Why pigs are so valuable for medical research

The medical world is abuzz over the lifesaving potential of a most unlikely source:

Pigs.

Researchers from the National Institutes of Health last month announced that they'd successfully transplanted hearts from genetically engineered pigs into baboons, potentially paving the way for pig-to-human organ transplants in the future.

And on May 7, genome scientist Craig Venter partnered with United Therapeutics Corp. to develop pig lungs that could be compatible with the human body.

But pigs offer more than just a potential source for organ donation. For over 30 years, scientists have been using pigs in a number of medical fields, including dermatology, cardiology and more. Recently, scientists were even able to re-grow human leg muscles using implants made of pig bladder tissue.

So what makes this farm animal so valuable for medical research? Outwardly, pigs and people seem drastically different; we share only three scientific classifications, and we certainly don’t look alike.

"If [something] works in the pig, then it has a high possibility of working in the human."

— Michael Swindle, author of "Swine in the Laboratory"

It just so happens that, despite our differences, many of the pig’s biological systems are very similar to our own.

“They have a number of anatomic and physiologic similarities to humans in different systems,” Dr. Michael Swindle, retired veterinary researcher and author of “Swine in the Laboratory,” said in an interview. “They are what’s known as a translational research model, so if [something] works in the pig, then it has a high possibility of working in the human.”

According to Swindle, many of the pig’s organ systems are 80 to 90 percent similar to the corresponding systems in humans – both in anatomy and function. The system that matches up best may be the cardiovascular system, as a pig’s heart is about the same size and shape as a human heart. Pigs develop atherosclerosis – artery plaque buildup – in the same way that humans do, and they react similarly to myocardial infarction, the classic heart attack.

Because of these similarities, scientists have long used pigs to test interventional catheter devices and methods of cardiovascular surgery, as well as to understand how the heart works in general. And tissues derived from pig hearts have been used to replace defective heart valves in humans, lasting upwards of 15 years in the human body.

Beyond their closely related hearts and blood vessels, another characteristic humans and pigs share is their diet. Both eat meat and plants to survive.

“The pig is a true omnivore like we are,” Swindle said. “It can eat and drink anything. And because of this, the physiology of digestion and the metabolic processes in the liver are also similar to humans. They’re used in a lot of dietary type of studies, as well as oral absorption studies of drugs.”

The similarities don’t stop there. Pig kidneys are comparable in size and function to human kidneys, lending themselves to renal research. And pigs have been one of the standard plastic surgery models for decades, as their skin wounds heal similarly to humans’ skin.

And there’s more. Diabetics who needed daily insulin injections relied on pork insulin until the 1980s, when manufacturers started making biosynthetic insulin through recombinant DNA technology. The insulin-producing cells in a pig’s pancreas are similar to humans’, so a significant amount of research on diabetes has been aimed at isolating those cells and harnessing them for future treatments.

No one really knows why the organs and anatomical systems of pigs are so similar to humans. Swindle theorizes that millions of years ago, they were even more similar, but then the species diverged genetically and independently developed similar characteristics through evolution.

“Along the evolutionary tree, my own personal belief is that they were true omnivores, so the metabolism and hormones that go along with being an omnivore may have led them on a pathway that gave them these similar characteristics [to humans],” Swindle said.

Because of these striking similarities in organ systems and the growing problem of donor organ shortages, pigs have been targeted as potential heart and lung donors for humans. Though primates such as baboons and chimpanzees are more closely related to humans, pigs provide a more attractive option for organ donation, as they are much more readily available and are already used for food.

“For another organ source, if you use a different species, they have to be available in large numbers and they have to be ethically acceptable,” said Dr. Soon Park, division chief of cardiac surgery for University Hospitals Case Medical Center in Cleveland.

“So if it’s something like the baboon, we may be closer to them than to pigs, but there are some ethical and moral issues there, and they are probably not acceptable for use. They’re also hard to raise in large numbers.”

Transplanting pig organs into humans – a process known as xenotransplantation – has been difficult, as the presence of pig organs causes the human immune system to go into hyperacute rejection. But with the success of the NIH’s study on pig-to-primate organ transplantation, pigs are once again being considered as a potentially viable option for transplants.

So in the world of medicine, it’s likely the pig’s popularity will only continue to rise.