Mixing species--and crossing a line?
Israeli scientist Ronald Goldstein wasn't all that surprised when TV comedians started making fun of his work implanting cells from human embryos into chicken eggs. He had suspected the public would find the work bizarre--"people have a visceral reaction to this experiment," he says. But he did not expect it to pose problems for his fellow scientists.
Goldstein wanted to trace how well human embryonic stem cells turn into specific tissues and organs--the very shape-shifting quality that could make these cells an endless source of transplant tissues to treat diseases like Parkinson's and diabetes. He did not let the eggs hatch, and he made sure the human cells could not mingle with all of the developing chicks' tissues to create true chimeras. But even some of his scientific colleagues expressed disapproval recently when he presented his work, saying he had crossed a contentious ethical line. "This is a very, very gray area right now," Goldstein says.
Man or mouse? Yet other researchers are pursuing far more intimate marriages of human and animal tissue. South Korean scientists say they've created early mouse embryos marbled with human embryo cells--the very outcome Goldstein set out to avoid with his chicks. And U.S. researchers have proposed slipping another kind of stem cell into a mouse fetus to make an animal with a brain formed entirely of human neurons. Crossing species barriers in the lab is nothing new--scientists have created mice with human immune-system cells and organ-donor pigs with human genes. To many people, though, implanting stem cells in an animal is a different matter because these cells are the clay from which human beings are made.
Such concerns have transformed the debate over stem cells, which once focused on whether it's ethical to harvest stem cells from embryos. Now that scientists have the cells to work with--albeit a smaller, less varied supply than many would like--the arguments have shifted to how scientists should study these versatile cells in lab animals. An article about the "moral confusion regarding social and ethical obligations to novel interspecies beings" in the current American Journal of Bioethics is drawing unprecedented response. And a project to look at the ethics of stem cells has formed a special panel on chimeric brains, which will hold its first meeting next month.
"We are very confused about how we understand ourselves as human beings in contradiction to nonhuman species," says Ruth Faden, a bioethicist at Johns Hopkins University, who has teamed up with stem-cell expert John Gearhart to create the panel. The threshold of unease about cross-species research seems to vary depending on the kind of stem cell being studied, the kind of animal, and the age of the embryo or fetus in which the cells are implanted. And where one expert sees a routine study, another might see a risky bid to endow an animal with a touch of humanity.
Late last year at a stem-cell conference in New York, researchers pondered adding human embryonic cells to early mouse embryos to create chimeric animals. "It was pretty hotly debated, even among the scientists," recalls Fred Gage, a biologist at the Salk Institute for Biological Studies in La Jolla, Calif. To some, the scientific payoff seemed appealing. Putting human embryonic stem cells in these early embryos, called blastocysts, might prove better than any other lab test how well these cells produce all cell types.
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