Doctors have transplanted pig valves into human hearts for years, and scientists have injected human cells into lab animals for even longer. But the biological co-mingling of animal and human is now evolving into even more unsettling mixes of species, evoking the Greek myth of the monstrous chimera, which was part lion, part goat and part serpent.
In the past two years, scientists have created pigs with human blood, fused rabbit eggs with human DNA and injected human stem cells to make paralyzed mice walk. Particularly worrisome to some scientists are the nightmare scenarios that could arise from the mixing of brain cells: What if a human mind somehow got trapped inside a sheep's head?
Few human-animal hybrids are as advanced as the sheep created by another stem cell scientist, Esmail Zanjani, and his team at the University of Nevada-Reno. They want to one day turn sheep into living factories for human organs and tissues and along the way create cutting-edge lab animals to more effectively test experimental drugs.
Drawing ethical boundaries that no research appears to have crossed yet, the National Academies recommend a prohibition on mixing human stem cells with embryos from monkeys and other primates. But even that policy recommendation isn't tough enough for some researchers.
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