A team of scientists from China and the United States has reached a breakthrough in the field of stem cell research. The scientists injected human stem cells into monkey blastocytes, an early structure in embryonic development in mammals. The team around lead researcher Juan Carlos Izpisua Belmonte then managed to keep some of the embryos made up of two different genetic materials alive for up to 20 days. A "mixed" organism like this is also known as chimeric, or an interspecies chimera.

Izpisua Belmonte and his team at the Salk Institute for Biological Studies in California cooperated with a group of Chinese researchers led by Weizhi Ji at Kunming University of Science and Technology in Yunnan. Their study on chimeric human-primate embryos was published on Thursday in the renowned natural science journal Cell.

Chimeras in mammals have been made since the 1970s to study early developmental processes. The difference: Back then, scientists used rodents, and the interspecies organisms usually didn't survive for very long. The big step that made the new study possible came last year when the Chinese team at Kunming University developed technology that allowed monkey embryos to stay alive and grow outside the body for an extended period of time.

"Historically, the generation of human-animal chimeras has suffered from low efficiency and integration of human cells into the host species," Izpisua Belmonte said. The advances now published in Cell will help scientists understand better how chimeras work and in turn, how to improve them for further research.

That's how it's done ― but what about the why?

Mixing human stem cells with animals' genetic material is a significant intervention into nature's course, but the researchers say they have good reason for it.

"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," Izpisu Belmonte explained.

In Cell, the study's authors further explain that their work generating chimera with human stem cells "might constitute a promising strategy for various regenerative medicine applications, including the generation of organs and tissues for transplantation."   

The work of Izpisu Belmonte and his colleagues has been harshly criticized, too, though.

"I think the research is of very low quality," Alfonso Martinez Arias, an affiliated lecturer in the department of genetics at the University of Cambridge, said in a comment on the study provided to the independent Science Media Centre in the UK. From looking at the researchers' data, Martinez Arias said, "it is impossible to see what they say is there."

Cell is a peer-reviewed journal, meaning that each study that is published undergoes a rigorous process in which a panel of scientists reviews the material and results provided by the study's authors. The fact that Izpisu Belmonte's work was published in Cell means that none of the experts reviewing it shared Martinez Arias' concerns.

What is a human being?

That still leaves ethical questions. In the current study, the embryos didn't survive past 20 days. But what happens when science eventually gets to the point that chimeras like these turn into fully formed beings?

"This research opens Pandora's box to human-nonhuman chimeras," Julian Savulescu, Director of the Oxford Uehiro Centre for Practical Ethics at the University of Oxford, said in his comment to the Science Media Centre. To him, the key ethical question relates to what kind of creatures these chimeras would be ― to what extent can they think and feel? Would it be acceptable to take organs from them that were grown for transplant purposes?  

"What is the moral status of these novel creatures?" Savulescu wrote. "Before any experiments are performed on live-born chimeras, or their organs extracted, it is essential that their mental capacities and lives are properly assessed."

Anna Smajdor, associate professor of practical philosophy at the University of Oslo, even went a step further.

"This breakthrough reinforces an increasingly inescapable fact: biological categories are not fixed ― they are fluid," she said in her comment to the Science Media Centre. "The scientists behind this research state that these chimeric embryos offer new opportunities, because 'we are unable to conduct certain types of experiments in humans'. But whether these embryos are human or not is open to question."