Embryos containing cells from different species are called “chimera embryos”. This sounds like the crazy idea of evil scientists; but in fact, many scientists hope that this technology can be used to solve an important practical problem-organ transplantation.
There are a large number of patients waiting for organ transplant operations. In China alone, there is a demand for as many as 300,000 organ transplants every year. However, the existing organ donation is far from meeting this demand. Scientists are exploring alternative methods, such as 3D printing, growing organoids in vitro, directly transplanting organs from other animals, or allowing chimeric animals to “grow” human organs. If a pig can grow a human heart, then the rejection of this heart when it is used for transplantation may be much less than when the pig heart is used directly.
Mouse embryos grew out of rat hearts
From left to right: mouse, mouse after chimerism with rat, rat after chimerism with mouse, rat.
In rodents, this theory has become a reality. In 2010, Keikai Nakauchi’s team at the University of Tokyo created a rat-mouse chimera, allowing mice to grow rat pancreas, and these pancreas can secrete insulin.
But to produce organs that can be used in humans, this technology has just begun.
In the past few years, foreign research teams have tried to create human-pig embryos, human-sheep embryos, etc., but the results were not satisfactory. For example, in the human-pig embryo study in 2017, only about 12% of chimeric embryos were able to continue to develop, and human cells accounted for less than 0.001% of all cells. This may be because pigs and sheep are far apart from people.
To understand what happens to human cells in chimeric embryos and how to improve the success rate of chimerism, researchers speculate that we may be able to learn from human-primate chimeric embryos.
Making human-monkey embryos
The production of human-monkey embryos, in simple terms, is the injection of human pluripotent stem cells into cynomolgus monkey embryos cultured in vitro. In theory, pluripotent stem cells have the same differentiation potential as embryonic stem cells and can differentiate into all tissues and organs.
Crab-eating monkeys as experimental animals
Pictures of different stages of in vitro culture of chimeric embryos
In this experiment, the researchers injected 25 human pluripotent stem cells into each cynomolgus monkey embryo that was fertilized in vitro and developed to the sixth day. One day after the injection, human-derived cells appeared in all 132 monkey embryos. By the 10th day of embryo development, there are still 111 chimeric embryos that can grow normally. Over time, the survival rate of chimera embryos gradually decreased, and dropped sharply on the 15th day. On the 19th day, only 3 chimeric embryos survived, which is also the limit of the current technology for culturing cynomolgus monkey embryos in vitro. On the 20th day, all the embryos died and the embryo culture in vitro ended.
In addition to the increased survival rate, human cells accounted for 2% to 7% of human-monkey embryos, and appeared in both epiderm and hypoderm. Epiderm and hypoderm are the two structures of blastocysts (a stage of the early embryo); as the embryo develops, these two parts of cells will further develop into ectoderm, mesoderm, endoderm, etc., and finally differentiate to produce all of the animal body organ.
In other words, if someone continues to cultivate or even transplant human-monkey embryos, so that they will eventually be born, then there may be human-derived cells in various organs in the chimera monkey.
In this study, human-monkey chimera embryos were only cultured until the 20th day, avoiding greater ethical issues. Tan Tao, the first author of the paper and a professor of biology at Kunming University of Science and Technology, said that they have no plans to transfer embryos. As for whether to continue culturing embryos, follow-up extensive discussions and ethical proofs are needed.
National regulations vary
Making embryos that are “half human, half animal” is a great ethical challenge. In order to standardize this type of experiment, many countries have formulated corresponding specifications, but the current specifications vary from country to country.
According to the regulations of the National Institutes of Health, experiments on human-primate chimera embryos cannot get their funding support; in 2015, they announced the suspension of funding research on human-non-primate chimeric embryos.
In Japan, on the contrary, they deregulated in 2019. Originally, animal embryos containing human cells could not be cultured for more than 14 days. However, with the introduction of new regulations, under scientifically reasonable and necessary conditions, chimeric embryos can not only continue to be cultivated after 14 days, but also allowed to be transplanted into surrogate animals, but they cannot be combined with human germ cells in any way.
When chimera embryos encounter primates, the problem becomes more complicated—because of the closer kinship, the chimerism of human monkey embryos is more likely to succeed; but also because of the closer kinship, primates may With more advanced wisdom and emotions, experiments involving them always take into account more ethical factors.
Tan Tao said that their research was conducted in accordance with national regulations. In my country, experiments with chimeric embryos are not prohibited, but ethical approval is required. This research is in cooperation with the Salk Institute for Biological Research in the United States. In the early, mid and after the research, China and the United States have conducted several rounds of consultation and review on the scientificity and ethics of the research.
Ethical standards need to be refined
If we take “whether the chimera embryo continues to develop” as the standard, this time the human-monkey embryo has not stepped on the ethical red line. These embryos are only cultured in vitro since fertilization, and only until the 20th day. One of the corresponding authors, Juan Carlos Espizua Belmonte of the Salk Institute for Biological Research in the United States, stated that their goal was not to create any monsters, and they did not do so.
The English chimera of chimera comes from ancient Greece. It is a fire-breathing monster with the upper body like a lion, the middle like a goat, and the lower body like a poisonous snake.
Another corresponding author, Niu Yuyu, a professor of biology at Kunming University of Science and Technology, said: “Our entire experiment was performed in vitro, without entering the body. In vitro culture is like culturing cells, and no individuals are produced. It is important to our work. Said that this is an important ethical boundary.”
Juan Carlos Espizua Belmonte is also the head of human-pig embryos in 2017.
But what about the future? If chimeras are to be used to make mature, transplantable organs, it is likely that the embryos of the chimeras will continue to develop and even be born. As the “Cell” review article said, biomedicine is increasingly facing a dilemma: we need better models to understand human physiology and diseases, but in order to obtain a model close to the human body, it is often a challenge to ethics.
If you want chimera organisms to produce transplantable organs, an ideal situation is that scientists can have a way to strictly control the position of human cells in the chimera, so that they appear only where they are most necessary-the deadlock of ethics and technology , Perhaps can be solved by the newer technology.
However, some scientists questioned this research. Developmental biologist Alfonso Martínez Arias of the University of Cambridge believes that the data in this study is not convincing enough. The survival rate of embryos declines rapidly after 15 days, indicating that the condition of these chimeric embryos is not good. . In addition, if you want to cultivate organs that can be used for transplantation, you can use in vitro organoid culture, which can avoid the ethical problems of experimental animals; and even if chimera animals are used, animals such as pigs and cattle are more promising than primates. , And will not challenge ethical issues.
Niu Yuyu also believes that from an ethical point of view, the future that can be used to produce organs is likely to be human-nonprimate chimeras; the current experiment is more about understanding what happens in human cells during the mosaic process. To this end, they evaluated the developmental process of human-monkey chimera embryos in detail, and identified those gene expression and signaling pathways that were altered in the chimeric embryos. This information is helpful for the research of human-nonprimate chimera embryos and the research of regenerative medicine. Tan Tao said that the more important issue in the follow-up is to study in detail how to improve the efficiency of mosaic.