Researchers at the National Heart, Lung and Blood Institute (NHLBI) of the National Institutes of Health will publish their results in the September issue of The Journal of Thoracic and Cardiovascular Surgery, though their findings were discussed several months ago at a conference. According to the study, the researchers experimented with different degrees of genetic modification in the pigs. They prevented all of the piglets from producing certain enzymes known to cause organ rejection in baboons (and, by extension, humans) but were given different gene alterations to keep blood from clotting, which is another common issue.

The most successful group had the human thrombomodulin gene added to their genomes. The expression of this gene prevented clotting, lead investigator Muhammad M. Mohiuddin said in a statement. While the average survival of the other groups were 70 days, 21 days and 80 days, the thrombomodulin group survived an average of 200 days in the baboon abdomen. And three of the five grafts in the group were still alive at 200 to 500 days since their grafting, when the study was submitted for review.

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Why do we care about pig hearts in baboon stomachs? The ultimate goal is to use these hybrid organs for human transplants. According to the NHLBI, there are about 3,000 people waiting for a heart transplant in the United States -- and only about 2,000 donor hearts available each year. Other scientists are focusing on creating organs from scratch (by 3D printing a scaffolding and populating it with the host's own stem cells, for example), but it could be that genetically modified pigs provide a faster solution.