There are already dozens of laboratories that are trying to create the model that most closely resembles a human embryo. Jun Wu, a stem cell specialist, looked at a group of cells in a cavity surrounded by a ring of cells under a microscope. However, this was not an embryo, but a similar structure in which cells were missing and there were others that should not be there. In addition, these structures would die abruptly. Each model is somewhat different, depending on the group of researchers who make it and who aim to learn more about the biology of the beginning of embryonic development.
Nicolas Rivron, a developmental biologist at the Institute of Molecular Biotechnology of the Austrian Academy of Sciences in Vienna, says that this important stage “is shrouded in mystery.” Because these embryos are so small that they cannot be seen on an ultrasound scan, and because of technical and legal limitations and respect for established ethical standards, they cannot be studied after 14 days of fertilization.
The purpose of creating these models is to increase knowledge about the causes of infertility, to find out why around a third of embryos die in the first weeks of life, and to determine whether a drug is safe. However, when embryonic models are created that, due to their complexity, can achieve the necessary perfection for events as important as a heartbeat to occur, ethical aspects become even more important.
Embryonic models are “practically the hottest topic right now,” says Insoo Hyun, a bioethics consultant at the Broad Institute of MIT and Harvard in Cambridge, Massachusetts. For this reason, since the meeting of these models in February this year, various groups of scientists have created companies that research new therapeutic molecules, methods to improve fertility or test new drugs.
The fertilization of the oocyte by the sperm gives rise in the first week to the so-called blastocyst: a hollow sphere formed by about 100 cells, made up of three groups that will give rise to the embryo, the placenta and the yolk sac. This embryo will then implant itself in the endometrium and, after two weeks, the gastrula will form with the three cell layers that will then differentiate into the various organs. “The embryo is never static, it goes through enormous and dramatic changes” says Naomi Moris, a developmental biologist at the Francis Crick Institute in London. This is what scientists are trying to recreate in the laboratory, achieving, for the moment, some phases of the process. First, in 2014, they succeeded in getting human embryonic stem cells to form the precursors of the embryo and placenta. Then, in 2020, the models were already three-dimensional and managed to form a tube-like structure as formed in gastrulation. In 2021, two groups unveiled models similar to the human blastocyst, the stage at which they are usually transferred to the mother in the technique of artificial fertilisation; for this reason they are called blastoids. They were the first so-called complete or integrated models. “They are not perfect, but they are quite good,” says Marta Shahbazi, a stem cell and development biologist at the MRC Laboratory of Molecular Biology in Cambridge, UK. Another stem cell biologist named Miguel Esteban, who works at the biotechnology company BGI Cell in Shenzhen, China, together with his collaborators, made a model similar to the 8-cell embryo that forms on the third day of fertilisation. More recently, biologist Du Peng of Peking University in Beijing produced blastomeres that spontaneously give rise to blastoids. Blastomeres are a type of undifferentiated embryonic cell that results from the segmentation of the zygote.
When the first embryonic models were created, bioethics professionals tried to guide this work through criteria based on respect for human dignity. Specifically, the International Society for Stem Cell Research (ISSCR) developed guidelines in 2021, and various countries are considering their own guidelines, as well as legal aspects. Australia has the strictest standards. For this reason, when biochemist José Polo, director of a research group based at Monash University in Melbourne and the University of Adelaide, informed the relevant Australian regulatory authority (the National Health and Medical Research Council) that he had obtained blastoids, he was told to stop the work, because the authority wanted to clarify whether blastoids could be considered embryos according to current legislation, which defines the embryo as a biological entity with the capacity to develop until the stage in which the so-called primitive streak appears and is projected onto a body plan. The final answer was that, given the theoretical ability to develop the primitive streak, the same limits had to be applied to blastoids as to embryos. Therefore, the research group had to obtain a specific license to work with human embryos that prevents them from culturing blastoids to continue studying the gastrula phase and organogenesis. Polo says: “I think they made a mistake.”
