ABOVE: In this image of a nine-day-old human-monkey chimera, human EPS cells are labeled in red. The trophectoderm layer, which contains placental precursors, is labeled in green and gray.

There are hundreds of different cell types that make up the human body, each derived from a single origin: the fertilized egg. Researchers investigating how this complexity arises have made human-animal chimeric embryos by introducing human pluripotent stem cells into the embryos of other animals, such as mice and pigs. By tracking the outcomes of the human cells, they can discern how capable these cells are of differentiating into various cell types and contributing to the embryo. In work published in 2017, for instance, human cells contributed up to 1 percent of embryonic cells in a human-mouse chimera.

In a study published today (April 15) in Cell, researchers describe their progress in producing a human-monkey chimeric embryo in an effort to determine whether having a more closely related host would allow the human cells to have a greater presence, thus paving the way for a better understanding of what cell types they can become and, possibly in the far future, a potential way to cope with the shortage of human organs available for transplant. The scientists made the chimeras by injecting human extended pluripotent stem (EPS) cells—also known as expanded potential stem cells—into early embryos of cynomolgus monkeys (Macaca fascicularis). The team grew the chimeras for up to 20 days in culture and found that up to 7 percent of the embryos’ cells can trace their lineage to the human EPS cells.

EPS cells—generated by treating pluripotent stem cells with factors that help them both maintain pluripotency and contribute to embryonic and extraembryonic tissues—were developed by Juan Carlos Izpisua Belmonte, a stem cell biologist at the Salk Institute for Biological Studies, and colleagues in 2017.

“A precise confirmation of these cells’ developmental potency can only be achieved in vivo . . . but chimeric assays are not easy,” says Berna Sozen, a stem cell biologist at Yale University who was not involved in the study. In the past, researchers have had limited chimeric success, at least in part due to the evolutionary distance between humans and mouse or pig hosts, she adds.

In the new study, Izpisua Belmonte’s team collaborated with Tao Tan of the Kunming University of Science and Technology in China and colleagues to tackle the challenge of evolutionary distance. They used a strategy described in two studies in 2019 by Tan’s group and another team that allows researchers to grow monkey embryos in culture for up to 20 days.

The researchers injected each of 132 six-day-old monkey embryos with 25 human EPS cells. The next day, they found human cells in all of the embryos. Where those cells were found within the embryos shifted over time. At 15 days old, the 38 surviving chimeras had the highest contribution of human cells (about 7 percent) in the outermost layer of embryonic cells, and at 19 days old, the three surviving chimeras had the greatest proportion of human cells (about 5 percent) in the innermost layer. The team did not see much human-cell presence in the layer that would become the extraembryonic tissues, such as the placenta.

Next, the researchers analyzed a readout of which genes were active during chimera development and observed a different suite of genes than they did in monkey embryos that weren’t injected with human cells. “We plan to take a closer look at the molecular pathways we identified as involved in interspecies communication and to determine which ones are critical to the success of this process,” Izpisua Belmonte writes in an email to The Scientist. In other words, how are monkey and human cells interacting with one another during the embryo’s development?

“These advances have accelerated our ability to study developmental and regenerative processes,” he continues. “This knowledge could also be instrumental to advancing the goals of regenerative medicine, such as developing tissues to address the critical shortage of organs for transplant.”

“For this to become a reality, there are a number of obstacles,” says Jacob Hanna, a stem cell biologist at the Weizmann Institute in Israel who was not involved in the work. The primary obstacle is that, whether human cells are being transplanted into mouse, pig, or monkey embryos, “the integration efficiencies we see in all cross-species experiments are very, very low.”

On the other hand, “it’s incredible that now we see so much evidence that human cells can survive and differentiate in other organisms,” he adds. The next question to address in pursuit of making human organs for transplant is, “how can we make integration either directed to a certain organ or . . . try to make the cells more competitive?”

Researchers around the world are “trying to understand [EPS] cells, but we need to collect much more data before we can actually start to discuss any potential clinical applications,” agrees Sozen. “It is really hard to say that it will ever be possible to grow organs for transplantation by creating these animal-human chimeras, but this research should continue for us to understand whether we will ever achieve this.”

T. Tan et al., “Chimeric contribution of human extended pluripotent stem cells to monkey embryos ex vivo,” Cell, doi:10.1016/j.cell.2021.03.020, 2021.