The US National Academy of Sciences is studying the ethical limits of research with chimeras, or mixed-species animals, an issue which is becoming more urgent with the development of stem cell biology. Scientists are already developing animals which have human organs and cells: pigs with human blood, sheep with largely human livers and hearts; mice with some human neurons. They are useful for research on the development of cells and could become medical resources as well. Some scientists are growing partially human organs in animals, for instance.

However, there are concerns about how far mixing species should go. As Stanford University ethicist Henry Greely told the Academy last month, “there is a non-trivial risk of conferring some significant aspects of humanity” on animals. At one end of the ethical scale is adding human genes to animals so that they produce human proteins, such as insulin. At the other end is injecting human stem cells into an animal embryo and transferring the chimeric embryo into an animal’s womb. But what if the human stem cells developed into sperm and eggs? It could happen that a human embryo would form in the uterus of a mouse.

Perhaps the most daring experiment is being proposed by Irving Weissman, of Stanford, an expert in chimeras who has created mice with a nearly complete human immune system, a development which has been invaluable in AIDS research. Now he is toying with the idea of making chimeric mice whose brains are 100% human. If they appeared to have a “human architecture”, they could be killed.

According to the Washington Post, some prominent scientists and bioethicists have no ethical objections even to radical proposals such as placing human embryos in the uterus of animals or making mice with human brains. A bioethicist at the University of Wisconsin, Robert Streiffer, for instance, told the Post that creating a human-chimpanzee hybrid might not be a bad idea. “There’s a knee-jerk reaction that enhancing the moral status of an animal is bad,” he said. “but if you did it, and you gave it the protections it deserves, how could the animal complain?”

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