Monkey embryos containing human cells have been produced in a laboratory, a study has confirmed, sparking new debate about the ethics of such experiments.
Embryos are known as chimeras, organisms whose cells come from two or more “individuals”, and in this case different species: a long-tailed macaque and a human.
In recent years, researchers have produced pig embryos and sheep embryos that contain human cells; research that they say is important as it could one day allow them to grow human organs within other animals, increasing the number of organs available for transplantation.
Now scientists have confirmed that they have produced macaque embryos that contain human cells, revealing that the cells could survive and even multiply.
Additionally, the researchers, led by Professor Juan Carlos Izpisua Belmonte from the Salk Institute in the US, say the results offer new insights into the communication pathways between cells of different species, work that could help them in their efforts to make chimeras with species that are less related to ours.
“These results may help better understand early human development and primate evolution and develop effective strategies to enhance human chimerism in evolutionarily distant species,” the authors wrote.
The study confirms the rumors reported in the Spanish newspaper The Country in 2019 that a team of researchers led by Belmonte had produced monkey-human chimeras.
The word chimera comes from a beast in Greek mythology that was said to be part lion, part goat, and part snake.
The study, published in the journal Cell, reveals how scientists took specific human fetal cells called fibroblasts and reprogrammed them into stem cells. They were then introduced into 132 long-tailed macaque embryos six days after fertilization.
“Twenty-five human cells were injected, and on average, we observed about 4% of the human cells in the monkey epiblast,” said Dr. Jun Wu, a co-author of the research now at the University of Texas Southwestern Medical Center.
The embryos were allowed to develop in Petri dishes and were terminated 19 days after injection of the stem cells. To test whether the embryos contained human cells, the team engineered human stems to produce a fluorescent protein.
Among other findings, the results reveal that all 132 embryos contained human cells on day seven after fertilization, although as they developed, the proportion containing human cells decreased over time.
“We showed that human stem cells survived and generated additional cells, as would normally happen as primate embryos develop and form the layers of cells that eventually lead to all the organs of an animal,” Belmonte said.
The team also reports that they found some differences in cell-cell interactions between human and monkey cells within chimeric embryos, compared to human cell-free monkey embryos.
Wu said he hopes the research will help develop “transplantable human tissues and organs into pigs to help overcome the shortage of donor organs around the world.”
Professor Robin Lovell-Badge, a developmental biologist at the Francis Crick Institute in London, said at the time of the El Pais report that he was not concerned about the ethics of the experiment, noting that the team had only produced one ball of cells. But he noted that enigmas could arise in the future if embryos are allowed to develop further.
The published study has rekindled such concerns. Professor Julian Savulescu, director of the Uehiro Center for Practical Ethics at Oxford and co-director of the Wellcome Center for Ethics and Humanities at the University of Oxford, said the research had opened a Pandora’s box to human and non-human chimeras.
“These embryos were destroyed within 20 days of development, but it is only a matter of time before human-non-human chimeras develop successfully, perhaps as a source of organs for humans,” he said, adding that a A key ethical question is what is the moral status of such creatures.
“Before experiments are performed on live-born chimeras or their organs are removed, it is essential that their mental capacities and lives are properly assessed. What looks like a non-human animal can be mentally close to a human, ”he said. “We will need new ways of understanding animals, their mental lives and their relationships before they are used for human benefit.”
Others expressed concern about the quality of the study. Dr Alfonso Martínez Arias, Affiliate Professor in the Department of Genetics at the University of Cambridge, said: “I don’t think the conclusions are supported by solid data. The results, insofar as they can be interpreted, show that these chimeras do not work and that all the experimental animals are very sick ”.
“Importantly, there are many human embryonic stem cell-based systems for studying human development that are ethically acceptable and, in the end, we will use this instead of chimeras of the type suggested here.”