How the pig became one of biomedicine's most important model organisms

Most model organisms earn their place in a lab for the same reason: something about their biology is easy to work with. Yeast divides fast. Mice breed quickly and come in genetically standardised strains. Zebrafish embryos are transparent. The pig's route into biomedical research is different, and in some ways more surprising, because the trait that matters most is one that has nothing to do with convenience: pigs are, roughly, the same size as us.

A pig heart pumps against a similar pressure to a human heart. Pig skin has a similar layered structure to human skin. Pig organs sit in a similar range of dimensions to human organs, at a similar body mass, with broadly comparable physiology. None of that makes pigs easy to house or breed at the scale of a mouse colony. What it does is make them useful in exactly the situations where a mouse's small size stops being an advantage and starts being the whole problem: surgical training, organ replacement, and disease models where the scale of an organ or a tissue is part of what determines the disease process. That single fact, more than any deliberate research programme, is what pulled the domestic pig out of agriculture and into medicine.

Hearts, valves and the operating table

The pig heart's usefulness to cardiology starts with something almost coincidental: pig heart valves are close enough in size and structure to human heart valves that they can be used as replacements. The first successful use of a pig aortic valve to replace a diseased human valve was carried out in Paris on 23 September 1965 by the surgeon Jean-Paul Binet, working with Alain Carpentier, and by January 1968 the same group had performed more than 60 such replacements. Those early valves were preserved in mercury salts and later formalin, relatively crude chemical methods that tended to fail early, through tearing and calcification of the tissue.

The technique improved substantially once Carpentier introduced glutaraldehyde as a tissue-fixing agent in 1969. Glutaraldehyde cross-links the collagen in the valve tissue, which stabilises it, reduces the immune response against it, and slows the degeneration that had limited the earlier heterografts. That innovation, combined with mounting the treated valve tissue on a flexible stent, is essentially the design still used in porcine bioprosthetic valves today. Xenograft valves, made from pig or cow tissue, now account for a large share of the more than 300,000 heart valve replacements carried out worldwide each year, valued particularly for older patients who don't need the decades of durability a mechanical valve offers and who benefit from not needing lifelong anticoagulation.

The same anatomical similarity that makes pig valves usable as replacements also makes pig hearts useful for something much less glamorous but just as important: practice. Pig hearts are close enough in size, chamber arrangement and vessel layout to human hearts that they are a standard tool for surgical training and for testing new valve designs and delivery devices before they reach a patient, cheaper and more available than cadaveric human tissue and without the ethical and logistical complications of using primates.

Skin, burns and wound healing

Skin is the other tissue where pig anatomy happens to line up unusually well with human anatomy. Porcine skin has a similar epidermal and dermal thickness and structure to human skin, and pigs are an inexpensive, readily available source of it, which is why pig skin xenografts became a standard temporary burns dressing from the 1960s onwards. Treated pig skin, whether fresh, frozen or chemically preserved, doesn't heal into the wound permanently, but it protects an open burn while it heals, reduces fluid loss and pain, and buys time when a patient's own skin isn't yet available to graft.

That use was always a stopgap, because untreated pig tissue is eventually rejected by the human immune system in the same way any foreign tissue is. Genetic engineering has since narrowed that gap. In October 2019, surgeons at Massachusetts General Hospital in Boston, working with genetic modifications originally developed by David Sachs, applied living skin from a pig with a pig-specific gene removed directly onto a patient's burn wound, alongside a conventional cadaveric skin graft, as part of an FDA-cleared phase one trial led by Jeremy Goverman. After five days, the genetically modified pig graft and the human cadaveric graft looked indistinguishable from one another, and laboratory testing found no transmission of porcine endogenous retroviruses to the patient. It was the first time gene-edited pig tissue had been used directly on a human wound, and it pointed toward a longer-term goal: a living pig skin graft that lasts as long as a human donor graft, without the chronic shortage of donor skin that burns units routinely face.

Genetically modified pigs and the rise of xenotransplantation

Skin grafts and heart valves are both, in the end, dead or inert tissue by the time they're implanted. The much harder problem is a living, functioning pig organ, and that problem is dominated by one molecule: a sugar called alpha-gal (galactose-alpha-1,3-galactose), which sits on the surface of normal pig cells and is recognised almost immediately by pre-existing human antibodies. Any unmodified pig organ transplanted into a human triggers hyperacute rejection within minutes, as those antibodies and the complement system attack the donor tissue.

