But such cross-species research raises red flags among some ethicists, who fear a slippery slope leading to the abuse of human-animal hybrids.
Of particular concern are experiments that include human sperm, eggs, or reproductive cells in animal hosts.
Last year, Canada passed the Assisted Human Reproduction Act to help regulate such research.
"The legislation prohibits the creation of a chimera, defined as a human embryo into which a cell of a nonhuman life form has been introduced," said Francine Manseau, with Health Canada's Assisted Human Reproduction Implementation Office. (The Chimera is a creature in Greek mythology that sports a lion's head, a goat's body, and a serpent's tail.)
"It also bans [human] embryos that consist of cells from more than one embryo, fetus, or human being."
The U.S. National Academy of Sciences issued a series of voluntary research guidelines in April. They reject mating between animals bearing human eggs or sperm. The academy also proposed limits on the amount of animal "humanization" that should be permitted.
But experts disagree on where such lines should be drawn.
Many plants and animals are engineered to express human genetic information. Bacteria produce insulin for diabetics in this way. Polly the sheep, successor to the famous clone Dolly, makes human blood-clotting factor, a coagulant, in a similar manner.
"The difference [with the mice with human brain cells] is that, instead of splicing in bits of DNA, scientists are stuffing in cells," said McGee, the bioethicist.
"A few thousand human brain cells will not turn a house pest into Mickey Mouse," he continued.
"Critics of this research would have you believe that to grow our cells in other creatures is repugnant and inhumane. Mice already grow human ears and are used in many experiments to grow colonies of other human cells," McGee said.
"The key to preventing some kind of new ethical problem is to watch in careful ways to ensure that the mice are—behaviorally—mice."
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