Researchers in California have created living mice with functioning human stem cells in their brains. The feat could boost stem cell therapy research on brain diseases like Parkinson's and Alzheimer's—and raises the specter of animal-human hybrids. RELATED Geneticist Fred Gage injected embryonic human cells into two-week-old fetal mice as they developed in the womb. When the mice matured, some human stem cells survived and became functional components of the mice's brains and nervous systems. Less than one-tenth of one percent of the test mice's brain cells are human. "When we characterized these cells two months later, we found that [they] had the [form and structure] and characteristics of mouse cells," said Gage, co-director of the genetics laboratory at the Salk Institute for Biological Studies in San Diego. "It is truly amazing that these human stem cells, although they are very immature, can still … respond to different cues in their environment and can fit right in with their mouse neighbors." "This illustrates that injecting human stem cells into mouse brains doesn't restructure the brain," Gage added. The study was published in yesterday's issue of the journal Proceedings of the National Academy of Sciences. Medical Promise or Ethical Peril? Some scientists hope that stem cells, which may develop into many different kinds of human cells, could someday be used to replace missing or damaged neurons in people with degenerative nerve diseases. "This research is significant because it suggests that it will be possible to create mouse 'models' of human brain tissue, enabling scientists to try out both stem cell interventions and other potential cures on living human brain cells without having to use humans in the process," said Glenn McGee, director of the Alden March Bioethics Institute at Albany Medical College in New York State. In short, it's a new way to test potential cures for human diseases, without harming human test subjects in the process. 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." 
O'Donnell In 2007: Scientists Have Created Mice With Human Brains!
Mice With Human Brain Cells Created
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