Human and Animal Hybrids: Chimeras

On Nov. 19, 2004 the Washington Post published an astonishing article that told us, among other things, "In Minnesota, pigs are being born with human blood in their veins. In Nevada, there are sheep whose livers and hearts are largely human. In California, mice peer from their cages with human brain cells firing inside their skulls." Less than a year later (June 27, 2005), Scientific American followed up with a more in-depth story on the same issue, human-animal chimeras, individuals who are composed of two genetically distinct types of cells, e.g. human and mouse): "In Greek mythology, the chimera was a monster that combined the parts of a goat, a lion and a serpent. With such a namesake, laboratory-bred chimeras may sound like a bad idea born of pure scientific hubris. Yet they may be unavoidable if stem cells are ever to be realized as therapies. Researchers will need to study how stem cells behave and react to chemical cues inside the body. Unless they are to do those risky first experiments in humans, they will need the freedom to test in animals and thereby make chimeras."