Researchers in China and the United States have injected human stem cells into primate embryos and grown chimeric embryos for up to 20 days. Despite ethical concerns, the scientists says that the procedure has potential for providing insights into developmental biology and evolution, into disease and transplants, and into drug development.
"As we are unable to conduct certain types of experiments in humans, it is essential that we have better models to more accurately study and understand human biology and disease," says senior author Juan Carlos Izpisua Belmonte, of the Salk Institute, in California.
In the current study, six days after the monkey embryos had been created, each was injected with 25 human cells. The cells were from an induced pluripotent cell line which has the potential to contribute to both embryonic and extra-embryonic tissues.
After one day, human cells were detected in 132 embryos. After 10 days, 103 of the chimeric embryos were still developing. Survival soon began declining, and by day 19, only three chimeras were still alive. Importantly, though, the percentage of human cells in the embryos remained high throughout the time they continued to grow.
Clearly there are ethical issues involved in creating hybrid embryos. What if the hybrid embryos grew into hybrid human-monkeys? As The Economist noted: “Experiments involving human cells can break deep-seated taboos about human dignity, human exceptionalism and—among the religious—stir up worries about interfering with God’s creation.”
“The complicated thing is that we need better models of human disease, but the better those models are, the closer they bring us to the ethical issues we were trying to avoid by not doing experiments in humans,” Nita Farahany, of Duke University School of Law, told STAT. “Remarkable steps forward require urgent public engagement.”
The International Society for Stem Cell Research will soon publish revised guidelines for stem-cell research, including nonhuman-primate and human chimaeras.
Human-monkey chimeras do raise worries, addressed in a report released last week by the National Academies of Sciences, Engineering, and Medicine: “In the specific case of neural transplantation and chimera research, the slippery slope concern is that if small increments in mental capacities develop in transplants or chimeras, there will be no logical point at which the research should be stopped, or it may not be possible later to institute policies to block research that could result in nonhuman animals with unacceptable human capabilities.”
However, the chimeras in this study do not have a nervous system. They “can’t experience pain and aren’t conscious,” bioethicist Katrien Devolder of the University of Oxford, told Science magazine. “If the human-monkey chimeras were allowed to develop further,” she says, “that would be a very different story.”
Michael Cook is editor of BioEdge
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