When a piece of skin, muscle, bone, nails, or hair gets removed or damaged, those usually grow back like nothing happened. So, why isn’t it possible for hands, or even something smaller like a finger to regrow? Or would a piece of meat just regrow there instead of something useful?
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actually in many cases bone, muscle, and even skin can’t regrow properly.
simple example: a skin scar is visible because it’s a different type of tissue, specifically connective tissue, which is weaker than normal epidermis in many ways.
more in general: as we grow up in the womb, our body parts are built starting from a mass of all-same “stem cells”. Stem cells *differentiate* into other types of cells in a very organized, multi-step process that takes place mostly in the womb. We are born with some niches (small groups) of partially-stem cells (= some steps before the final one in the differentiation) in some parts of the body, but mostly we can only regenerate wounds by making more of the types that are already around the wound.
To make a hand, you would need the cells to “know” that they have to make only this many cm of bone, then build an articulation, then another shorter bone, another articulation, all the muscles and tendons and nerves required to control them, and finally a nail. It’s extremely complex and the cells don’tknowhow to do it anymore.
Axolotl and a few other species can do it and we still don’t know how they do.
The genes for regeneration exist, and the evidence for this can be seen in species as different as a lobster and a salamander. So if the common ancestor of lobsters and salamanders was able to regenerate, and its safe to assume that we evolved from the same thing, or something closely related to it, why don’t we have regeneration?
The only conclusion we can draw here is that evolution shut it down for some reason. Which would imply that individuals who had regeneration were dying before they reproduced. Maybe the regeneration genes caused cancer, maybe they caused new, extra limbs to sprout when the body was simply nicked in the wrong way.
Whatever the specific reasoning, something about regeneration reduced the fitness of animals who were related to our ancestors, so much so it caused any ones who had regeneration to die off. Both cancer and extraneous limbs would be considered ‘neoplasmic’ and something about this neoplasm was killing their hosts rather than helping them.
First of all, it is possible for children to regrow their fingertips, the liver can regenerate, and the hippocampus can undergo neurogenesis.
Because fingernails are always regrowing, stem cells are located in the nail beds of the finger nails. And children are in the life cycle phase of their lives when they are literally growing their bodies. The increased presence of stem cells in the nail beds is what makes it possible for children to regrow their fingertips.
But if you cut off too much finger or even an entire hand, there isn’t a convenient bed near the injury where stem cells would naturally gather to regrow the lost body part, and we don’t know what other conditions may be needed to make that regrowth happen once your body has decided that it is finished growing that part of the body. If these secrets are someday unlocked, it may become possible to regrow more than just hands.
I would imagine that on an evolutionary time scale, humans haven’t been able to as reliably survive a lost limb without serious medical intervention. There would be no trait to pass if it were 100% death to lose an arm.
Wasn’t there some research on when they sprinkled pig embryo stem cells into wounds on the end of people’s fingers instead of just scarring over the body would actually regrow the entire tip? Seemed like some good research for fixing wounds.
Imagine you have a huge army of millions of soldiers in a foreign country, and you are trying to build a large encampment or something like the great wall of china. You don’t have a map, but you have a good heirarchy, and you know that as long as you spread out with that hierarchy in mind, you’ll be fine. But the only way you can communicate is with a bunch of battery-powered megaphones, and you don’t have any extra batteries.
You start off with one massive army, and you use the megaphone to shout, okay, everyone spread out until you can’t hear me anymore.
So they do, and your army branches into several large groups. And then your commanders start using their megaphones, telling their groups to do the same thing, but to stop if they start hearing another group or if they hit water.
And so it goes, your army branches out until everybody is settled in a little camps, all coming from one single starting point. Your megaphones have mostly run out of power by now, so you’ve stored them away. If an invasion happens, and a camp is damaged but not destroyed, you shout out loud, and you can pretty much rebuild the broken areas, though they might not look exactly the same.
You keep in contact with the rest of the original army by sending runners to relay messages back and forth, but you don’t really have a map so you rely on your neighbors to pass the message on and hope they know where the message should go.
Then one day the enemy comes, and wipes out half of the south branch. You want to rebuild the same structure, but you don’t have any megaphones anymore, and many of the commanders from the early days have retired. And you never had a map, so all you know if the camp was vaguely branching out from the south side. You try to spread out, but without the megaphones, you can’t get too far out before chaos ensues.
During human development, many cells start secreting signal molecules, kind of like shouting with a megaphone. We don’t really have a blueprint for the entire body — more like a series of directions and timings, most of which are relative to an existing signal molecules gradient from previous cells. Are you close to the “Wnt” signal? OK, grow away from that signal, then start secreting FGF8, make some blood vessels, and so on. As each of these signals have achieved their purpose, the cells stop sending those signals, and usually stay that way, because endlessly telling cells to grow leads to cancer.
At that point, you can repair small structural defects with some small-scale signals. Sometimes, since these signals aren’t as structured, you end up with weird shapes, like bulging scars, but the defect is vaguely repaired.
On the other hand, most of the massive architectural growth needed to regrow a limb relies on pre-existing directional signal chains that are no longer present, and the older cells in your body aren’t willing to create those signals anymore.
And since we don’t have a blueprint, but instead use relativity to create structure, that means we no longer “know” how to create a limb.
It gets a lot more complex than that, with differentiation and whatnot, but that’s part of the reason why we can have a blueprint of the body, but also “forget” how to regenerate body parts.
If your hands grew like nails hair and skin, then we would need to cut them off when they got too long.. If they didn’t fall off on their own.
So if they knew how it would be fixed, it can be fixed?
It is possible, human kids before the age of 9 roughly can regrow the tips of their fingers. Dr. Michael Levin works with bio electricity and has been able to stimulate the regrowth of amphibian limbs that don’t typically have that available to them in the wild and has hinted that this isn’t unique to amphibians.
The human immune system can fight diseases. If you get cut, your platelets will form a clot to repair the blood vessel and your skin can form new tissue. Bones can mend after a break—this ability is exploited for limb-lengthening procedures. However, a bone that is truncated or removed can’t reform.
Species like salamanders can regrow limbs and, since all species on Earth have common ancestors, it’s possible humans have dormant genes for similar functions. So scientists are trying to learn more about how this process works. The hope isn’t to help humans grow back arms tomorrow but to help burn victims or people suffering from macular degeneration.
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