On 23 September, NIH issued a notice saying that it will not fund such research “while the Agency considers a possible policy revision in this area.” And it has invited scientists and bioethicists to a meeting on 6 November to discuss the ethical questions raised by such experiments. Izpisúa Belmonte took the news in stride. “I’m not upset. Quite the opposite. I think it is great that we openly discuss this and hope that a conclusion is reached,” he says.
The mixtures of cells under debate are called chimeras, named for a monster of Greek mythology that had the body and head of a lion, a fire-breathing goat’s head on its back, and a snakelike tail. In ancient Greece, the chimera was a bad omen, appearing before shipwrecks, volcanoes, and other disasters.
Scientists see the term in a more positive light, using animal chimeras in a range of developmental biology and stem cell experiments. Chimeras that combine animal and human cells, especially those that involve pluripotent human cells, raise ethical questions, however. Pluripotent cells are a powerful type of stem cell that can become any cell type in the body. Some worry that such human cells, when combined with animal embryos, could develop into brain cells, sperm, or egg cells in the chimeric offspring.
The U.S. National Research Council and the Institute of Medicine recommended limits on such research in 2005, among them that no human stem cells be added to primate embryos and that animal- human chimeras not be allowed to breed. Current NIH funding guidelines, finalized in 2009, reflect those recommendations. They prohibit breeding animals in which human stem cells might have become sperm or eggs, and they rule out primate-human experiments. They do not, however, prohibit injecting human pluripotent cells into the embryos of other animals and letting the chimeras develop.
That is what several groups of researchers are now trying to do. Their goal is to learn how to coax stem cells to become specific tissues or organs. Doing that in the lab, by recreating the 3D environment of a developing organ and reproducing all the signals it receives, is very difficult. “We don’t know how to guide the cells to become the cells we want,” Izpisúa Belmonte says. Instead, he and his colleagues want “to use the animals as an incubator. We don’t know how they do it, but every day [developing animals] produce perfect organs.”www.elisakitgs.com