Could Animal-Human Chimeras Be On the Way?


On November 13th 2002, stem cell researchers convened by the New York Academy of Sciences gathered together to probe what vistas could be opened up by human stem cell biology. It has often been stated that once embryonic stem cells are removed from a blastocyst (an embryo at the 64-200 cell stage) and cultured, these cells cannot form a fetus. While that is true in the purest sense, these embryonic stem cells may be used as "building blocks" to make "transgenic" (or "chimeric") animals. Such a use of embryonic stem cells may be especially attractive to scientists desiring to insert a new gene into the germ line. The stem cell researchers gathered posed the question: How should human embryonic stem cells be used in research? In particular, how much "mixing" between humans and animals should be permitted?

One alleged deficiency of adult stem cells has been their reduced pluripotentiality. Pluripotentiality is the ability of a stem cell to form many tissue types and has been widely touted as the hallmark of embryonic stem cells. However, the essential experiment(s) to prove if human stem cell lines--whether adult or embryonic--are pluripotent have not yet been done. We should do these experiments, right? Before hastily offering an affirmative answer to this question, we must recognize what constitutes the gold standard for proving pluripotency. Stem cells are injected into a developing embryo at the blastocyst stage (the same stage used to isolate embryonic stem cells), and the resulting developing fetus is then examined (usually at autopsy) to determine how many tissues were derived from the injected stem cell lines.

Thankfully, scientists recognize that it would be unethical to do such an experiment with human embryos! However, this recognition leaves us with the prospect of using mice to determine to what extent human stem cells are pluripotent. Can scientists inject human stem cell lines into a mouse blastocyst as a means of discovering how many tissues in the resulting mouse are of human origin? Though this type of experiment has not yet been attempted, it may hypothetically provide a way to produce--in an animal model--complex human tissue, and possibly even organs, needed for therapeutic purposes by replacing all the mouse genetic material in certain tissues with human genetic material. Until such an experiment is actually conducted, there is no way of knowing if human stem cells could even produce tissue in a mouse, if such tissue would grow normally and function, or if all types of animal tissue could be converted to human tissue (the intriguing question emerges here of whether we would feel differently about human stem cells that contributed to the liver, rather than, say, to the brain). In the face of such ambiguity, the overarching ethical question is: Should we even begin these types of experiments?

For twenty years, transgenic mice containing human genes in their genomes have been scurrying about in laboratory cages. These human genes were placed into the mouse genome and passed on to subsequent mice generations. However, several technical obstacles have limited the amount of human genes that are expressed in a mouse: only a few human genes could be successfully inserted into the mouse genome at a time without interrupting essential mouse gene functions or creating a fatal combination. This may not be the end of the story, though, as the wedding of human embryonic stem cell biology and mouse developmental biology may allow for the creation of even more fascinating--and more mixed--human-mouse chimeras. Human stem cells could potentially be placed in a mouse embryo with the result that human genetic material alone would drive developing "human" tissues and organs in a mouse.

What principles may Christians invoke to guide them in formulating a response to the possibility of such animal-human chimeras? Some concern should certainly be expressed for the experimental animal's suffering; however, Christians do believe that they have been given stewardship over animals and are permitted to use them to benefit humanity. Another concern would be zoonotic transmission of disease, which occurs when pathogens cross the traditional species barriers of disease transmission. When human and animal tissues are intertwined so closely, potential mutations of once species-specific pathogens may gain a unique ability to infect organisms of other species. A more fundamental Christian concern involves violation of the divinely created order. The Bible tells us that God designed procreation so that plants, animals, and humans always reproduce after their own kind or seed. (Gen 1:11-12, 21) In the biblical view, then, species integrity is defined by God, rather than by arbitrary or evolutionary forces. The fusion of animal-human genomes runs counter to the sacredness of human life and man created in the image of God.

The creation of animal-human chimeras as a means of deriving human tissue and organs highlights the deeper issues facing our generation: the new biological genomic revolution and the resultant power that may permit scientists to redesign various species and biological life. We must not allow such an ability to outstrip the ethical analysis that must accompany it. 

Editor's Note: The story referenced in this article is "Human-Rodent a Reality?" which can be found at  (Accessed January 2, 2003). For further information, see also "Scientists Produce Viable Pig and Goat Sperm from Mice" in Denver Post.