The turning point came in 2002, when Liangxue Lai and colleagues, working across the University of Missouri and Immerge BioTherapeutics, used nuclear transfer cloning to produce pigs with one copy of the alpha-1,3-galactosyltransferase gene knocked out, and follow-up work extended this to pigs lacking the gene entirely. Removing the enzyme responsible for building the alpha-gal sugar meant pig organs no longer displayed the epitope that caused hyperacute rejection, and pig-to-primate organ survival, previously measured in hours, began to be measured in months.

That genetic groundwork eventually made its way to human patients. In September 2021, a team led by Robert Montgomery at NYU Langone transplanted a kidney from an alpha-gal knockout pig into a brain-dead patient being maintained on a ventilator, with the family's consent, and observed no signs of hyperacute rejection over the following days. In January 2022, surgeons at the University of Maryland Medical Center, led by Bartley Griffith, transplanted a genetically modified pig heart into 57-year-old David Bennett, a patient in end-stage heart failure who did not qualify for a conventional donor heart and had run out of other options.

"It was either die or do this transplant. I want to live. I know it's a shot in the dark, but it's my last choice."

Bennett said this the day before his surgery. The transplanted heart functioned well for close to seven weeks with no obvious signs of rejection, though Bennett died two months after the operation; a later investigation identified a latent pig cytomegalovirus in the donor heart as a likely contributing factor, a finding that has since shaped how donor pigs are screened for future procedures. Pig-to-human xenotransplantation is still experimental, but the distance travelled between the 2002 knockout pigs and a functioning heart in a living patient twenty years later is a fair measure of how far genetic engineering has taken the field.

Sequencing the pig genome

None of the genetic engineering above would have been practical without a reference genome to work from, and the pig's is a comparatively recent addition to the list of sequenced mammals. The Swine Genome Sequencing Consortium was formed in September 2003 to coordinate international efforts to sequence Sus scrofa, and a high-quality draft genome, assembled from a female Duroc pig and compared against wild and domestic pigs from Europe and Asia, was published in Nature in November 2012 by Martien Groenen and colleagues, drawing on work from more than 50 institutes including the University of Illinois, Wageningen University and the University of Edinburgh.

The paper did more than provide a reference sequence. It identified numerous candidate disease-causing gene variants shared with humans, reinforcing the pig's value as a genetic model for human disease, and it produced a detailed catalogue of the porcine endogenous retroviruses (PERVs) carried in the pig genome, information that has since fed directly into the safety screening used in xenotransplantation programmes, including the retrovirus testing done after the Massachusetts General Hospital skin graft trial. A genome sequence turned the pig from an organism people knew anatomically into one they could engineer deliberately, and it's difficult to imagine the alpha-gal knockout work or the more recent multi-gene edited transplant pigs progressing at the pace they have without it.

A better model for cystic fibrosis

The pig's usefulness isn't limited to transplantation and surgery. It also shows up in disease modelling, and cystic fibrosis is the clearest example of why. Cystic fibrosis is caused by mutations in the CFTR gene, and mice with the equivalent mutation have been bred for decades, but CF mice never develop the lung disease that defines the human condition. Mouse airways simply don't have the density of submucosal glands that human airways do, so the pathology that matters most in human CF patients has nothing to model against in a mouse.

In 2008, Christopher Rogers, David Stoltz and colleagues at the University of Iowa, working with Randall Prather's group at the University of Missouri, reported CFTR-knockout pigs that developed hallmark features of human CF at birth, including meconium ileus and exocrine pancreatic destruction. Pig airways, unlike mouse airways, have a density of submucosal glands similar to the human lung, and pig CFTR shares roughly 93% amino acid identity with the human protein, which is why the disease process in these pigs tracks the human condition far more closely than any mouse model has managed.

That gap between mouse and pig biology is exactly why CF researchers describe the pig model as one of the more faithful reproductions of the human disease available in a non-primate animal, useful for studying how the disease actually progresses in the lung and pancreas over time, and for testing therapies aimed at that progression rather than a proxy for it.

What ties it together

Cardiac surgeons picking up pig valves in the 1960s, burns units using pig skin dressings, geneticists knocking out alpha-gal in 2002, and CF researchers breeding CFTR-null pigs in 2008 were not working from a shared plan. Each field arrived at the pig independently, for a reason specific to its own problem, and in every case that reason traces back to the same basic fact: a pig is roughly the size of a person, with organs, tissues and physiology that scale accordingly. Mice remain unmatched for genetic tractability and low-cost, high-throughput work. But wherever a research question genuinely depends on scale, whether that's a valve that needs to sit in an adult chest, a graft that needs to cover a burn, an organ that needs to filter a person's blood, or a lung that needs the right density of mucous glands to get sick in the right way, the pig has become the animal medicine reaches for.