Scientists said the long-range goal is to better understand and treat an array of human diseases.
The researchers hope to ultimately cultivate human tissue that can be given to patients awaiting transplants.
But that’s a long way off, said Jun Wu, who worked on the research.
“This study is reporting an important first step,” said Wu, a staff scientist at the Salk Institute, in La Jolla, Calif.
That step, specifically, was to insert human stem cells into pig embryos. Weeks later, some of the embryos showed signs that the human cells were beginning to mature and turn into “tissue precursors.”
Such embryos are known as chimeras, and they are controversial.
In 2015, the U.S. National Institutes of Health declared a moratorium on funding chimera research while officials assessed the ethical issues that the work raises. Last year, the agency proposed changes to that policy — though, it said, certain restrictions would still apply.