Why are our bodies not capable of regrowing limbs but can regenerate skin after a scrape or bone after a fracture?


Why are our bodies not capable of regrowing limbs but can regenerate skin after a scrape or bone after a fracture?

In: Biology

The cells that specialize into long bones and that sort of muscle aren’t _everywhere_ in your body. It happens initially that your body grows one of everything, but not extra parts.

The localized re-growth is because there _are_ fix-it cells that are in the area of big patches of skin, and in (some) bone to re-grow it. Arm bones, leg bones, yes. Skull, not so much.

There are no mammals which can regrow lost limbs. That’s an incredibly complicated evolutionary function which requires architecture our lineage just doesn’t have. So we can’t, even though we would benefit from it.

When you are just a teeny tiny clump of cells, there are certain chemicals present that will activate certain parts of the cells’ DNA to grow a limb (google limb buds). Those early cells share the same DNA as the cells you have in your body right now as an adult, but not all of the DNA is active at any stage. So, the DNA required to grow new limbs has closed and tightened a long time ago. Even if you introduced the chemicals that stimulate growth of a limb, that DNA is no longer activated and nothing will grow.

The DNA to grow skin is always on once it is activated, it is much harder to turn off. So, you are always growing more skin every second of every day, skin cells deep down in the first layer of skin are spitting and being pushed up into the surface so every day more cells are developing and by the time they reach the surface they are fully grown.

Animals that do have the ability to regrow limbs will never fully deactivate this DNA but their body’s nervous system is much less complex meaning that a full grown adult’s nervous system is very similar to what it was when it was a fetus and so the DNA that grows limbs still works and is compatible with the adult set up of the body. This is not the case in humans and if the DNA was left on to regrow limbs they most likely would not grow right and come out deformed because human DNA is set up to grow a limb from a ball of cells and it actually “sculpts” the limb like someone making a statue would. The body slowly kills the cells on the outside of the ball of cells, similar to how someone would create a limb from an ice sculpture. This makes it so the body has plenty of time to regulate and fix any mistakes its made. because the cells are growing and splitting quickly.

Once the animal is full grown though the body can’t use this method to sculpt anything because the cells are no longer splitting and creating new cells as quickly because it would take too much energy for this and natural selection has found that for humans saving that energy for something else (like brain function) is better in the end.