Efforts to create human-animal chimeras have rebooted an ethical debate after reports emerged that scientists have produced monkey embryos containing human cells.
A chimera is an organism whose cells come from two or more “individuals,” with recent work looking at combinations from different species.
The word comes from a beast in Greek mythology which was said to be part lion, part goat and part snake.
The latest report, published in the Spanish newspaper El Pais, claims a team of researchers led by Juan Carlos Izpis Belmonte from the Salk Institute in the US have produced monkey-human chimeras.
The research was conducted in China “to avoid legal issues,” according to the report.
Chimeras are seen as a potential way to address the lack of organs for transplantation, as well as problems of organ rejection.
Scientists believe organs genetically matched to a particular human recipient could one day be grown inside animals.
The approach is based on taking cells from an adult human and reprogramming them to become stem cells, which can give rise to any type of cell in the body. They are then introduced into the embryo of another species.
Izpis Belmonte and other scientists have previously managed to produce both pig embryos and sheep embryos that contain human cells, although the proportions are tiny: In the latter case, researchers estimate that only one cell in 10,000 was human.
Pig-human and sheep-human chimeras are attractive in part because pigs and sheep have organs about the right size for transplantation into humans.
Details of the work reported this week are scarce: Izpis Belmonte and colleagues did not respond to requests for comment.
However, Alejandro De Los Angeles, from the department of psychiatry at Yale University, said it was likely that monkey-human chimeras were being developed to explore how to improve the proportion of human cells in such organisms.
“Making human-monkey chimeras could teach us how to make human-pig chimeras with the hope of making organs for transplantation. It could teach us which types of stem cells we should be using, or other ways of enhancing what’s called ‘human chimerism levels’ inside pigs,” he said.
De Los Angeles pointed out that, as with previous work in pigs and sheep, the human-monkey chimeras have reportedly only been allowed to develop for a few weeks — ie, before organs actually form.
Robin Lovell-Badge, a developmental biologist at London’s Francis Crick Institute, agreed.
“I don’t think it is particularly concerning in terms of the ethics, because you are not taking them far enough to have a nervous system or develop in any way — it’s just really a ball of cells,” he said.
However, he added that if chimeras were allowed to develop further, it could raise concerns.
“How do you restrict the contribution of the human cells just to the organ that you want to make? If that is a pancreas or a heart or something, or kidney, then that is fine if you manage to do that. [But] if you allow these animals to go all the way through and be born, if you have a big contribution to the central nervous system from the human cells, then that obviously becomes a concern,” Lovell-Badge said.
The news of the monkey-human chimeras came shortly after it was reported that Japanese researchers, including Hiromitsu Nakauchi, received government support to create mouse-human chimeras.