Spurred on by the specter of miraculous cures to horrible diseases, Irving Weissman, director of Stanford’s Institute for Stem Cell Biology and Regenerative Medicine, is working on experiments combining human brains and mice. The Stanford Daily reports that Dr. Weissman “has worked with the transfer of human neurons to the brains of mice for several years now. He has already bred mice whose brains are composed of 1 percent human neurons, finding that transplanted human brain cells could successfully connect to a mouse brain.” Such experiements yield what are known as “chimeras,” the creation of organisms composed of material from multiple species.

But what a mere 1% can’t tell you, Dr. Weissman bets the other 99% will. Dr. Weissman “wants to initiate a new experiment by transplanting human brain-stem cells to an inbred strain of mice whose natural brain cells die before the mice’s birth. Human brain cells would then replace the mice’s own, creating a breed of mice whose brains are composed entirely of human neurons.”

This kind of transplantation seems to be a rather different from other kinds of brain transplantation that has been discussed before, since the brain-stem cells would develop once they had been implanted in the mice. With respect to the possibility of the transplant of a completely developed brain, Dr. Ben Carson, who has been director of pediatric neurosurgery at the Johns Hopkins Medical Institutions for over 20 years and is a member of the President’s Council on Bioethics, says, “As a brain surgeon, let me say we don’t have to worry about transplanting human brains into other animals because we’re already dealing with billions and billions of neurons and hundreds of billions of interconnections, and it’s not going to happen.”

But even this supposedly less complex kind of transplant proposed by Dr. Weissman has been acknowledged by him that it “may not even work at all.” For the time being, Dr. Weissman’s proposals are on hold until a clear scientific consensus is reached on the ethical dimensions of the research.

Recent testimony given by members of the President’s Council on Bioethics attests to the diversity of opinion on these issues. Dr. William Hurlbut, himself a consulting professor in human biology at Stanford University and who was not quoted in the Stanford Daily article, says:

It is possible using certain technologies to transplant whole modules of developing portions of the embryo from one species to another. This has been done by Le Dourian and Balabon, where they actually transplanted a portion of the developing brain, early neurologic system at that stage, and got the crowing capacities of a quail put into a chick.

And so he actually transplanted a unit of behavior. Just to draw that a little farther, I think we should also be careful to not do that with elements of human form. In other words, it isn’t just a matter of cognition that we’re concerned about. The categories of our world, the conceptual categories that organize our world provide an intelligible world to us. These are not to be taken lightly.

The way we understand our world is by the separations within the world. For very serious purposes we might mix those, but I think we should be careful not just to see that as a matter of inner psychological or cognitive functions, but we need to preserve the human form, the dignity of the human form.

Diana J. Schaub, a professor of political science at Loyola College in Maryland, discusses a study on the subject and argues that “transplanting human neural stem cells into a mouse no more transforms the mouse than transplanting a pig heart valve into a person transforms the person. All of the rules that the authors recommend seems to me sensible, and although they don’t acknowledge it, those rules are based on preserving species integrity. Transfer the smallest number of cells necessary; use dissociated human stem cells rather than larger tissue transplants; and select host animals carefully, preferring distant relations over our nearer primate cousins.”

When Dr. Alfonso Gómez-Lobo, professor of metaphysics and moral philosophy at Georgetown University, discusses the possibility, he says:

Let me start with the most extreme and highly unlikely case. Suppose human neurological stem cells are transplanted into a primate so that the animal acquired some key human features. It seems to me that this would be morally troublesome in spite of the often heard argument that there’s nothing wrong with enhancing the capabilities of an animal.

In my opinion, this procedure should be viewed the other way around. It is not that an animal is thereby enhanced, but rather that what is essentially human is really debased. It is closer to the production of a human being in the wrong body.

And I often imagine what it would be like to wake up one day only to realize that I have the body of a chimpanzee. Luckily, we’re told that this is virtually impossible because the human body as we know it seems to be absolutely necessary for the development of the human mind, and I’m thinking about size of the brain, the cranial space, et cetera.