First published May 28, 2021.
In April 2021 a team of scientists in the United States and China reported that they had grown macaque monkey embryos containing human cells in the lab for up to 19 days. The paper, published in the journal Cell, did not create a “half-human, half-monkey” creature, and no embryo was ever placed in a womb. What it did show was that human stem cells could survive and integrate inside an early primate embryo far better than they had in earlier experiments with pigs, which made it an important, and controversial, step in research aimed at growing human organs and understanding early development.
What the researchers actually did
The work was led by Weizhi Ji of Kunming University of Science and Technology in Yunnan, China, with Juan Carlos Izpisua Belmonte of the Salk Institute for Biological Studies in La Jolla, California, as senior author. Ji’s group had developed a method, published the year before, for keeping monkey embryos alive outside the body for longer than had previously been possible.
Six days after the monkey embryos were created, the team injected each one with 25 human extended pluripotent stem cells, a type of cell that can in principle become almost any tissue. The results, as described in the press materials released with the paper:
- After one day, human cells were detectable in 132 embryos.
- After ten days, 103 of these chimeric embryos were still developing.
- By day 19, only three were still alive, and the experiment ended there.
The proportion of human cells stayed relatively high in the embryos that kept growing. That was the key finding: in earlier attempts with other species, human cells had barely taken hold.
Chimera, not hybrid
Headlines in 2021 often called these “hybrid” embryos, but the correct term is chimera. In a true hybrid, every cell carries genetic material from both species, as with a mule. In a chimera, each individual cell is either human or monkey; the two kinds of cell simply live side by side in one embryo. The name comes from the fire-breathing monster of Greek myth that was part lion, part goat and part serpent.
The distinction matters because it shapes what the research can and cannot do. Nobody was mixing human and monkey DNA inside a cell, and the embryos were nowhere near the stage where organs, let alone a nervous system, form.
Why scientists want human cells in animal embryos
The long-term hope behind this field is to grow human organs inside animals such as pigs, whose organs are a similar size to ours. In the United States alone, more than 100,000 people are on the national transplant waiting list, and many die before a match is found. If a pig embryo could be engineered to lack, say, a pancreas, and human stem cells filled that gap, the animal might one day grow a pancreas made mostly of human cells.
That goal is still distant. The Salk group had tried human cells in pig embryos in a study published in 2017 and found that only a tiny fraction of the cells were human. In 2019 a team in China reported piglets born with a small share of monkey cells; reportedly, none survived beyond a week. Monkeys are much closer relatives to humans, so the macaque work was designed to understand why human cells struggle in other species, not to grow organs in monkeys.
Izpisua Belmonte said the team was trying to understand how cells from different organisms communicate, adding: “Our goal is not to generate any new organism, any monster.” The researchers also listed other uses, including studying very early human development, modelling disease and screening drugs.
The ethical debate
The paper reignited an old argument. Supporters, such as Case Western Reserve and Harvard bioethicist Insoo Hyun, told NPR the study itself was not ethically problematic and could lead to fewer experiments on whole animals. The researchers said ethical reviews had been carried out at their institutions and with outside bioethicists before the work began.
Critics asked broader questions. Kirstin Matthews, a fellow for science and technology at Rice University’s Baker Institute, put it bluntly to NPR: “My first question is: Why?” She worried that science was pressing ahead without a proper public conversation about limits. Stanford bioethicist Hank Greely co-wrote a commentary in the same issue of Cell, accepting that this experiment was done ethically while warning about where the field could lead.
The main concerns usually raised are:
- Brain development: could human cells ever contribute to an animal’s brain in a way that changes its cognitive abilities?
- Reproductive cells: could human cells end up in an animal’s eggs or sperm?
- Animal welfare: primates are intelligent, social animals, and their use in research is tightly limited for good reason.
The rules around this research
Most scientific guidelines have long applied a “14-day rule” to human embryos grown in the lab. These monkey embryos were not human embryos, so the rule did not strictly apply, which is part of why the 19-day experiment drew attention. A month after the paper, in May 2021, the International Society for Stem Cell Research updated its guidelines, relaxing the strict 14-day limit for some studies while calling for careful oversight and stating that chimeric embryos should not be transferred to a uterus to develop further. Countries differ: Japan, for instance, approved some human-animal embryo research in 2019 under strict conditions.
Common myths about the study
- “Scientists created a human-monkey creature.” No. The embryos were tiny clusters of cells in a dish, ended by day 19.
- “Lab-grown human organs are coming soon.” Not from this work. It was basic research into how cells interact, years away from any transplant use.
- “It was done in secret.” The study was peer reviewed, published openly and accompanied by an ethics commentary.
The real story is more modest and more interesting: a carefully limited experiment that showed human cells can thrive in a primate embryo for a short time, and a debate about how far such research should go that is far from settled.
Sources: NPR (15 Apr 2021); Cell Press release via EurekAlert; Tan et al., Cell (2021); Wikipedia: Human–animal hybrid; earlier coverage by Futurism.
COMMENTS