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How can we transplant animal tissue into humans? And will we ever be able to grow customized organs? SciShow explains!

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Written by: Alexa Billow
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There are thousands of people who owe their lives to pieces of dead pig tissue -- and no, I’m not talking about the admittedly life-affirming taste of bacon.   For the last fifty years, we’ve been replacing damaged human heart valves with pig valves.   So, if we can transplant heart valves from pigs, why not entire hearts?    Or kidneys or livers or lungs?    How come you can you walk around with a pig’s heart-valve inside of you but you reject a kidney from another human?    And out of all the creatures in the world, why PIGS?   So many questions.   Transplantation of organs from one species to another is known as xenotransplantation, and it turns out to be pretty bizarre.    But since there’s a shortage of human organs, figuring out how to get a steady supply of animal organs and tissues that we could use is something we might want to figure out.   Transplantation from human to human is tricky enough - much more difficult than matching blood types -- the new organ is often rejected by the recipient, because of a protein called human leukocyte antigen, or HLA.    Almost every type of human cell has different kinds of these proteins on its surface, and normally, they’re the immune system’s undercover agents, marking targets for it to attack.    So if you transplant an organ from one person to another, and the recipient’s immune system recognizes the donor’s HLA as foreign, it’ll attack the transplanted tissue. This can lead to severe blood clots, failure of the new organ, and all kinds of painful symptoms that, put together, amount to transplant rejection.   The problem with these HLAs is that they’re incredibly diverse, with thousands of variants, which makes it very hard to find a donor whose chemistry matches a recipient.   Some people spend years on the waiting list. Others never get a match at all.   But there are a few ways around the problem, and one of them is to skip humans and move to pigs.   You might not find this too flattering, but you have a lot in common with a pig. Your immune system functions in a similar way, and your organs mostly do the same things.    And pigs are the right size. You wouldn’t want a heart valve transplant from a mouse. Or a beluga whale.   The main thing is that a heart valve doesn’t need to be alive to do its job. It’s really just a piece of tough, flexible connective tissue that acts as a glorified piece of plumbing to direct blood flow.   So if you blast the pig valve with a chemical called glutaraldehyde, you can preserve it while killing all its cells in the process. Dead cells can’t produce those HLA proteins, so there’s much less risk of rejection.   The risk is not zero, though. These heart valves can wear out over time, since they’re dead and unable to replace damaged cells. Also, sometimes the patient’s immune system still gets nervous about having this foreign thing in the body.    The dead cells still have some antigens on their surface, and patients -- especially those with stronger immune systems -- can react to them. The valve may eventually fail.   So scientists figured out how to minimize even that small risk.    They identified the main antigen that our immune systems attack in pig tissue, called galactose-alpha 1,3-galactose, or alpha Gal for short.    Then, they genetically engineered pigs not to express alpha Gal, so the human immune system can accept a pig valve much more readily.   But why stop there?   Why not use pigs to grow not just heart valves, but whole hearts, or anything else you might want? That could solve the organ shortage.    Well, scientists are working on it.   Knocking out alpha Gal alone is not enough to convince the human body to accept a whole organ from a pig, but some more extensive genetic engineering could help.   For example, you could remove the gene that tells the cell to produce some of its other antigens, or throw in some human genes to make the tissue seem more familiar to the human immune system.   They’ve tested these genetically engineered pig organs in non-human primates like baboons, which only survived for a short time. But the technology is advancing quickly.   The downsides? Some people aren’t comfortable with the idea of genetically engineering pigs to express human genes, even if it’s only to trick the human immune system.   Plus, cloning enough pig cells for the technique to work is mega-expensive. For now, these genetically engineered pig organs would be too costly to reach any patients who aren’t at, like, the private jet level of wealth.   There are alternatives in the works, though.   One concept hinges on the idea that there is virtually no risk of rejection when an organ is made up of the patient’s own cells.    So, say you take an organ -- like a lung -- from a human or animal donor, and remove all of its cells except the connective tissue that gives the lung its shape.    Then, you replace the donor cells with stem cells from the patient. Cells love to stick to that connective tissue, so they’d move in and start dividing and differentiating into the proper kinds of cells.   This, too, has been tested in animal models, but researchers still have a way to go before they can build a completely functional organ.    So the science of xenotransplantation still has a long road ahead of it.   But eventually, the day might come when we will be able to make organs to order. And it will likely be thanks to our porcine friends.   Thank you for watching this SciShow Dose, brought to you by our patrons on Patreon who help make SciShow possible. If you want to help us keep making videos like this and get some pretty cool stuff, you can go to!