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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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.
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