Scientists are now working on a technique that would allow human organs to be grown inside pigs. The DNA within a pig embryo that enables it to grow a pancreas is deleted, and human stem cells are injected into the embryo. These stem cells have the ability to develop into any type of cell within the body, and previous experimentsusing rats and mice suggest that they will automatically fill the gap created by the missing pancreas genes and form a pancreas that consists of predominantly genetically human cells.The idea of transplanting organs from pigs into humans is not new. Transplants between different species, or xenotransplantation, was considered promising in the 1990s but fell from favor due to the challenges of preventing the human immune system from rejecting pig organs, and concerns about possible transmission of infectious diseases from pigs to humans. Modern gene editing techniques may help alleviate both concerns: Rejection is less likely since the organ will more closely resemble a human one, while other scientists have demonstrated that CRISPR can also be used to delete retroviruses from the pig genome.

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