By Rick Weiss
Washington Post Staff Writer
Saturday, November 20, 2004; Page A01

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 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. 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 Institutes 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."