Monkey embryos containing human cells have been developed in a laboratory, a study by a global team of researchers confirms.
The development sparks new debate on the ethics of experiments on "chimera" embryos, whose cells come from two different species.
In the new study, the researchers, led by Prof Juan Carlos Izpisua Belmonte from the Salk Institute in the US, detail their experiments with cells from a long-tailed macaque and a human. They reveal that the cells can survive and even multiply when matched together.
Despite the controversy surrounding such works, the researchers say their research may one day allow human organs to grow inside other animals, allowing for a new source of transplant organs.
The researchers also say their latest study contributes to a greater understanding of the way in which chimera cells link, allowing them to potentially experiment with other species.
"These results may help to better understand early human development and primate evolution and develop effective strategies to improve human chimerism in evolutionarily distant species," the authors wrote.
Reprogramming human fetal cells
The paper details how the scientists took human fetal cells and reprogrammed them to become stem cells. These were subsequently introduced into 132 embryos of long-tailed macaques, six days after fertilization.
The embryos developed for 19 days before they were terminated by the researchers. The researchers engineered the human stem cells to produce a fluorescent protein in order to indicate that the embryos did contain human cells.
Though all 132 embryos contained human cells seven days after fertilization, the study shows that the proportion of human cells fell as the days passed.
In recent times, scientists have experimented with human-pig and human-sheep chimera, as well as monkey-pig hybrids. Perhaps the most controversial of these was the monkey-pig hybrid experiment, conducted at the State Key Laboratory of Stem Cell and Reproductive Biology in Beijing, which saw the birth of two piglets with monkey cells, who subsequently died two weeks later.
Dr. Jun Wu, a co-author of the new paper, said he hoped the research would help develop "transplantable human tissues and organs in pigs to help overcome the shortages of donor organs worldwide." Critics argue that we should be focusing on other methods, such as additive manufacturing, to meet the same demand.