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Scientists have begun blurring the line between human and animal by
producing chimeras—a hybrid creature that's part human, part animal.
Chinese
scientists at the Shanghai Second Medical University in 2003 successfully
fused human cells with rabbit eggs. The embryos were reportedly the first
human-animal chimeras successfully created. They were allowed to develop
for several days in a laboratory dish before the scientists destroyed the
embryos to harvest their stem cells.
In
Minnesota last year researchers at the Mayo Clinic created pigs with human
blood flowing through their bodies.
And at
Stanford University in California an experiment might be done later this
year to create mice with human brains.
Scientists feel that, the more humanlike the animal, the better research
model it makes for testing drugs or possibly growing "spare parts," such
as livers, to transplant into humans.
Watching how human cells mature and interact in a living creature may also
lead to the discoveries of new medical treatments.
But
creating human-animal chimeras—named after a monster in Greek mythology
that had a lion's head, goat's body, and serpent's tail—has raised
troubling questions: What new subhuman combination should be produced and
for what purpose? At what point would it be considered human? And what
rights, if any, should it have?
There
are currently no U.S. federal laws that address these issues.
Ethical Guidelines
The
National Academy of Sciences, which advises the U.S. government, has been
studying the issue. In March it plans to present voluntary ethical
guidelines for researchers.
A
chimera is a mixture of two or more species in one body. Not all are
considered troubling, though.
For
example, faulty human heart valves are routinely replaced with ones taken
from cows and pigs. The surgery—which makes the recipient a human-animal
chimera—is widely accepted. And for years scientists have added human
genes to bacteria and farm animals.
What's
caused the uproar is the mixing of human stem cells with embryonic animals
to create new species.
Biotechnology activist Jeremy Rifkin is opposed to crossing species
boundaries, because he believes animals have the right to exist without
being tampered with or crossed with another species.
He
concedes that these studies would lead to some medical breakthroughs.
Still, they should not be done.
"There
are other ways to advance medicine and human health besides going out into
the strange, brave new world of chimeric animals," Rifkin said, adding
that sophisticated computer models can substitute for experimentation on
live animals.
"One
doesn't have to be religious or into animal rights to think this doesn't
make sense," he continued. "It's the scientists who want to do this.
They've now gone over the edge into the pathological domain."
David
Magnus, director of the Stanford Center for Biomedical Ethics at Stanford
University, believes the real worry is whether or not chimeras will be put
to uses that are problematic, risky, or dangerous.
Human Born to Mice Parents?
For
example, an experiment that would raise concerns, he said, is genetically
engineering mice to produce human sperm and eggs, then doing in vitro
fertilization to produce a child whose parents are a pair of mice.
"Most
people would find that problematic," Magnus said, "but those uses are
bizarre and not, to the best of my knowledge, anything that anybody is
remotely contemplating. Most uses of chimeras are actually much more
relevant to practical concerns."
Last
year Canada passed the Assisted Human Reproduction Act, which bans
chimeras. Specifically, it prohibits transferring a nonhuman cell into a
human embryo and putting human cells into a nonhuman embryo.
Cynthia
Cohen is a member of Canada's Stem Cell Oversight Committee, which
oversees research protocols to ensure they are in accordance with the new
guidelines.
She
believes a ban should also be put into place in the U.S.
Creating chimeras, she said, by mixing human and animal gametes (sperms
and eggs) or transferring reproductive cells, diminishes human dignity.
"It
would deny that there is something distinctive and valuable about human
beings that ought to be honored and protected," said Cohen, who is also
the senior research fellow at Georgetown University's Kennedy Institute of
Ethics in Washington, D.C.
But,
she noted, the wording on such a ban needs to be developed carefully. It
shouldn't outlaw ethical and legitimate experiments—such as transferring a
limited number of adult human stem cells into animal embryos in order to
learn how they proliferate and grow during the prenatal period.
Irv
Weissman, director of Stanford University's Institute of Cancer/Stem Cell
Biology and Medicine in California, is against a ban in the United States.
"Anybody who puts their own moral guidance in the way of this biomedical
science, where they want to impose their will—not just be part of an
argument—if that leads to a ban or moratorium. … they are stopping
research that would save human lives," he said.
Mice With Human Brains
Weissman has already created mice with brains that are about one percent
human.
Later
this year he may conduct another experiment where the mice have 100
percent human brains. This would be done, he said, by injecting human
neurons into the brains of embryonic mice.
Before
being born, the mice would be killed and dissected to see if the
architecture of a human brain had formed. If it did, he'd look for traces
of human cognitive behavior.
Weissman said he's not a mad scientist trying to create a human in an
animal body. He hopes the experiment leads to a better understanding of
how the brain works, which would be useful in treating diseases like
Alzheimer's or Parkinson's disease.
The
test has not yet begun. Weissman is waiting to read the National Academy's
report, due out in March.
William
Cheshire, associate professor of neurology at the Mayo Clinic's
Jacksonville, Florida, branch, feels that combining human and animal
neurons is problematic.
"This
is unexplored biologic territory," he said. "Whatever moral threshold of
human neural development we might choose to set as the limit for such an
experiment, there would be a considerable risk of exceeding that limit
before it could be recognized."
Cheshire supports research that combines human and animal cells to study
cellular function. As an undergraduate he participated in research that
fused human and mouse cells.
But
where he draws the ethical line is on research that would destroy a human
embryo to obtain cells, or research that would create an organism that is
partly human and partly animal.
"We must be cautious not to violate the integrity of humanity or of animal
life over which we have a stewardship responsibility," said Cheshire, a
member of Christian Medical and Dental Associations. "Research projects
that create human-animal chimeras risk disturbing fragile ecosystems,
endanger health, and affront species integrity." |
