Slouching Toward Chimeras

What happens when you cross a human and a mouse? Sounds like the beginning of a bad joke but, in fact, it's a serious high tech experiment recently carried out by a research team headed by a distinguished molecular biologist, Irving Weissman, at Stanford University's Institute of Cancer/Stem Cell Biology and Medicine. Scientists injected human brain cells into mouse fetuses, creating a strain of mice that were approximately 1 percent human. Dr. Weissman is actively considering a follow-up experiment that would produce mice whose brains are 100 percent human.

What if the mice escaped the laboratory and began to proliferate in the outside environment? What might be the ecological consequences of mice who think like human beings, let loose in nature? Dr. Weissman says he would keep a tight rein on the mice and if they showed any signs of humanness he would kill them. Hardly reassuring.

In a world where the bizarre has become all too commonplace, few things any longer shock the human psyche. But, experiments like the one that produced a partially humanized mouse at Stanford University stretches the limits of human tinkering with nature to the realm of the pathological.

The new research field at the cutting edge of the biotech revolution is called chimeric experimentation. Researchers around the world are combining human and animal cells and creating chimeric creatures that are part human and part animal, reminiscent of the ancient Greek chimeric myths of human-animal hybrids.

The first such chimeric experiment occurred many years ago when scientists in Edinburgh, Scotland fused together a sheep and goat embryo-two completely unrelated animal species that are incapable of mating and producing a hybrid offspring in nature. The resulting creature, called a Geep, was born with the head of a goat and the body of a sheep.

Now, scientists have their sights trained on breaking the final taboo in the natural world-crossing human and animals to create new human-animal hybrids of every kind and description. Already, aside from the humanized mouse, scientists have created pigs with human blood running through their veins and sheep with livers and hearts that are mostly human.

The experiments are designed to advance medical research. Indeed, a growing number of genetic engineers argue that human-animal hybrids will usher in a golden era of medicine. Researchers say that the more humanized they can make research animals, the better able they will be to model the progression of human diseases, test new drugs, and harvest tissues and organs for transplantation into human bodies. What they fail to mention is that there are other equally promising and less invasive alternatives to these kinds of bizarre experiments including sophisticated computer modeling to study diseases and test the effectiveness and toxicity of drugs as well as in vitro tissue culture, nanotechnology, and artificial prostheses to substitute for human tissue and organs. When it comes to chimeric experimentation, then, the question is, at what price?

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