thescientist | Late last year, Steve Goldman of the
University of Rochester and his colleagues reported that they had
transplanted immature glial cells from donated human fetuses into the
brains of immunodeficient mouse pups. These human glial cells matured
into astrocytes and developed as the primary astrocyte population in the
newborn mouse brain. One unexpected outcome of the team’s research,
published in the Journal of Neuroscience
(34:16153-61), was that these human-mouse chimeras outperformed normal
mice almost fourfold in a variety of cognition tests, underscoring the
importance of astrocytes in regulating synaptic plasticity and neural
connectivity to enhance learning and memory. But the study also raised
important ethical considerations—namely, what biological properties
differentiate Homo sapiens from other organisms, and when should such “humanized” animals be afforded the rights that people currently enjoy.
Goldman is quick to state that the enhanced memory and learning
performance of these human-mouse chimeras did not make the mice more
human. “It’s still a mouse brain, not a human brain, but all the
non-neuronal cells are human,” Goldman told New Scientist
at the time of the publication. “This does not provide the animals with
additional capabilities that could in any way be ascribed or perceived
as specifically human. Rather, the human cells are simply improving the
efficiency of the mouse’s own neural networks. It’s still a mouse.”
At the same time, the team had ethical reservations about repeating
these types of experiments on monkeys, presumably following the National Academies’ guidelines
that no human embryonic stem cells should be introduced into nonhuman
primates at any stage of fetal or postnatal development. Is there really
an ethical difference in performing these experiments on mice as
opposed to monkeys? The scientists have not addressed this question,
perhaps because it is a difficult one to answer.
Human intelligence, as difficult as it is to define, is often thought
to be one of the most important characteristics that differentiate Homo sapiens
from all other organisms. However, the capacity to humanize animals has
the potential to complicate this assessment of being human. For
example, should the definition of human or humanlike intelligence be
psychometric, based on a constellation of cognitive processes, or should
it be neurophysiologic or neurogenetic because it is inextricably
linked to the brain? The question of distinguishing human and nonhuman
characteristics has arisen regarding our close primate relatives. Last
October, a New York Appeals Court announced that it will consider the
issue of whether chimpanzees are entitled to “legal personhood.”
Similarly, in December, an appeals court in Argentina recognized
orangutans as having basic legal rights, stating that these primates
deserve living quarters in a sanctuary and not in a zoo.
