Where are the ethical boundaries of biomedicine? Chinese-American Monkey Hybrid Embryo Research Stirs Controversy

On April 16, a scientific team co-led by Chinese and American scientists announced that they had created the first embryo composed of human and monkey cells together, and published the results in the journal Cell. 

This paper, according to some scientists, is a watershed moment in the science of stem cells and cross-species chimeras. At the same time, the journal Nature examined the research's potential ethical implications.

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What is a human-monkey hybrid embryo?
Cross-species chimeras have been studied by scientists since the 1970s in order to better understand how early development occurs or to uncover potential medical uses. In 2017, scientists transplanted a mouse stem cell-derived pancreas into a rat embryo, curing the latter of diabetes. However, in the same year, Juan Carlos Izpisa Belmonte of the Salk Institute in the United States failed to get positive results in more distantly related animals—he transplanted human stem cells into pig embryos. After a few weeks, about one out of every 100,000 cells was human.

Belmonte is one of the leaders of this human-monkey hybrid embryo project. In this study, the scientists used animals that are more closely related to humans—monkeys, who first isolated fertilized eggs from the monkeys and grew them in a petri dish. Six days after fertilization, they injected 132 monkey embryos with human expanded pluripotent stem cells (capable of growing into different cell types inside and outside the embryo). The researchers then observed the development of these embryos to see if the two types of cells could coexist.

Ten days after fertilization, 103 chimeric embryos were still continuing to develop, nearly 80%. However, after that, the survival rate of the embryos declined rapidly, and by day 19, only 3 chimeric embryos remained alive.

From a different perspective, a high percentage of human cells are maintained in each of these embryos. These human cells have begun to differentiate into different cell types that can further develop into different organs. "Overall, there were human cells in each embryo and these cells proliferated and differentiated to different degrees." Belmonte said.

What purpose do human-monkey hybrid embryos serve?
Belmonte noted that through transcriptome analysis, the study identified a number of novel communication pathways between human and monkey cells in mixed embryos. The incorporation of human cells into host tissues has been ineffective in previous chimera studies. And if the role of these pathways can be understood, it is expected to overcome the barriers between different species and improve the efficiency of generating chimeras for future studies. In the long term, researchers hope to use these chimeras to study early human development, develop disease models, screen for potential new drugs, and create cells, tissues, or organs that can be used for transplantation.