Scientists at the Salk Institute inject human stem cells into a pig embryo in research to create organs for transplantation.

The cells made up just a tiny part of each embryo, and the embryos were grown for only a few weeks, researchers reported.

Such human-animal research has raised ethical concerns. The U.S. government suspended taxpayer funding of experiments in 2015. The new work, done in California and Spain, was paid for by private foundations.

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In the current study, Belmonte, Wu and their colleagues used gene-editing techniques known as CRISPR/Cas9 to generate mice embryos lacking a pancreas. Then they inserted rat stem cells that contained a gene for the pancreas into these mutant embryos. Once implanted, the stem cells developed into a rat pancreas within a mouse embryo that ultimately (and importantly) grew into a healthy mouse with a normal lifespan. Taking their idea a few steps further, the researchers used the same method to develop rat eyes and rat hearts within mice embryos. First Human-Pig Embryo Has Been Successfully Grown in Lab “We demonstrated the robustness of this system,” Wu said. By “genetically disabling” the mouse host, they proved it was possible to generate rat organs within a host species. Unexpectedly to the researchers, the mice delivered bonus organs after the injection of rat stem cells: gallbladders, which are present in mice but not in rats.

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After years of trial and error, scientists have finally done something incredible: They have successfully grown human stem cells in a pig embryo. Why would anyone do this? Turns out, many scientists have been working on growing the organs of one animal inside of a different type of animal. For example, scientists recently reported the successful growth of mouse pancreases inside of rats.” Says Tech Insider

Ethical concerns

“The interesting thing is, it was not funded by American taxpayer money,” Hyun said, at least not directly. The primary funding came from private Spanish sources. One reason for this is the US National Institutes of Health examined chimera science in a 2015 workshop and “accompanied that with a pause” on funding of a “very narrow subset of animal-human chimera research,” said Carrie D. Wolinetz, the institutes’ associate director for science policy. (CNN)