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What are chimeras and why are they created?
Chimeras (pronounced ki-MER-ahs) result from the mixing of at
least two different bodies in one. They occur naturally, such
as in the case of twins, which can carry some of the same cells
as their siblings with which they shared a womb. And most mothers
carry in their blood some cells from each child they have born.
People who have received transplants from other humans or animals,
such as heart valves from pigs or cows, are also chimeras.
 Chimeras
are also produced by blending human and animal genes. Although
it sounds like the stuff of science fiction, many scientists believe
the closer an animal resembles a human, the better research model
it makes to test medications or grow parts for transplanting into
humans. So researchers have been injecting human cells into lab
animals for years, but more recently have been creating more exotic
hybrids, such as pigs with human blood and mice with human brain
cells.
The term "chimera" comes from Greek mythology and refers
to a creature that had the head of a lion, a goat's body and a
serpent's tail.
Where is
research on chimeras being conducted?
Research using chimeras is going on in the United States and
abroad. Chinese scientists at the Shanghai Second Medical University
reportedly created the first human-animal chimeras by successfully
fusing human cells with rabbit eggs in 2003, according to National
Geographic News.
A year later, researchers at the Mayo Clinic in Minnesota created
pigs with human blood pumping through their veins. Not only do
the pig blood cells flow with human cells, but some of the cells
themselves have merged, creating hybrids. The research gave scientists
a better understanding of how viral infections can pass from animals
to humans.
Several universities are performing chimeric research, including
at Stanford University. Irving Weissman, Stanford professor and
cofounder of the biotech company StemCells Inc., has transplanted
human-brain stem cells into the brains of mice. The human cells
make up less than 1 percent of the mouse brain and are being used
to study neurodegenerative diseases, according to the Pittsburgh
Post-Gazette. He has proposed creating mice with 100 percent human
neurons. Stanford law professor Hank Greely chairs an informal
ethics committee and is overseeing Weissman's work.
Although scientists have been mixing animal species for more
than a dozen years and mixing cells and organs in adult animals,
the more recent blending of human stem cells with animals has
generated a heated debate among those who say the research is
vital to producing medical advancements and those who are concerned
the research may breach basic human ethics.
Are there
any ethics guidelines on chimeric research?
There are no U.S. federal laws governing chimeric research. But
in April 2005, the National Academy of Sciences released suggested
ethics
guidelines for stem cells, which included a section on chimeras.
The NAS said no embryonic stem cells should be transplanted into
a human blastocyst- a ball of cells that results from the fertilization
of a human egg. The NAS also said Embryonic Stem Cell Research
Oversight Committees should be created and approve any human embryonic
stem cells put into an animal.
 The
guidelines also said no animal into which human embryonic stem
cells have been placed should be allowed to breed, and no human
embryonic stem cells should be put into nonhuman primate blastocysts.
The NAS panel said human embryonic stem cells should only be
introduced into nonhuman mammals when no other experiment can
provide the information needed, and "experiments in which
there is a possibility that human cells could contribute in a
'major organized way' to the brain of an animal require strong
scientific justification."
The panel also suggested the formation of an independent body
to regularly review whether the guidelines need to be updated
in light of scientific advances and changing public opinion.
The President's Council on Bioethics is also studying the topic.
And legislation may be introduced in Congress to address this
burgeoning area of research.
Canada banned chimeras in 2004. The Assisted Human Reproduction
Act prohibits transferring a nonhuman cell into a human embryo
and putting human cells into a nonhuman embryo.
Didn't
a mouse grow a human ear on its back several years ago?
In 1995, researchers at the University of Massachusetts took
a prototype human ear made of a biodegradable polyester fabric
covered with human cartilage cells, and implanted it on the back
of a hairless mouse. The mouse's blood nourished the "ear,"
and the cartilage cells grew to eventually replace the fiber.
Although the cartilage grew into the shape of an ear, it was not
functional since it lacked the internal structure and wasn't connected
to a nervous system. The purpose of the experiment was to fine
tune technology to help doctors grow ears and noses for people.
-- Compiled for the Online NewsHour by Larisa Epatko
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