Researchers say they've grown sheep embryos containing human cells—but the benefits that the work could bring are still a way off.
The news: Scientists report that they've fused human stem cells into sheep embryos, and grown the resulting "chimeras" into fetal animals over the course of 28 days.
The big idea: It's hoped this kind of approach could be used to grow animals with human organs, as a source for transplants.
Too few cells: The new chimeras are just 0.01 percent human by cell count. That's better than the 0.001 percent of the human-pig embryos, but growing human organs will require the proportion to be nearer 1 percent.
Plus: The technique remains highly controversial. Attempts to push beyond 28 days of development will test the ethical limits of regulators.