Concerns About Chimera Research

If a chimera's brain is comprised of mostly human neurons with an appropriate structure, experts are questioning whether a human brain—and mind—could develop within an animal or human-animal body. The National Academies Guidelines for Human Embryonic Stem Cell Research cautions that experiments in which there is a possibility that human cells could contribute in a "major organized way" to the brain of an animal require strong scientific justification.

Projects such as the following give rise to questions regarding chimeras with human brains:
* Researchers at Stanford University are injecting human neural stem cells into mouse fetuses to study development of neurologic diseases (see project description in Definition section, above). Initial results were mice with brains that were about 1% human, but researchers could increase that percentage to 100% human brains in the laboratory mice. They anticipate that the structures of the brain, and certainly the size, would still be mouse-like even though the neurons of the brain would be 100% human. Responding to concerns that the experiment could result in brains that were more human than mouse, Stanford University's ethics committee has approved the research on the condition that if the mice developed indicators of human intelligence, such as improved memory or problem-solving, it would be time to stop the project.

* At St. Kitts Biomedical Foundation in the Caribbean, scientists are transplanting immature human brain cells into the brains of vervet monkeys, generating a human-monkey chimera.xxi This kind of research could create human-primate chimeras, with human brain cells.

An expert panel was convened by John Hopkins University in 2005 to study the potential for just such human-primate experiments to significantly change the cognitive and emotional capacities of primates—in effect, to humanize them.

The Working Group on Interspecific Chimeric Brains consisted of primatologists and other scientists, ethicists, and lawyers. They concluded that any shift in brain capacities toward the human end of the spectrum would be more likely to occur if human brain cells were introduced early in the development of the brains of great apes. Experiments in which small numbers of human cells were grafted into healthy adult brains of the most distantly related monkey species were the least likely to raise concerns.

If human embryonic stem cells were to be injected into an animal embryo very early in its development, the human cells might spread through every organ system in the animal embryo. Conversely, animal embryonic stem cells could do the same thing when introduced into an early-stage human embryo. The results of such techniques could be a humanized animal (or animalized human), with physical features and behaviors that are distinctly human.

Considering this possible outcome, the National Academies' Guidelines for Human Embryonic Stem Cell Research say that no animal embryonic stem cells should be transplanted into a human blastocyst (early embryo). In addition, approval by an Embryonic Stem Cell Research Oversight committee should be secured before any human embryonic stem cells are put into an animal.

If a female chimera with human ova (egg cells) mated with a male chimera with human sperm cells, scientists believe that it would be possible for a human fetus to be the result. Potentially, a chimera could give birth to a human. To address this issue, the National Academies Guidelines for Human Embryonic Stem Cell Research states: “No animal into which human embryonic stem cells have been introduced at any stage of development should be allowed to breed.”

While already being used a t St. Kitts Biomedical Foundation (mentioned above), primates are likely to be the animal of choice in some human-animal chimera projects because of their close genetic similarities (especially in chimpanzees) to humans. While the guidelines state that “no human embryonic stem cells should be put into nonhuman primate blastocysts [early embryos],” they do not exclude chimera research at a later stage of development. While some oppose this research outright, others are assuming that human-primate chimeras will become more widespread in research and question whether they will be treated as humans or animals.

"What is a human? This is not a question of the moral dilemma to define a human but is a legal requirement as to how much human material a chimpanzee must have before it is declared a part human and therefore falls under the protection of the Thirteenth Amendment….We must move beyond moral and philosophical discussions of hybrid human-nonhuman animals and be prepared to tackle the difficult legal questions that will attend not-so-distant attempts at creating such hybrids say for organ harvesting, for use as a subhuman species to perform hard manual labors, or simply for curiosity’s sake.”