In today’s scientific world, it is often uncommon to find experiments that have been done on nonhuman primates, as there are strict research ethic rules that protect them from being used.

A recent article published in the scientific journal Cell, though, highlighted the successful growth of monkey embryos containing human stem cells, and the possible applications that these results might have in regenerative medicine.

At the beginning of the study, a group of scientists led by Juan Carlos Izpisua Belmonte  fertilized eggs that had been extracted from cynomolgus monkeys (Macaca fascicularis).

6 days after the process of fertilization, scientists injected 132 monkey embryos with human stem cells, or induced pluripotent stem cells. Scientists selected to use these specific stem cells because they can vary in cell types inside and outside of an embryo.

11 days after fertilization, scientists noted that 91 of the monkey-human embryos were still alive, which dropped to 12 embryos on day 17, and only three on day 19.

In order to study the communication between human and animal cells, the scientists grew the mixed embryos in test tubes for approximately 20 days through a process called in vitro fertilization.

Through this research, scientists hope that these chimaeras — or a single organism that is composed of cells with no distinct phenotype — could provide better insight into the effects of new drugs and aid in growing human organs that can be used for transplant, as well as to learn more about human development and disease progression.

“The overall message is that every embryo contains human cells that proliferate and differentiate to a different extent,” said Juan Carlos Izpisua Belmonte, a developmental biologist at the Salk Institute for Biological Studies.

The scientists selected to use the macaque monkey because their embryonic development is nearly identical to the embryonic development of humans.

“These chimeric approaches could be really very useful for advancing biomedical research not just at the very earliest stage of life, but also the latest stage of life,” Belmonte said. 

According to researchers, thousands of people die each year waiting for such transplants, as the demand for organ donors is much higher than the supply of viable organs.

“These embryos were destroyed at 20 days of development, but it is only a matter of time before human-nonhuman chimeras are successfully developed, perhaps as a source of organs for humans,” stated the director of the Oxford Uehiro Centre for Practical Ethics, Julian Savulescu, in response to the success of the experiment.

If these experiments continue to result in successful results, then scientists look to use these chimeric organs on those who are in desperate need of them.

Although, these experiments were not actually the first experiments to create chimeric models.

In fact, back in 2017, Dr. Izpisua and the Salk Institute had successfully created human-pig chimeras, and Portugese researchers were able to create a chimera virus, more specifically, a mouse virus with a human viral gene.

Not only have they injected human stem cells into other types of animals, they have also introduced the idea that some chimeras occur naturally.

Twins who absorb some of their sibling’s DNA in embryonic development have also been seen as chimeras.

While having been successful in these trials, there are still others who are of the opinion that these experiments are not entirely ethical.

“These scientists behind this research state that these chimeric embryos offer new opportunities,” said Dr. Anna Smajdor, an ethics specialist at the Norwich Medical school at the University of East Anglia. “But whether these embryos are human or not is open to question.”

Members of the Society of Stem Cell Research  see the use of monkeys for experiments like these as unethical because of the genetic similarities that are shared between monkeys and humans.

“Meanwhile, international guidelines are catching up to the field’s advances — next month, the International Society for Stem Cell Research is expected to publish revised guidelines for stem cell research. These will address non-human primate and human chimaeras,” said Hyun, the leader of an ISSCR committee that has recently been discussing the use of chimaeras.

Despite the negative feedback about these experiments, scientists have high hopes that in future medical practices, these chimeric organs will be able to be used in organ transplantation, drug practice, and even other medical disorders.