After years of trial and error, scientists have finally done something incredible: They have successfully grown human stem cells in a pig embryo. Why would anyone do this? Turns out, many ...

hoping that the pigs' kidneys would have human cells. Most recently, experiments by Juan Carlos Izpisua Belmonte and his team at the Salk Institute created a human-monkey chimera embryo.

Researchers investigating how this complexity arises have made human-animal chimeric embryos by introducing human pluripotent stem cells into the embryos of other animals, such as mice and pigs ...