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Scientists debate blending species
WASHINGTON – 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.
These are not outcasts from “The Island of Dr. Moreau,” the 1896 novel by H.G. Wells in which a rogue doctor develops creatures that are part animal and part human. They are real creations of real scientists.
Biologists call these hybrids chimeras, after the mythical Greek creature with a lion’s head, a goat’s body and a serpent’s tail. They are products of experiments in which human stem cells were added to developing animal fetuses.
Chimeras are allowing scientists to watch, for the first time, how nascent human cells and organs mature and interact – not in the cold isolation of laboratory dishes but inside the bodies of living creatures. Some are already revealing deep secrets of human biology and pointing the way toward new medical treatments.
But with no federal guidelines in place, an awkward question hovers above the work: How human must a chimera be before more stringent research rules should kick in?
The National Academy of Sciences, which advises the federal government, has been studying the issue and hopes to make recommendations by February.
“We need to establish some kind of guidelines as to what the scientific community ought to do and ought not to do,” said James Battey, chairman of the National Institutes of Health’s Stem Cell Task Force.
Chimeras (pronounced ki-MER-ahs) – meaning mixtures of two or more individuals in a single body – are not inherently unnatural. Most twins carry at least a few cells from the sibling with whom they shared a womb, and most mothers carry in their blood at least a few cells from each child they have born.
Scientists for years have added human genes to bacteria and farm animals – feats of genetic engineering that allow those critters to make human proteins such as insulin for use as medicines.
“Chimeras are not as strange and alien as at first blush they seem,” said Henry Greely, a law professor and ethicist at Stanford University who has reviewed proposals to create human-mouse chimeras there.
But chimerism becomes a more sensitive topic when it involves growing entire human organs inside animals. And it becomes especially sensitive when it deals in brain cells, the building blocks of the organ credited with making humans human.
In those experiments, Greely told the academy, “there is a non-trivial risk of conferring some significant aspects of humanity” on the animal.
In one ongoing set of experiments, Jeffrey Platt at the Mayo Clinic in Rochester, Minn., has created human-pig chimeras by adding human-blood-forming stem cells to pig fetuses. The resulting pigs have both pig and human blood in their vessels. And it’s not just pig blood cells being swept along with human blood cells; some of the cells themselves have merged, creating hybrids.
It is important to have learned that human and pig cells can fuse, Platt said, because he and others have been considering transplanting modified pig organs into people and have been wondering if that might pose a risk of pig viruses getting into patient’s cells. Now scientists know the risk is real, he said, because the viruses may gain access when the two cells fuse.
In other experiments led by Esmail Zanjani at the University of Nevada at Reno, scientists have been adding human stem cells to sheep fetuses. The team now has sheep whose livers are up to 80 percent human – and make all the compounds human livers make.
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