In his State of the Union address, President Bush named "human-animal hybrids" as one of "the most egregious abuses of medical research." That declaration made a lot of people wonder:Who is creating such hybrids, and why?
Chimeras—organisms composed of cells from two or more animals—have actually been a staple of experimentation for decades. In 1988 mice with human immune systems helped scientists prove that HIV causes AIDS, says Irving Weissman of Stanford University, and implanting human tumors in mice has allowed scientists to identify the cells that cause several cancers.
More recently, researchers have suggested that chimeric sheep could grow human organs for transplantation. Stem cells injected into lamb fetuses have created livers that are up to 10 percent human, says Esmail Zanjani of the University of Nevada at Reno.
Biologists are also moving stem cells across species boundaries to test therapies for seemingly incurable diseases. Human fetal stem cells injected into the brains of vervet monkeys may help treat the simian equivalent of Parkinson's, says Eugene Redmond of Yale University. Partially paralyzed rodents walk almost normally after human embryonic or fetal brain stem cells repaired their spinal cord injuries in recent studies. The FDA recently approved the first clinical trial of a human neural stem cell therapy, developed by StemCells, a company in Palo Alto, California, to slow the progress of a fatal childhood neurodegenerative disorder called Batten disease.
"We know people who are dying of these diseases," Weissman says. "Which of these treatments should we not study as hard and as fast as we can?"