Why dont limbs grow back?


I’m always amazed by the fact that human body (or any creatures including plants) have an amazing design and system to fight off infections (immune response) and repairs for any damages.

I was particularly interested in why limbs don’t grow back if my finger that had a nasty cut recovered. If my cells know how to recover the damages on my finger why wouldnt they grow back a limb?

In: Biology

Think of repairing a cut as a result of a function, rather than the body consciously knowing it needs to repair it. When you break the skin barrier and hit blood, the blood is being pumped by the heart and bleeds out of the cut, where it clots and hardens when it is exposed to the air. Then over time your body regenerates skin cells from it’s base layer and that clotted blood falls off. That’s why a scar forms for deep cuts as the base layer that skin grows from becomes deformed due to damage.

However, the design and function of entire limbs have many base layers of cells that are removed (like bones, nails, hair, skin, and blood vessel designs and things of that sort). Theoretically, stem cells may give us the ability to regrow limbs, but for now these base layers of cells just have a function and our body has mechanical functions (in a sense) to keep the body alive.

There’s also the release of hormones that cause our body to have responses (like swelling to stabilize a damaged site, adrenaline to speed up our reflexes and increase our pain tolerance, and other things) which cause different functions to occur that aren’t occurring constantly (like blood pumping and skin growing).

If we effectively could utilize stem cells to behave similarly to hormones, then this could be a possibility. But for now it just doesn’t happen naturally.

Skin can grow back over a wound because it’s just one type of tissue making more of itself to fill a gap. Same goes for e.g. a torn muscle or broken bone: as long as you’re filling in a (small-ish) gap with the same tissue, the body can generally do it.

In general, your body replaces most of your tissues regularly. Skin is constantly being shed and replaced, and most of your cells are renewed every few years, with very few exceptions. So your body is naturally able to make more of what’s already there.

Losing a limb is a different story though. To regrow a limb, you have to grow several new bones from scratch, as well as new muscles, new skin, new ligaments, new everything. And it’s not just filling a gap, it’s growing these tissues in the correct shapes too. This is an ability that your body has during development in the womb, but not after.

Ver very condensed version. Skin and muscle can be regrown to some extent. Bones can be repaired by patching in the cracks. Nerves can be repaired if both ends are intact.

Our bodies’ healing and regen capabilities are such that if both ends are present it can attach them together. It cannot create new muscle, bone or nerves. Our bodies simply do not have the ability