Scientists want to harvest them as tissue transplants to treat ailments ranging from Parkinson's disease, caused when certain brain cells die, to the type of diabetes caused when the immune system destroys pancreatic cells. James Robl, Jose Cibelli and colleagues took some of Cibelli's cells, either from inside his cheek or from his leg, hollowed-out cow eggs and used a pulse of electricity to get the nuclei of the human cells to fuse into the cow eggs. The cells started developing just as if they had been fertilised. Other scientists at the University of Wisconsin beat them to the punch last week, by growing such stem cells from embryos donated at fertility clinics. They, too, hope to grow tissues for transplantation.
Critics say both groups have already gone too far.
In April Rifkin and cellular biologist Stuart Newman of New York Medical College applied for a patent to cover human-animal chimera technology in the hope of preventing just such experiments. The developing embryo is a human embryo inside a cow egg. We don't know what kind of creature could develop from that. There is no precedent in history. Robl says the human genes would take over and only a little bit of mitochondrial DNA from the cow would remain.