The ability to regrow severed limbs is present in reptiles and amphibians, and some comic book superheroes, but why can’t humans regrow their limbs?
One of the most surprising and, at the same time, least understood processes in nature is regeneration, that is, the ability of an organism to regenerate the body parts it has lost. Among vertebrates, this ability is more common in reptiles and amphibians, but why can’t we humans do the same?
The answer is in our DNA. All animals have genes that contain the instructions for building and maintaining their bodies. These instructions are stored in chromosomes, long strands of DNA that wind up inside the nucleus of each cell. In most cases, when a body part is lost or damaged beyond repair, the information needed to develop a new one is no longer present in the animal’s DNA.
But some animals have developed the ability to turn on certain genes after injury, genes that were inactive until then, allowing them to regrow lost body parts. Researchers believe that vertebrates developed this ability more than 375 million years ago. Today, this ability is mostly found in reptiles and amphibians, which can regrow lost tails, toes, and parts of their jaws and spinal cord. Even some mammals, such as rats and mice, can regrow lost fingers, part of the tail, and fingertips.
And the humans? Unfortunately for us, we don’t have the ability to regrow lost body parts. This is because the information needed to grow a new member is no longer present in our DNA. In fact, when we lose a limb, some of the genetic instructions for building it are destroyed. Scientists believe this loss of information could be due to a process called methylation.
Methylation is a chemical reaction that occurs throughout our lives and plays an important role in embryonic development. But it also has a dark side: Methylation can silence genes – in other words, prevent them from being expressed. This silencing can be passed from one cell generation to another. As a result, over time, some genes may become permanently silenced and unable to function again, making regeneration impossible.
There are two types of cells in our body: embryonic and adult. Embryonic cells are capable of infinite division and multiplication, and this is how we develop from a single cell to a fully formed human being. Adult cells, on the other hand, are much more specialized. They can divide and multiply, but not as much as embryonic cells. That’s why cuts and scrapes heal, but we can’t regrow lost limbs.
Another mechanism by which we can’t regrow lost limbs is that our blood vessels don’t branch out enough. In order for new tissue to grow, it needs a blood supply. Our blood vessels branch a lot when we develop in the womb, but they stop branching when we are born. This is why wounds heal from the inside out: blood vessels have already grown in the area and can bring oxygen and nutrients to the healing tissue.
The last reason we can’t grow new limbs is that our nerves don’t regenerate well. Nerves are important for two things: sending signals from the brain to the muscles to make them move, and sending sensations to the brain so we know what’s going on in our bodies. When we lose a limb, the nerves that went to that limb are still there, they end abruptly at the injury site. Over time, some of these nerves grow back (this is how amputees sometimes regain feeling in the lost limb), but they never grow back perfectly. This is why prosthetics are so important, helping amputees regain some of their function by connecting the remaining nerve endings and the artificial limb.
This does not mean that regeneration is impossible for us; after all, we already have the ability to regenerate lost body parts, such as nails and skin. But when it comes to growing new members, it seems like at the moment we’re sticking with what we have.