August 1, 2025

Andrew Georgy 

9th Grade

Fountain Valley High School

Axolotls, lizards, and sea cucumbers, what do all of these organisms have in common? No, this isn’t an exaggerated version of Animal Bingo; rather, all of them can regenerate parts or limbs. The most famous example is, of course, the starfish, where we were all morbidly fascinated to learn in the third grade that they can regenerate their entire body from just a single limb (if you didn’t, the more you know). 

Now, you might be thinking, “Well, why can’t I regenerate limbs? How come I have to wait months or even years for my body to recover from a single muscle tear?” The simple answer is that environmental adaptism isn’t perfect; our ancestors who survived thousands of years ago didn’t have the ability to regenerate limbs, yet since their offspring survived, we never needed to. However, what are some of the requirements needed if we were to regenerate body parts, and what is the process of regenerating limbs in the first place?

Let’s answer the second question using amphibians as our example (it’ll make our life easier when answering the first question). However, I do want to leave just a quick little note; not all animals regenerate limbs in the same way. Just because both starfish and axolotls, for example, can renew their arms/legs if severed, doesn’t mean that they use the exact same techniques to regenerate. 

The major first step in regeneration is actually something our bodies do every day: clotting. Clotting, usually done through platelets (although it can vary from animal to animal) is probably the most important part of the regeneration cycle. If the wound isn’t cleaned and sealed, the organism may very likely die as they will be constantly exposed to bacteria in their respective environments. You also can’t build off of something if there’s no base (in this case, known as the wound epidermis). 

The next step is usually where different animals will diverge in terms of the first stage of reconstruction. Most limb-regenerating amphibians, including lizards, will use blastema cells (in a process known as epimorphosis), which are usually undifferentiated cells that keep dividing near the injury site. These cells can be dedifferentiated (which is the process of a specialized cell becoming less differentiated in order to reproduce) or original stem cells, which were the original cells that created the organism. Each has its own advantages and disadvantages; stem cells are important for renewals in organs and other body tissues, and you only get a limited amount of them at birth. Using dedifferentiated cells means you take away trillions of cells from their primary function just to rebuild, but you leave stem cells intact, which is why animals who are capable of limb regeneration usually use a combination of both. 

After clotting has happened and the stem cells/dedifferentiated cells have been “recruited” through signals, the blastema forms. The blastema is designed to have the same function as a limb bud, which is what most infants use to generate their own limbs when they’re in the womb. Already existing nerves supply oxygen and nutrients to the blastema, which consistently grows into a new limb into a couple of weeks. When growth is finally complete, it serves as a fully-functioning limb, without even the trace of a scar.

While it may seem we already know most of the regeneration process, there is a vast amount still left uncovered. For example, stem cells are supposed to continually degenerate throughout your lifetime, and scientists are still studying how the signals amphibians and other types of animals release “revert” the aging process of stem cells. Researchers are also puzzled on how the blastema figures out what part of the limb they’re regenerating or how much growth is needed; without a stop sequence present, the blastema could very well become cancerous and unhelpful as the limb extends beyond its normal range of function. In fact, even the absence of a scar is still highly-debated; there’s no clear reason on why certain animals’ regeneration almost never leaves one. 

What we do know, however, is that not all regeneration needs to be for only full limbs. For example, deer use most of the regeneration a salamander does to grow out its antlers every year, while some types of mice can fully heal hair, skin, and appendages without the trace of a scar. Even humans use a little bit of regeneration; we can restore the tips of our fingers if cut off, although it’s more present in children and only in certain scenarios (this is by no means an advertisement to try it out, readers!). 

Now, we can finally answer our first question: we simply don’t have the cells or chemicals to do so. Our stem cells age as we grow, and no known hormone can truly revert them to be suitable for regeneration. We also don’t usually dedifferentiate cells, and even if we did as some sort of treatment, there’s no way to be able to control the growth caused like salamanders do. However, research is still being done on how we can implement some of the regeneration strategies used by animals in humans, so who knows; maybe we won’t be that afraid of sharks in a couple of years!