Scientists from China and the United States have generated chimeric human-monkey embryos that survived well beyond the expected survival period, with some embryos reaching up to 19 days.
This breakthrough follows advances reported in 2019 where the same team of scientists reported research on human-animal chimeras.
"Historically, the generation of human-animal chimeras has suffered from low efficiency and integration of human cells into the host species," senior author, Professor Izpisua Belmonte said in a statement. "Generation of a chimera between human and non-human primate, a species more closely related to humans along the evolutionary timeline than all previously used species, will allow us to gain better insight into whether there are evolutionarily imposed barriers to chimera generation and if there are any means by which we can overcome them."
In the new study, published in the journal Cell, Belmonte and team created monkey embryos that on day 6 of survival received an injection of 25 human extended pluripotent stem cells. The specific human stem cell line that was used can contribute to both normal and embryonic tissue under the correct conditions. After just one day, they detected the presence of human cells within 132 of the newly created human-monkey chimeric embryos. After 10 days, 103 of the embryos were still developing. However, after this time frame, some of the embryos started dying and by day 19 there were only three embryos still alive. One important point the researchers noted was that the percentage of human cells in the embryos remained high throughout the experiment.
Speaking to IFLScience, Professor Belmonte explained the importance of the survival rate they saw with the chimeric embryos. "The current technical limit on maintaining healthy, non-chimeric monkey embryos in a dish (ex vivo) is 20 days. Chimerism adds another level of complexity; it is very difficult to keep chimeric cells alive in a dish for even a few days," he said.
These results are both pioneering and controversial, and many argue for ethical reasons we should not be generating and experimenting with human-monkey chimeric embryos.
“Substantial advances are being made in embryo and stem cell research, and these could bring equally substantial benefits. However, there is a clear need for public discussion and debate about the ethical and regulatory challenges raised," Sarah Norcross, director of the Progress Educational Trust, who was not involved in the study, said.
“Issues raised by this particular research concern the combination of human and nonhuman material (although the embryos, in this case, were predominantly nonhuman) and how long embryos may be cultured for in the laboratory."
Belmonte told IFLScience the work they are doing has important applications, however, from drug testing to organ transplants.
"This type of research is important for developing insights into cellular communication and biological development during embryogenesis and evolution as well as for a variety of research and regenerative medicine applications," he said. "For example, it may one day result in the ability to grow replacement human tissue, such as heart and kidney tissues. It may also help test candidate drugs for human diseases more accurately than traditional animal models."
The ability to generate potential replacement organs for transplant is a lofty goal, but there's no denying the Frankenstein element of the research. So, can these embryos actually survive and grow to term?
"We do not know whether fully formed human-monkey embryos would ever be biologically possible, but our goal in chimera research is to better understand human development to inform therapies for human diseases, not to develop new organisms." Professor Belmonte said.