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Regeneration is a process by which wounds or even limb loss are healed rapidly and limbs are replaced. It is much more widespread among animals than people think! Animals that regenerate are fascinating examples of healing and regrowth.
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How does regeneration work?
If you’ve ever chased a lizard or watched it run away from something, you might have noticed it drop its tail and run away. In a few weeks, it may grow back its tail. Lizards have the remarkable ability to grow back an entire limb!
Lizards are not the only organisms to regrow limbs. Across the animal kingdom – from cockroaches to crickets, axolotls to anemones, regeneration is possible.
Regeneration is essentially superpowered healing. It is the process of restoring tissues, limbs, and organs to a state that mimics the original. Healing cannot replace the original and is marked by scars and a loss of function. In other words, regeneration is like the original document and healing is like a badly scanned copy.
Stem Cells are the key to regeneration. The cells that make up our hair, skin, liver, heart, and every other part of our body are as different as the final tissues are. These are specialized cells. Hair and skin grow back when these specialized cells divide, but hair can’t turn into skin, and bones can’t turn into muscle cells. Stem cells are entirely undifferentiated cells, which have the potential to become anything! Embryos are made mostly of stem cells, though as the embryo develops the number of stem cells dwindle. As adults, the only stem cells we have lie within our bone marrow, but even these are partially specialized to give rise to the cells that make up our blood.
When a lizard or axolotl is injured, the body sends stem cells to the site of the injury. Since stem cells have the potential to become anything, they can replace the bone, muscle, and layers of skin that made up the limb almost perfectly. In contrast, if we lose a limb, the best our skin can do is form a toughened layer – a scar, to protect us.
Examples of Regeneration in Animals
Surprisingly, humans can also regenerate! Until the age of three, humans can grow back their fingertips above the first knuckle if they lose it. In adulthood, our skin regenerates constantly when there is normal wear and tear. Our liver cells are only partially differentiated. This means that an entire liver can be grown from a sliver of a liver! When injured, the liver can grow back to its original size, if not its original shape. This also means livers can be transplanted much more easily than other organs.
Zebrafish are one of biology’s favorite model organisms. These fish are hardy and grow in almost any condition. They also go from embryo to hatched eggs in only 48 hours.
Zebrafish can regenerate their livers, eyes, fins, and even heart tissue. One reason why zebrafish may be so talented at regeneration is that they grow throughout their lifespan. This means developmental pathways never switch off, allowing regeneration to take place throughout the lifespan of the fish.
Crickets can grow back limbs that are injured or harmed. When injured, a clump of stem cells gathers at the lost limb. These differentiate and begin to form the limb again. While the leg is functional, crickets are not as good at regeneration as other animals like the axolotl. The limb grows back slowly, and the cricket can walk. However, the shape of the limb tends to be changed forever. Crickets regrow their exoskeleton throughout their lifespan, and their ability to regenerate limbs is thought to be linked to this.
Sea lilies are similar to starfish and live attached to rocks or the sea floor. They typically live close to the surface, in areas like coral reefs. Sea lilies are spectacular because not only can they grow back any of their limbs perfectly, but they can also regenerate their entire body from a single organ!
The mechanism is slightly different than in crickets or zebrafish, where stem cells go to the site of injury. Instead, tissues turn back into stem cells and then replace the limbs lost.
Hydra are miniscule freshwater organisms that live in lakes and streams. These tiny blobs have tentacles above, which drift in the water, gathering nutrients and hunting even smaller animals.
If you thought growing a body back from a single organ was incredible, imagine growing a body back from a single cell!
Regeneration in hydra depends entirely on stem cells. When injured, stem cells migrate to the site of injury. If only one cell is left, this turns back into a stem cell which then differentiates again to replace the entire organism. The hydra’s incredible ability to regenerate is similar in origin to the zebrafish – it grows throughout its lifespan.
Regeneration in Planaria
Although they are still tiny, planaria are small freshwater flatworms thousands of times larger than hydra. These worms are fully visible to the naked eye. Similar to hydra, planaria can regenerate its entire body from a single cell. Adults are also filled to the brim with stem cells, which are ready to migrate to any injury and restore the planaria back to full health.
Exoskeleton: A hard outer covering that protects an animal’s body, most common in insects
Development: The chemical and biological processes involved in an organisms’ growth from birth to adulthood
Model Organism: A very well studied organism in biology, because it is a good way to observe a certain trait or it is very easy to keep in the lab
Stem Cell: An undifferentiated cell that has the potential to become any specialized cell
Specialized: A change in structure to perform a particular function
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