The Stuart Little Syndrome

Thus begins E. B. White's classic tale of Stuart Little, the intrepid mouse-son of Mr. and Mrs. Frederick C. Little of New York City. I couldn't help thinking of Stuart Little as I read the recent report of the National Academies of Sciences (NAS), titled, "Guidelines for Human Embryonic Stem Cell Research."

If this seems like a peculiar claim, consider the section of the NAS report on "Interspecies Mixing," which highlights the fact that stem-cell research will inevitably require the creation of so-called chimeras. Taken from Greek mythology, the term chimera refers to the interspecies mixing of cells or tissues.

Although the mixing of human and nonhuman cells and tissue is common in research and medicine-think of heart valve replacement surgery in humans where pig heart valves are used-the NAS sought to draw attention to a little-discussed reality: stem-cell research will require the creation of chimeras because it will be necessary to test treatments developed through stem cells to ensure that they do not produce inappropriate cell types or tumors. Such testing, the report states, "will inevitably be required by the Food and Drug Administration (FDA) en route to any application of [human embryonic stem] cells or their derivatives or, indeed of adult stem cells in therapeutic applications."

The report notes that interspecies mixing is common in animal stem-cell research and cites the transfer of mouse embryonic stem cells into chicken embryos as an example. Then comes a passage that concentrates the mind: Transferring human stem cells, for example, human neural cells, into mouse embryos, the report states, "raises other [moral] issues because potentially the inner cell mass, the progenitor of the fetus, would consist of a mixture of human and mouse cells. It is not now possible to predict the extent of human contribution to such chimeras."

The idea that we will soon have a lot of Stuart Littles running around retrieving rings out of drains and unsticking piano keys is far-fetched. Still, it is worth asking what we should make morally of the creation of human-nonhuman chimeras that stem-cell research will inescapably involve.

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