Scientists announced Monday that they had created mice with small amounts of human brain cells in an effort to make realistic models of neurological disorders such as Parkinson's disease.
Led by Fred Gage of the Salk Institute in San Diego, the researchers created the mice by injecting about 100,000 human embryonic stem cells per mouse into the brains of 14-day-old rodent embryos.
Those mice were each born with about 0.1 percent of human cells in each of their heads, a trace amount that doesn't remotely come close to "humanizing" the rodents.
"This illustrates that injecting human stem cells into mouse brains doesn't restructure the brain," Gage said.
Still, the work adds to the growing ethical concerns of mixing human and animal cells when it comes to stem cell and cloning research. After all, mice are 97.5 percent genetically identical to humans.
"The worry is if you humanize them too much you cross certain boundaries," said David Magnus, director of the Stanford Medical Center for Biomedical Ethics. "But I don't think this research comes even close to that."
Researchers are nevertheless beginning to bump up against what bioethicists call the "yuck factor."
Three top cloning researchers, for instance, have applied for a patent that contemplates fusing a complete set of human DNA into animal eggs in order to manufacturer human embryonic stem cells.
One of the patent applicants, Jose Cibelli, first attempted such an experiment in 1998 when he fused cells from his cheek into cow eggs.
"The idea is to hijack the machinery of the egg," said Cibelli, whose current work at Michigan State University does not involve human material because that would violate state law.
Researchers argue that co-mingling human and animal tissue is vital to ensuring that experimental drugs and new tissue replacement therapies are safe for people.
Others have performed similar experiments with rabbit and chicken eggs while University of California-Irvine researchers have reported making paralyzed rodents walk after injecting them with human nerve cells.
Doctors have transplanted pig valves into human hearts for years, and scientists have injected human cells into lab animals for even longer. But the brain poses an additional level of concern because some envision nightmare scenarios in which a human mind might be trapped in an animal head.