Scientists wanting to do experiments on humans that would fail to meet ethical standards think they've found a loophole - do them on human-monkey hybrids instead.
Researchers in the US and China injected 25 human stem cells into 132 six-day-old macaque embryos. Ten days later, most of them - 103 - were still developing, using both the monkey and human cells.
Carrying out experiments on human embryos has posed ethical dilemmas. After 19 days, developing babies are still blastocysts - just a bundle of cells - but still have the potential to become people.
"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," said Juan Carlos Izpisua Belmonte of the Salk Institute for Biological Sciences, who in 2018 was named one of the most influential people in healthcare by Time magazine.
In his study, conducted with scientists in China, survival of the hybrids - or chimera - began declining after day 10, and 19 days after the 132 hybrids were created, only three were still alive.
"Historically, the generation of human-animal chimeras has suffered from low efficiency and integration of human cells into the host species," said Dr Izpisua Belmonte.
"Generation of a chimera between human and non-human primate, a species more closely related to humans along the evolutionary timeline than all previously used species, will allow us to gain better insight into whether there are evolutionarily imposed barriers to chimera generation and if there are any means by which we can overcome them."
Stem cells have the ability to turn into any kind of cell a growing creature needs.
The goal is to create lasting chimera which can be used to study early human development, model diseases, find new medicines and generate transplantable cells, tissue and even entire organs.
The news of the experiments was reported in 2019 by a Spanish newspaper, though Dr Izpisua Belmonte and his team kept quiet at the time. Friday's publication of the findings in journal Cell is the first time the research has been publicly acknowledged.
Dr Izpisua Belmonte was also behind the first animal-human hybrids, in 2017. Those were pigs, and the human stem cells were introduced during gestation. They ended up contributing to the growing embryos' muscles and organs, but not the brain - and not very much, with just one human cell developing for every pig 100,000 cells.
"One concern with the creation of human/animal chimeras is that the chimera will be too human - researchers don't want human cells to contribute to the formation of the brain," Dr Izpisua Belmonte said at the time.
Those embryos were destroyed after spending four weeks inside surrogate mothers. The monkey-human blastocysts were grown outside of the womb.
In an accompanying editorial, experts said creating chimera wouldn't solve every ethical dilemma faced by researchers, and potentially raise new ones.
"To the authors' credit, they recognised their work has important ethical and social implications and, as they describe in detail, followed existing rules, guidance, and oversight, with additional input from external bioethics experts and panels," wrote bioscience law expert Henry Greely of Stanford University and Nita Farahany, a Duke University expert in the legal ramifications of emerging technology.
They said in past research involving injecting cells of one species into another, the cells' spread has been limited to particular regions or organs. This latest study takes it to the next level however.
"With the introduction of [human embryonic stem] cells into blastocysts, the human cells may integrate throughout the embryo or differentiate into more or less morally sensitive cell types. But that is true only if the embryo is allowed to grow and if the human cells grow with it. The embryos here were not transferred into a uterus, and thus could not lead to living chimeric animals or even fetuses...
"It remains altogether unclear whether such results are, or ever will be, possible. And yet, those future experiments are now at least plausible. We must begin to think about that possibility."