A Scientist Crossing a Boundary: A Step into the Bioethical Issues Surrounding Stem Cell Research

First, I would like to note that comments made herein are based on my personal training as a basic neurobiologist and therefore do not represent the views of other scientists in the field of stem cell biology.

The opening sentence of Jason Scott Robert and Françoise Baylis's "Crossing Species Boundaries" (2003)—"Crossing species boundaries in weird and wondrous ways has long interested the scientific community, but has only recently captured the popular imagination beyond the realm of science fiction"—gave me pause. For while Robert and Baylis do describe the seemingly bizarre results of experiments involving the transfer of tissue or genetic material from one organism to another of a different species, most of their striking examples were not created for the sake of creating chimeras. (Kac's green-fluorescent-protein bunny is the exception.) A review of the literature Robert and Baylis cite shows that each chimera was created for clear, justifiable scientific reasons. I might add that they are not by any means an outdated approach to ask functionality of a protein or an organ as a whole unit.

However, their provocative opening invites consideration of an interesting and sophisticated concept of species—one based on several theoretical approaches—asking what distinguishes Homo sapiens from other species. Robert and Baylis note how genetically close human beings are to worms, fruit flies, rodents, and primates. They then explore other biological attributes that might prove to be uniquely "human": the human nervous system, intelligence, emotion, physical appearance. They conclude that scientific evidence alone cannot offer a viable definition of species. They ask why, then, should we debate "crossing" species boundaries between human beings and nonhuman animals. I agree with Robert and Baylis that there is no consensus among scientists about what species integrity might mean or what changes might indicate that a species boundary has been crossed. Nonetheless, without clear and practical definitions, "the creation of novel human-animal beings" does not scientifically alert "creators" themselves.

In addition to the lack of a basic reference point, discrepancies between media and scientific interpretation of discoveries in the stem cell field seem to add another level of complexity. For example, the news report by Krieger (2002) cited by Robert and Baylis has a striking headline and covers the transplantation of human central nervous system stem cells (CNS-SC) into a neonatal mouse brain. Krieger's article covers but does not specifically cite the research published by Tamaki and colleagues (2002) and Uchida and colleagues (2000). Krieger suggests that the presence of significant levels of CNS-SC in the host mouse can be interpreted as "a bit of man in a mouse brain" (Krieger 2002). However, a careful reading of the original research indicates that the survival of the injected human CNS-SC was verified by a number of methodologies, that a human-specific nuclear protein (a general marker for human cells) was found, that a mitotic marker protein (evidence for cell division) was found, and that a general marker for neuronal and glial cytoskeletal proteins was found. While these studies are important and will have a significant impact on human neural stem cell biology and future medical science, the evidence details the survival and mitotic ability of only a fraction of the injected human stem cells (the authors do not comment on absolute numbers remaining in the host) and some progenies expressing marker proteins for neurons and glial cells several months after the transplantation procedure. As brain cells bear far more complex biochemical and physiological properties in a region-specific manner, the functionality of the CNS-SC-derived neurons and glial cells in the host mouse brain is highly speculative. The researchers emphasize that further analysis of the cells is required.

In other words, the researchers' claim for the "survival of human stem cells whose functional specificity...