How are octopi, salamanders, etc. able to regrow limbs, and why can’t all animals do it?
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Our DNA has a blueprint of where every cell must be and what they must do in every stage of development. In early stages, most cells are pluripotent (stem cells), meaning that they can become any type of specific cell . As development goes on, the cells lose that ability, and can only duplicate to make cells of the same type.
So, losing a limb should not be a problem, you just keep duplicating the cells of the same type until the limb is complete again, right? Well, If you have joints and multiple, different bones, you need something that can start duplicating into that joint and bone tissue. But the cells on what remained of the limb know only to build that specific part of the limb, not the other parts.
Lizards, contrary to most people think, cannot really regrow limbs. Some of them can only replace the tail, and that tail can only be replaced a finite number of times. They have special system with defined places of breakage, which allow them to significantly reduce the loss of blood associated with losing a body part. Those breaking points are situated along the tail so if they break it too close to the body, they lose the other breaking points closer to the tail, reducing the number chances to replace it.
Salamanders can regrow whole limbs, but it seems they have a way of creating new stem cells for that situation.
Even in humans, scar tissue is not a regrowth of skin, but rather a replacement, as the new tissue does not have the same characteristics as the original skin (cannot grow hair, sweat glands, etc).
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