system
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human
Eli5 how do the blood vessels and nerves reattach themselves when reattaching a severed limb?

I just saw a post about organ transplant, but how can the nerves of a limb still allow it to function properly if nerves cant really be reattached?
gpt
The first thing is that the surgical team has to basically lock the two ends of the blood vessels and the nerves together so that they can heal, a lot like setting a broken bone.  After that, it's mostly the "quick cells" growing.  In blood vessels, you have lining called epithelium that can repair and grow pretty quickly, so as long as it's sealed up so that the patient doesn't bleed out the blood vessel will probably heal up well.

As far as nerves go, while the nerve fibers can't recover if the cells inside are killed, as long as the nerve cells themselves are still alive, there actually are cells that can help reattach nerves.  These cells, called neuroglia ("nerve glue" in Latin,) are the cells that help support nerves when the nerves are healthy.  There are specific cells, called Schwann cells, that surround nerve fibers and make them conduct signals faster; those cells can support a damaged nerve cell as the cut part grows outward, searching for the part it lost.  If the cut parts find each other, the nerve cell can actually fuse the two parts back together and heal itself, but if it can't, the function of that cell is permanently lost.  Schwann cells and other cells involved in the neuroglia can actually help direct the two parts of the nerve cell in the hopes that it will find the other part of the cell.  As long as most of the cells in the nerve find their cut parts, the nerve can function pretty much as if it were never cut.  It's unlikely that 100% of the nerve cells will find their cut parts, so it's very rare to get back 100% of the function of a severed limb, but as long as the limb was just cut off and the surgical team reattaching it is skilled, most of the nerve fibers can get back where they belong and most of the limb function can survive.