Researchers have successfully grown human stem cells in pig embryos, setting in motion a path of research which could lead to human organs being grown in animals for the purpose of transplantation.

Interspecies blastocyst complementation, to give the process its full title, involves injecting human induced pluripotent stem cells (adult cells reprogrammed to resemble cells in early development) into animal embryos, in this case, a pig.

‘Intermediate’ cells grew best within pig cells, the researchers found. These cells were added to the pig embryo and implanted into sows. The embryos were allowed to develop in the pig for 3–4 weeks. 

The researchers discovered that human cells do survive in pig embryos, although not very efficiently. After 3–4 weeks of embryo development, the amount of human cells in the pig embryo was a very small fraction of the overall total cells. Co-lead researcher, Dr. Jun Wu explained to The New York Times that for further research advancement, this may not be a bad thing, “To the brain we observed little or no contribution at all, this is good news because we can guide the human cells to the organ we want.”

The researchers began their experiment with rats and mice.They used a rat with a deleted master gene for the pancreas (something which is not currently possible with a pig), and injected mouse stem cells into the rat embryo. Due to lack of pancreas gene, the rat’s embryo constructs a pancreas of pure mouse cells, making future transplantation to a mouse, possible. So, to hypothetically develop specific human organs in a pig, researchers would first have to construct a pig that cannot make a certain organ. 

The researchers acknowledged that transplanting organs from animals is a long way off, but their research, which found no traces of human cells in the brain or reproductive tissues, does allay some of the most common fears involving chimera research. Moral and ethical questions will likely arise with each advancing study. Last May, the National Institutes of Health announced a plan to lift a ban on government funding for chimera stem cell research, however the ban is still in place meaning any research needs private funding. 

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