Pigs: Source of Replacement Organs for Humans?
Today the demand for human organs for transplantation far outstrips the supply. Although an estimated 45,000 Americans under age 65 could benefit from a heart transplant each year, only about 2,000 human hearts are available. To close this gap, some transplant surgeons have called for “xenografts,” or transplanting organs and tissues from animals. Although non-human primates such as chimpanzees are genetically closest to humans, reducing the chances of graft rejection, primates are endangered in the wild and their use as a source of replacement organs raises ethical concerns because of their high level of intelligence. As an alternative, some have proposed using pigs as a source of organs because they have large litters (up to 10 offspring) and a short gestation time (four months), are anatomically and physiologically similar to humans, are already produced as a food source, and provide some replacement tissues, such as heart valves.
Nevertheless, xenotransplantation would have to overcome many obstacles – both technical and ethical – before it becomes a reality. First, pig cells have antigens on their surface, similar to ABO blood group antigens, that trigger a severe immune response called “hyperacute rejection.” To address this problem, scientists have inserted human genes into single-cell pig embryos in an attempt to “humanize” their cell-surface proteins so they are no longer antigenic. Even if this procedure reduces the risk of hyperacute rejection, however, other immunological barriers to xenotransplantation will still exist.20