The Laws of Man and Beast

New ethical guidelines seek to quell debate surrounding researchers' development of human-animal hybrids, but concerns continue to mount as scientists expand the boundaries of their work.

While the mythical Chimera is the stuff of fantasy, researchers across the country are developing their own real-life chimeras -- animals that are bred to incorporate the cells of other animals or humans -- in an effort to better study human diseases or to create more viable organs for people needing transplants. 

But as scientists continue to create more varied chimeras -- especially those that have some amount of human brain matter -- questions continue to rise from ethicists, religious groups, and even other biomedical researchers, about the types of limitations that should be set on the scientific community.

Those questions resulted in the National Academy of Sciences (NAS), which advises the government on various scientific matters, releasing a set of ethical guidelines on April 26 for the creation of such hybrid organisms. 

Published as part of a larger report on human embryonic stem cell research -- human embryonic stem cells are often implanted in animal embryos to create "chimeras" -- the NAS guidelines discourage the transplantation of animal embryonic stem cells into human embryos. Further, the report suggests that researchers should establish and seek approval from as-yet-unformed Embryonic Stem Cell Research Oversight committees, which should include legal and ethics experts, before transplanting human embryonic stem cells into an animal. 

The guidelines also say that any chimera possessing human cells should not be allowed to breed, and that human stem cells should not be put into other primates, such as chimpanzees, where a more human-like brain might be apt to develop. The introduction of human stem cells into the developing embryo of another animal, the guidelines say, should be done only in cases where no other alternatives exist.

While the restrictive suggestions may hamper certain types of research, Dr. Esmail Zanjani, chairman of the Department of Animal Biotechnology at the University of Nevada, sees the recent guidelines as "perfectly legitimate…the dos and don'ts are perfectly clear."

For Zanjani and his team, these rules wouldn't restrict the types of research they are conducting. They have introduced adult stem cells from human bone marrow into sheep embryos in hopes of creating a ready supply of organs, particularly livers, which could someday be transplanted into humans. These "humanized" sheep livers would be more likely to regenerate (just like a healthy human liver) and are less likely to be rejected by a human recipient -- since they would contain that person's own human cells.

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But creating chimeras isn't only about curing disease by creating more transplant-friendly organs and blood for human recipients. For many researchers, it's the only alternative to understanding the workings of human diseases -- and experimenting to find possible cures.

Researchers at the world-renowned Mayo Clinic in Minnesota announced last year that they had created pigs with human blood by introducing human stem cells into gestating pig fetuses. Meanwhile, to help in his research to better understand the human brain and develop treatment for degenerative diseases such as Alzheimer's and ALS, Dr. Irv Weissman, director of Stanford University's Institute of Cancer and Stem Cell Biology, has made mice that have brains which are one percent human.