[R]esearchers from the Salk Institute for Biological Studies in La Jolla, Calif., and China’s Kunming University of Science and Technology injected human stem cells—made by reprogramming mature skin or blood cells—into 132 embryos from macaque monkeys.
The new chimera experiment highlights a dilemma. When human stem cells are injected into an animal embryo at such an early stage of development, there so far is no way to control where they go or limit what type of adult cells they become.
In a glimpse of the potential effects, researchers at the University of Rochester in 2014 transplanted human fetal brain cells called astrocytes into young laboratory mice. They discovered that within a year the human cells had taken over the mouse brains. Moreover, standard tests for mouse memory and cognition showed that the altered mice were smarter.
In such ways, stem-cell chimeras have “the potential to radically humanize the biology of laboratory animals,” [bioethicist Dr. Insoo] Hyun says.
[Researcher Dr. Juan Carlos] Izpisua Belmonte says he welcomes oversight. At his urging, the recent experiment was reviewed beforehand not just by institutional review boards in the U.S. and China but also by three independent bioethics experts. “Not everything that we scientists can do should be done,” he says. “Experiments like this certainly raise many concerns.”