are decapitated.
These creatures regenerate via a stem cell-mediated process. They have a population of pluripotent stem cells that are constantly present in the body and are intermittently replacing damaged cells. These cells are effectively tasked to repair the missing structure when a significant amputation occurs, no matter how severe. Sea squirts, which are a type of marine invertebrate, also employ this method.
Classification: Urodela order
The salamander is an amphibian with short legs and a tail. The number of salamander species that we currently know of exceeds 700. Although all salamander species are capable of some degree of regeneration, certain species are more capable than others. Following the removal of the old tail to frighten away predators, certain salamanders can develop a new tail in a few weeks. The replacement limb performs all functions just like the old one.
Salamanders have earned praise for being masters of regeneration because of their astonishing capacity to create new tissues, organs, and even whole body parts, like their limbs. The methods by which salamander cells, tissues, and organs detect and restore missing or damaged pieces can provide key insights into the world of regenerative medicine.
Classification: Hydra genus
The hydra is a type of freshwater jellyfish that prefers to adhere to rocks throughout its life, similar to an anemone. These unique animals often go through a process of regeneration called morphallaxis.
In essence, these animals can shuffle their cells around and restructure what’s left of the tissue, creating a miniature replica that is completely formed and has all of the necessary features. They can also take this regeneration technique a step further. The mechanism of how they regenerate can change depending on how they are harmed. If they sustain more severe wounds, the hydra will also engage in the same process as the Axolotl, whereby a fresh batch of cells proliferates and dedifferentiates to fill in the gaps in the missing structure.
7. Sea Squirts
Classification: Ascidiacea class
Tunicates, sometimes referred to as sea squirts, are renowned for their extraordinary ability to regenerate their whole body. A sea squirt can restructure its residual tissues and rebuild a completely functioning body in a couple of days after being damaged or losing a large chunk of its body.
Genes that regulate cell division and differentiation are activated during the regeneration process in sea squirts. In order to create the required tissues and organs, the cells must then rearrange and differentiate. Sea squirts are a model organism for researching the genetic and molecular pathways of regeneration because of their exceptional capacity for regeneration. This capacity could also provide new ideas for regenerative medicine, like many of the entries on this list.
Classification: Slender danios
Even as older adults, zebrafish have the ability to regenerate their fins, spinal cord, retinas, heart, kidneys, and the telencephalon, the most advanced portion of the frontal lobe of the brain. It appears that different organs have different pathways for regeneration in this creature as well. The Axolotl or the starfish have comparable processes used for fin regeneration. However, just like the flatworm, regeneration of the zebrafish’s telencephalon relies on stem cells to intervene and ensure the fish’s brain is properly repaired.
9. Mexican Tetra
Classification: Astyanax mexicanus
Mexican tetras can repair heart tissue, much like zebrafish. Or rather, surface fish of this species can; populations of fish from caves no longer possess this ability. After damage, cave populations’ hearts develop scarring similar to how a human’s heart would. According to the latest research on this species, tetras have unregulated versions of many genes.
The surface specimens of the Mexican tetra, which live in rivers and streams, can regenerate tissue without leaving scars. Researchers are hoping that their research on the Mexican tetra will help them make advances in the treatment of cardiovascular disease. The Mexican tetra is not the only fish capable of regenerating heart tissue, though. Also capable of regenerating its heart with minimal to no scarring is the zebrafish.
Classification: Chamaeleonidae family
Chameleons are extremely fascinating creatures that are widely renowned for their extraordinary ability to alter their color in order to fit in with their surroundings. Chameleons can also grow new tails and limbs, in addition to their other abilities. During the healing process, they can also repair damaged skin and nerves.
A chameleon can sprout a new tail if it loses its original one. A blastema, or a collection of undifferentiated cells that will eventually become the new tail, is created throughout the process. Cells from the tail stump that dedifferentiate, or go back to a less specialized state, create the blastema. Following cell division and differentiation, the cells form the diverse tissues of the new tail. This remarkable capacity for regeneration is displayed by a few other species and is a subject of current study in the field of regenerative medicine.
Classification: Cervidae family
Deer antlers are the only organ in mammals that can totally regenerate. They lose their antlers each year and then re-grow into enormous, branching structures of bone and cartilage that are utilized for combat and exhibition.
Scientists are using the regeneration of antlers, which is started and maintained by stem cells generated from the neural crest, to mimic and research the regeneration of other animal organs. Only male deer (except for caribou) have antlers. Male deer grow antlers in order to compete with other males for females and to find food in the snow. Antlers develop at a very rapid rate of roughly one-fourth of an inch every day.
There are many animals that regenerate around the world. The animals we’ve listed above are just a few. Hopefully, one day science will be able to harness the processes of regeneration that these animals possess to apply to humans.
The photo featured at the top of this post is © iStock.com/NORRIE3699
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