If we make skin and muscle cells when we heal cuts and heal/generate bones after breaking them, why wouldn’t we be able to grow a finger if one is cut off?
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Scientists have been doing some very interesting research on these things called homeotic (Hox) genes lately which appear to tell the body where to grow certain parts. For example, editing a fruit fly Hox gene can cause it to grow a leg in place of an antenna. So perhaps in a few decades our continuing knowledge of Hox genes in combination with future stem cell voodoo and gene therapy may actually allow for us to have this sort of control over our bodies. Long way away but we’re starting to figure it out. [https://www.khanacademy.org/science/biology/developmental-biology/signaling-and-transcription-factors-in-development/a/homeotic-genes](https://www.khanacademy.org/science/biology/developmental-biology/signaling-and-transcription-factors-in-development/a/homeotic-genes)
Other people have talked about the regeneration mechanics, so I’ll take it a step further back. We evolved those specific regeneration mechanics because it was evolutionarily beneficial.
If you get a finger chopped off, you’re still 100% capable of living, feeding yourself, creating shelter, etc. with the other 9 fingers. At least long enough to create offspring, and that’s the important bit.
If regrowing a finger provided some tangible benefit to survival rate or reproductive rates, then there’s a chance that some human ancestor (millions of years ago) would get a very minor version of digit regeneration as a mutation, use that mutation to produce many children who all had a good chance to carry that same mutation, and on and on until it (maybe) evolved into full blown limb regeneration.
But it doesn’t really help. From an evolutionary perspective, you’d be spending a LOT of extra energy to regrow that digit, which means extra food for a long while. It’s more evolutionarily effective for the body to just close the wound as hastily as possible and keep moving… so that’s what we got.
When you’re young you kinda can.
Friend’s baby lost perhaps half his finger in an accident with a stroller quite a few years ago, and perhaps a year later it had grown back.
My wife lost the tip of her finger in an accident with a wedding gift knife a week or so after we were married. It was cut on the side, nearly down to the bone, through the nail. The docs pulled the rest of her nail off to stitch it back together (we’d kept the piece). The piece ended up not reconnecting and dried and fell off soon thereafter. But the tip regrew and the nail did too, and you would have to look closely to see the scar now, and the nail appears fully normal.
You can, to some extent. I just went to a medical conference in which they discussed this very thing. The presenter showed some before and after pictures of fingertips that were cut off, a few losing as much as back to the beginning of the fingernail. They showed slightly less than perfect results, maybe a little misshapen, but surprisingly good, even with a fingernail. The key, as was explained, is to not try to sew it up, but to have a good scab form over the end and just keep it protected and clean.
Funny seeing this come up, about two weeks ago I managed to do a slight amputation on my index finger. It cut off just above where the bone is, x-ray shows it might have actually grazed the bone but did not fragment it thankfully. When I went and saw the Ortho doctor he said that it should heal really nicely, look and function similar to how it was before. Probably have some scarring too but he wants me to use it more and get the nerves used to working again otherwise the basic touch could become painful after it’s fully healed. As for if it was the whole finger I can’t really answer that. I’ve been taking pics of the healing process because the way the tissue grows back is pretty cool!
What’s really important are HOX genes. They are the blueprints that lay out the basic body structure, like drawing a stick figure before filling in the finer details.
But they aren’t really active after development finishes, iirc. If we could find a way to reactivate them in a directed way, we could tell the body to grow a new finger.
Some animals in nature have this ability, like axolotls, (they can regrow entire limbs!) which may be a side effect of them being in a not-fully-developed state.
Read more about HOX genes to better understand how the body tells “what” to grow “where”.
update: found an old but cool video about how researchers learned about HOX genes.
[Hox genes](https://youtu.be/voQQ1dhCqZg)
The cells we have are not pluripotent (plants have cells like these which is why you can grow a whole plant from a price of it). Pluripotent cells contain all the instructions (dna) to make an entire organism. Our cells are specialized meaning each cell type has very specific instructions for specific functions (white blood cells cannot act as a liver cell).
When your body is first developing, your cells have access to all of the blueprints for every part of the body. Before exiting the womb, your cells essentially lock most of these blueprints up and lose the key. They keep a couple blueprints for healing, but none for complete regeneration.
On a cellular level, healing basically consists of surviving cells being stimulated to reproduce and rejoin together, which is something they know how to do well. If you’ve sustained a simple injury to your skin, like a laceration or something, the skin cells can just be like, “okay I will just reproduce more of me and eventually hopefully join up with other of my kind.” And that will work well. Depending on exactly how deep the injury was, you may or may not have a scar, but either way, it should heal pretty straightforwardly.
So, healing is cellular *reproduction*. That works well with something nice and uniform like skin. But of course much of our tissue is not uniform. A finger for example has all this highly specific muscle, bone, and nervous tissue that isn’t just a uniform copy of whatever is nearby. To create a finger, you don’t just need reproduction, you need *differentiation*. Differentiation is when cells “decide” that no matter what they just were a moment ago, they are now going to become pinky second-joint bone tissue, or pinky fingernail quick cells, or pinky first-joint connective tissue cells. Or whatever.
The thing about cellular differentiation is that in higher animals it is only really available during gestation. The power to differentiate is “shut off” permanently after all the cells in our bodies have successfully formed our future selves, never to be turned back on. Unless something goes wrong and you develop cancer.
Since being highly susceptible to cancer all the time as a species is much worse than a few members of the species irrecoverably losing fingers here and there, evolution has settled on the rule that differentiation is no longer possible after a fairly early stage of development. (As a compromise, we have 5 fingers on each of 2 hands, so if you’re not too much of a dumbass hopefully you will be okay losing a couple.)
However, as we gradually gain greater mastery over the devilishly intricate mechanisms of cellular signaling and differentiation, it has started to become possible to talk about somehow inducing a differentiation state in surviving tissue, so that it would basically re-enact fetal development and reproduce the missing finger or whatever. What exactly that would look like is anyone’s guess, but while it is still a far-flung theoretical concept it is also a highly realistic prospect and will probably become medical reality within the lifetimes of some people reading reddit today.
One slight correction. We do not make new muscle cells when healing cuts and wounds. Muscle tissue can only grow by increasing the size of the existing muscle cells.
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