| An ampoule containing a medium for stem cell storage at the UK Stem Cell Bank in London
Washington, Nov. 21: 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, stretching the boundaries of stem cell research.
Biologists call these hybrid animals chimeras, after the mythical Greek creature with a lion's head, a goat's body and a serpent's tail. They are the products of experiments in which human stem cells were added to developing animal foetuses.
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. Yet the range of opinions it has received so far suggests that reaching consensus may be difficult.
During one recent meeting, scientists disagreed on such basic issues as whether it would be unethical for a human embryo to begin its development in an animal's womb, and whether a mouse would be better or worse off with a brain made of human neurons.
'This is an area where we really need to come to a reasonable consensus,' said James Battey, chairman of the National Institute of Health's Stem Cell Task Force. 'We need to establish some kind of guidelines as to what the scientific community ought to do and ought not to do.'
Chimeras ' 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 given birth to.
Recipients of organ transplants are also chimeras, as are the many people whose defective heart valves have been replaced with those from pigs or cows. And scientists for years have added human genes to bacteria and even to 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 with brain cells, the building blocks of the organ credited with making humans human.
In experiments like those, Greely told the academy last month, 'there is a non-trivial risk of conferring some significant aspects of humanity' on the animal.
Greely and his colleagues did not conclude that such experiments should never be done. Indeed, he and many other philosophers have been wrestling with the question of why so many people believe it is wrong to breach the species barrier.
Does the repugnance reflect an understanding of an important natural law' Or is it just another cultural bias, like the once widespread rejection of inter-racial marriage'
Many turn to the Bible's repeated invocation that animals should multiply 'after their kind' as evidence that such experiments are wrong. Others, however, have concluded that the core problem is not necessarily the creation of chimeras but rather the way they are likely to be treated.
Imagine, said Robert Streiffer, a professor of philosophy and bioethics at the University of Wisconsin, a human-chimpanzee chimera endowed with speech and an enhanced potential to learn ' what some have called a 'humanzee'.
Los Angeles Times-Washington Post News service Service