Baboons with two hearts. Pigs with human DNA.

These things exist -- in labs, at least -- because researchers have engineered them to help answer some basic questions in the emerging field of xenotransplantation.

Xenotransplantation is the transplant of organs or tissues between species. And it might sound a little weird, but it has the potential to save a lot of lives.

According to the National Institutes of Health, there are about 3,000 people in the United States waiting for a heart transplant; but every year, only 2,000 hearts become available.

So for years, scientists have been trying to figure out how to
transplant pig hearts into humans.

Pig hearts and human hearts actually have a lot in common: They each have four chambers, four valves, and they're about the same size. Anatomically, they are highly compatible with our bodies. Genetically, they are not.

The biggest barrier to all organ transplantation is rejection, which is when the host's immune system attacks and kills the transplanted organ. Even human-to-human kidney transplants, the safest and most common kind of transplant in the world, have about a 15% rejection rate after one year.

And when you're talking about transplanting between species, rejection happens much faster and much more severely. Your body can tell when you stick something weird inside of it. Your immune system produces hundreds of thousands of different antibodies that can detect unfamiliar protein markers in your bloodstream.

The problem with transplanting pig tissues is that they express a protein that doesn’t occur in us primates. It’s called galactose oligosaccharide, also known as the Gal antigen. And our immune response to it is swift and toxic, resulting in the death of the organ within days, sometimes even hours.

But this week, the National Heart, Lung, and Blood Institute announced that scientists have managed to successfully keep pig hearts alive in baboons for over a year.

They did this by creating genetically engineered pigs that have human DNA. Their genes responsible for creating the Gal antigen were knocked out, and replaced with human genes that express antigens that are familiar to our immune systems. The baboon hosts were also given a cocktail of immuno-suppressant drugs to help the process along.

Baboons, like humans, are in the ABO blood group, which means that they produce the same antigens that govern blood type that we do. So, successfully transplanting a pig heart into a baboon suggests that the new technique would also work on humans.


I want to note, though, that these hearts weren’t replacing the baboons' hearts: They were actually implanted next to the heart, and then connected to the circulatory system so it could be nourished by the actual baboon heart. Just two hearts sitting there inside a baboon.

So now we know that a baboon can keep a pig heart alive. The next step is to find out if a pig heart can do the same thing for a baboon.

And another xenotransplantation breakthrough this week: Five human subjects have been found to grow new muscle, thanks to tissues transplanted into them from -- you guessed it -- pigs!

The research involved volunteers with a condition known as volumetric muscle loss, the traumatic loss of skeletal muscle, usually because of an injury, most often seen in military personnel.

Treatment has traditionally involved harvesting muscle tissue from another part of the body, and then bracing it in place on the injured limb. But even after extensive reconstructive surgeries, the limb is usually still very weak. And seven and a half percent of soldiers who have suffered major limb injuries still end up having amputations.

But a new technique developed at the University of Pittsburgh has managed to grow new muscle by transplanting a special part of pig muscle tissue known as the extracellular matrix. This is a material, made mostly of the protein collagen, that’s found outside of cells and provides a sort of support framework for growing tissue.

Surgeons transplanted this pig-muscle matrix into the site of the injury, and then flooded the matrix with stem cells.

And because each type of tissue has its own matrix, the stem cells were guided by the muscle matrix to develop specifically into muscle cells. This matrix also seems to have acted as what the physicians call a “homing device,” drawing new stem cells to the site of the injury.

After six months of physical therapy following the transplants, all five patients reported at least some improvement, and three of them grew enough muscle to see at least a 25% improvement in the function of their limbs.

Researchers say that this new technique would probably get even better results if used right after an injury occurs, like in field hospitals.

Xenotransplantation is a fascinating field that could potentially solve the problem of organ shortages all over the world, and eliminate the black market for human organs.

So next time you have the chance, hug a pig, thank them for making people’s lives better! With organs and with bacon. Thank you pigs!

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