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?
In: 7967
We lost our ancestral ability to regenerate limbs, now only our very young can regenerate, grownups can’t regenerate more than a fingertip. We traded regeneration for reduced chance of cancer and faster scar formation.
There’s some research into regeneration, for example with preventing scar tissue, which blocks normal tissue. Some people can now recover from spinal injuries thanks to our research on blocking scar tissue. We can also make a extracellular matrix scaffold and have our cells build on that. Nothing in principle prevents us from regenerating limbs, only that our body doesn’t work that way so we need some medical research to do it.
Your finger forms while you are still a fetus, the conditions to grow entire new one when you lose your finger as an adult are not there anymore. Interestingly enough, it’s sometimes possible to regrow tip of a finger provided a bit of nail is still intact from injury. There is continuous growth from stem cells going on in a nail and that can guide reforming the tip of a finger.
All these analogies and examples are complex and inaccurate. Simply put, all the body does is close holes. If the hole is small enough that it does not affect the shape of the structure, it will heal as good as new. If a finger is cut off, the “hole” at the stump will close off, but the regeneration of a complex structure is something else entirely.
Cells are great at looking at their buddy and saying hey whats your job again? Oh yeah i can do that. Lets do that together.
They arent good at saying what was there before and going back to the manual to figure out how to make that part from scratch. If humans could do this we would likely learned to grow more teeth rather than just 2 sets.
Some vertebrates (some salamanders) can regenerate limbs. I knew someone years ago who was researching salamander reproduction precisely so they could raise them in the lab to study this.
But one political party has long sought to build themselves up by tearing others down. My friend’s research lost their funding in response to [this political pressure.](https://en.wikipedia.org/wiki/Golden_Fleece_Award?wprov=sfti1)
Sadly, this continues. [Senators like Ron Paul](https://www.science.org/content/article/rand-paul-takes-poke-us-peer-review-panels) and [Senator Ron Johnson](https://www.jsonline.com/story/news/2021/06/28/wisconsin-senator-ron-johnson-promotes-views-odds-science/7418058002/) among others in public office have a weird obsession with being the only smart person, and will do stupid things to prove it, like insist that all science is wasting money because they don’t understand it, even when they haven’t tried.
Little kids actually can. (Source – I know a little kid who severed their finger in a accident. A year later she’s fine.)
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.
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.
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.
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)
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