David Cyranosky in Nature:
A Japanese stem-cell scientist is the first to receive government support to create animal embryos that contain human cells and transplant them into surrogate animals since a ban on the practice was overturned earlier this year. Hiromitsu Nakauchi, who leads teams at the University of Tokyo and Stanford University in California, plans to grow human cells in mouse and rat embryos and then transplant those embryos into surrogate animals. Nakauchi’s ultimate goal is to produce animals with organs made of human cells that can, eventually, be transplanted into people. Until March, Japan explicitly forbade the growth of animal embryos containing human cells beyond 14 days or the transplant of such embryos into a surrogate uterus. That month, Japan’s education and science ministry issued new guidelines allowing the creation of human–animal embryos that can be transplanted into surrogate animals and brought to term.
…But getting human cells to grow in another species is not easy. Nakauchi and colleagues announced at the 2018 American Association for the Advancement of Science meeting in Austin, Texas, that they had put human iPS cells into sheep embryos that had been engineered not to produce a pancreas. But the hybrid embryos, grown for 28 days, contained very few human cells, and nothing resembling organs. This is probably because of the genetic distance between humans and sheep, says Nakauchi.