eli5 : Why are there organs and limbs transplant but no brain and entire eye transplant?
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Complexity. Your nervous system is crazy complex and completely removing something from it, then trying to replace it with a different one, is beyond anything we can do right now. I mean, we can’t even reconnect severed nerves within the same body (like paraplegics due to spinal damage) so how would we connect cut nerves to a foreign organ?
Nerves are REALLY difficult to deal with. They do mot regenerate easily like skin or muscle. Even something like limb reattachments do not give back the full capacity they had. Anything even more complex and delicate, like the brain (including eyes) or spine, is basically beyond our capabilities right now.
Not to mention that to extract the living brain, you’d need to keep it alive, and that’s difficult enough even when it’s INSIDE our skull. There’s a reason why an operating theatre has all that equipment and staff.
Axons and dendrites do not heal back together once severed. If you could heal them together the connections would not be the same as those learned and created during infancy. It would be like trying to type on a keyboard where every key was moved around, except some of those keys have to be pressed or you die.
A brain transplant would not be a brain transplant. The brain is the person. It would be a body transplant
You can’t just transplant a brain. You have to transplant most of the Central Nervous system. This includes the eyes, ears, spinal cord, certain facial nerves, major tributary spinal nerves, possibly the vagus nerve system. While you’re excising all that, you can’t interrupt the blood supply to any part for more than 4 minutes, or it dies. Humans don’t have the ability to regrow CNS parts in a functional way once they’re seriously damaged. (Moderate damage can be rerouted, sometimes to bypass damaged pathways.)
Assuming that you just transplanted the brain itself without the rest of the CBS, and assuming you could quickly and accurately reconnect the major arteries and veins in around 3 minutes flat, then what? What you have an unpleasant case of profound “locked-in syndrome.” This is a disturbing scenario where a person may be awake and conscious, but has absolutely no sensory input from the outside world nor external output. Just a case of full-body phantom limb syndrome which isn’t a good experience when if it’s just an arm or leg that you’re missing.
So, doing so would be unethical.
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