But the cloning that led to the creation of Dolly the sheep in 1996 also failed far more often than it succeeded, and now cattle, sheep, pigs, and some pets are routinely cloned. Cells derived from rat pluripotent stem cells were enriched in the developing heart of a genetically modified mouse embryo using CRISPR. Because ethical guidelines advise against letting chimeras develop completely, the scientists gathered the 186 surviving embryos after 21 to 28 days. Wu and his team injected three to 10 of the human pluripotent stem cells into 1,506 pig embryos, each a few days old.Īfter growing the embryos in dishes for a few days, the scientists transferred them to 41 surrogate mother sows - 30 to 50 embryos each. They first created “induced pluripotent stem cells” by turning back the calendar on adult cells until they were embryo-like. The same technique - injecting pluripotent stem cells into early embryos - failed with other combinations: The scientists couldn’t create rat-pig chimeras, and although they produced human-cow chimeric embryos, they did not transfer them into cows to develop into fetuses.Īnd the human-pig chimeras proved much harder to create than the scientists expected, taking four years instead of the expected one. But those experiments are underway, Wu said.
The Salk team did not report using CRISPR in the human-pig chimeras to help the pigs develop more humanlike organs. NIH considering restarting funding for human-animal chimera research
Then, when the rat stem cells are injected, the ones fated to become heart cells had less competition and outcompeted mouse cells to form the particular organ, including the heart, eyes, and pancreas. He and his colleagues therefore used CRISPR to knock out, in fertilized mouse eggs, at least one gene crucial for the development of a particular organ, like the heart. “To enrich the donor cells in the host, you need to disable the genetic program in the host embryo that gives rise to a particular organ,” Salk’s Wu said. The credit for that likely goes to CRISPR, the powerful new genome-editing technology. The heart had the most rat cells - 10 percent. By age 2, the mice had some rat cells in their kidney, lung, pancreas, liver, and brain. Injecting rat pluripotent stem cells into mouse embryos produced rat-mouse chimeras. In the same paper, the scientists - working under Salk’s Juan Carlos Izpisua Belmonte - reported making rat-mouse chimeras, too. “The overall human contribution was very low, with what we estimate is less than 1 human cell per 100,000 pig cells,” and no human cells in the chimeras’ brains, biologist Jun Wu of the Salk Institute of Biological Studies, lead author of the Cell paper, said in an interview.īut that’s likely a floor, not a ceiling. And even the successes carried very few human cells. The attempts at human-pig chimeras failed more often than they succeeded. Policymakers likely have a lot of time to figure out their position. A White House spokesman did not immediately respond to a request for comment. It’s anyone’s guess as to what the new administration thinks about chimeras.
But a bill introduced late in the last Congress would prohibit chimera research, with penalties of up to $1 million and 10 years in prison. That has no effect on experiments supported with private or state money. But the proposal was not finalized before the start of the Trump administration, which this week directed federal agencies not to issue final guidelines or regulations on any topic, so the 2015 ban stands. The NIH proposed lifting that moratorium last August, requiring additional oversight of chimera experiments and barring the use of human cells to create chimeras. (It wasn’t funding such research at the time.) This is how the human-pig chimera was created, with funding largely from foundations. That and other concerns led the National Institutes of Health to announce in 2015 that it would not fund experiments that put human pluripotent stem cells, those with the ability to morph into almost any kind of tissue or organ, into the early embryos of other animals. Much of the bioethics focus has been on what would happen if an animal had enough human brain cells to think and feel like a person - but a person inside the body of a monkey, pig, rat, or mouse. We’ve created human-pig chimeras - but we haven’t weighed the ethics Exclusive analysis of biotech, pharma, and the life sciences Learn More
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