Wu and his colleagues detail the work in the Jan. 26 online issue of the journal Cell.
First, the researchers experimented with mouse-rat chimeras. They used gene-editing technology to alter mouse embryos — deleting some genes that are critical for the development of a particular organ, such as the heart, pancreas or eyes. The scientists then inserted rat stem cells into the embryos, to see if they would fill the developmental gap.
The resulting chimeric embryos were then implanted into surrogate mouse mothers, where they matured normally, the researchers found. And the rat cells did grow into mature organs.
That offers a “proof-of-principle,” Wu said. Achieving the same feat with human stem cells, and a larger animal, is another story, however.
Wu said his team chose to work with pig embryos because pig organs are similar in size to humans’. They also used adult human stem cells called induced pluripotent stem cells.
After the pig embryos were injected with stem cells, the researchers implanted them in sows. The investigators allowed the embryos to mature for only four weeks before removing and studying them.
According to Wu, some embryos showed signs that the human stem cells were beginning to mature and form precursors to tissue.
But, he said, the actual “contribution” of human cells to the organism was low, and that’s a hurdle that will have to be overcome.
Wu said scientists envision using human-animal chimeras in several ways.
Researchers have long used lab animals to study human diseases and test possible treatments — by, for example, growing human tumor cells in mice.
But actually creating lab animals that have human tissue could prove to be a better way, Wu said.
Then there’s the prospect of using animals as “hosts” to grow human organs for transplant purposes.
“We’re nowhere near that yet,” Wu stressed. No one knows, for example, how to direct human stem cells to grow into a specific tissue once they are in an animal embryo.
Even if those scientific challenges are overcome, at least some ethical issues will persist. And they relate to both human and animal welfare.
Some people, Robert said, will balk at the idea of using animals as “incubators” for human organs, for instance.
Others, he said, “will object to this on the grounds that animal experimentation is wrong.”
More information
The U.S. National Institutes of Health has more on human stem cell research.
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