In the United States alone, 22 people die every day waiting for a kidney, liver, heart,

or lung transplant.

There simply aren't enough donated human organs to go around, and we can't just go

around stealing them out of random healthy people.

But pigs' organs have roughly the same structure and proportions as human organs – and we

already kill half a billion of the creatures every year for food – so why can't we

just use their organs?

Here's the thing - our immune systems are really, really good at seeking out and destroying

anything foreign to our bodies, so even human-to-human organ transplants sometimes get rejected.

And pig organs are much foreign-er: they have a super-conspicuous sugar molecule on the

surface of their cells that's basically a “fight me” sign for humans' organ-rejecting

antibodies.

But we've actually been able to engineer pigs whose organs don't carry those sugars, so

assuming we could take care of pig organs' other “fight me” signs in similar ways,

we might one day be able to transplant pig organs into humans.

But there's a far more radical idea for pork-based organ farming: grow customized human organs

in pigs.

Actually, it's not just an idea – it's an ongoing experiment in real life.

First, we engineer a pig embryo that doesn't grow its own kidneys.

Then, we inject it with stem cells from the person needing a transplant.

Then, as the modified pig embryo develops, those human cells grow inside the oinkubator

into healthy kidneys made from the patient's own cells.

At least, that's the plan.

So far, we've only seen this experiment all the way through when the modified pig

embryos were injected with stem cells from other pigs.

We don't know whether it'll actually work with human kidneys, because we've halted

all experiments with the pig-human embryos after just four weeks of development, because...some

of those human stem cells could develop into other human organs elsewhere in the pig's

body - which raises a lot of ethical questions, like what to do with them.

Even if the cells don't migrate and these hybrid pigs end up producing lots of live-saving

organs, what rights do we give to a part-human pig, even if the part is just a kidney?

What if it's two kidneys?

Two kidneys, a liver, and a set of lungs?

In principle, we could use the same technology to grow human hearts, human spinal cords,

and even human brains - all inside of a pig.

And when we do, how will we know if that pig is still a pig?

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