In general, we can say that regulatory bodies are guided by the current regulations corresponding to human embryos when evaluating embryonic models, although each country is taking its own route. The key point is how each country defines the human embryo. This position goes beyond the scope of research and jumps into what is related to reproduction, regenerative medicine or the rights of people. In Spain, the embryo is defined based on fertilization, which rules out embryonic models, says Nienke de Graeff, a bioethicist at the Leiden University Medical Center (Netherlands). Other definitions value the embryonic model’s ability to transform into something else, which is why the ISSCR considers that models cannot be identified with embryos. Others have suggested reviewing the regulations on real embryos to include some of the embryonic models. A scientific corporation in the Netherlands has proposed a ban on the cultivation of models older than 28 days, and France is inclined to do the same. As for the United Kingdom, they have published so-called voluntary guidelines that set no limit on the time for culturing embryonic models. But both these guidelines and the ISSCR guidelines of 2021 prohibit the transfer of these embryonic models into the uterus.
As progress continues to be made in this area, the ISSCR reported last June that it had organized a task force to revise the previous guidelines in accordance with scientific progress on embryonic models since 2021. Of the approximately six groups that reported on post-implantation models, two received significant media coverage. One by Jacob Hanna, a stem cell biologist at the Weizmann Institute of Science in Rehovot, Israel, and another by Magdalena Zernicka-Goetz, a developmental biologist at the California Institute of Technology in Pasadena. Although they were called complete models, not everyone agreed. Rivron says, “They are not complete models.” One lacks trophoblast-like cells and the other lacks the organization that a real embryo has.
There is one criticism made by some researchers that I find very interesting because they question whether it is reasonable to seek the complete model. It is “a rather exquisite balancing act. We want to skate as close to the edge as possible, without falling off,” Hyun says. And that is because scientists try to build models as close to an embryo as possible, but not so close that they cannot be distinguished, so that they do not hinder their research. Precisely to avoid this, some alter their models so that they cannot give rise to an organism. For example, by inactivating the genes involved in the genesis of the heart and the brain. Others consider that if they achieve a model that reaches gastrulation without forming the primitive line, the ethical problem diminishes.
One model has sparked some optimism, both because of its resemblance to an embryo and its clinical applications. It was Mo Ebrahimkhani, a stem cell bioengineer at the University of Pittsburgh, Pennsylvania, who and his team noticed tiny islands of blood in their 3D models. When they analyzed them, they found that they contained blood cell progenitors, platelet-producing cells, haemoglobin-containing cells and macrophages (immune cells). He believes that these could be used to produce blood stem cells for transplants.
Some research groups have focused on organogenesis, which occurs three weeks into an embryo. To reduce the possibility of ethical limitations, they have developed models that only match part of the embryo. For example, one model shows the development of segments called somites, which give rise to the vertebrae. Another forms a model of the neural tube, which gives rise to the central nervous system. However, these models also have ethical problems because they include nerve cells.
Now the researchers are considering placing the embryonic models in an environment similar to the uterine endometrium and analyzing how they relate to each other. It has already been proven that, when the blastoids are placed on the cells that line the uterus, they penetrate and fuse well.
Liu Zhen, a developmental biologist at the Institute of Neuroscience of the Chinese Academy of Sciences in Shanghai, managed to develop monkey blastoids to the organogenesis stage in 2023. Liu transferred blastoids to eight monkeys: in three of them, a hormonal increase typical of the beginning of a pregnancy was observed and gestation sacs were created, but they did not go ahead. Something similar happens in other species. Current regulations do not prohibit this type of experiment, but Hyun is concerned that, if a living being is born from these animal models, research on human models will suffer a setback, because he says it is logical to mentally transfer to humans what was done in a monkey. And he comments: “In reality, it is not necessary to carry out the experiment with humans to have a fairly serious concern.”
The key question then is: When are we dealing with a real embryo? Currently, most scientists consider that current embryonic models are very different from a real embryo. So, the challenge is to establish “when an embryonic model would be considered equivalent to an embryo,” says Amander Clark, a developmental and stem cell biologist at the University of California in Los Angeles.
Since current ethical standards prohibit the implantation of an embryonic model in a human uterus, researchers are looking for other alternatives to find out if they are capable of giving rise to a living organism. The study of the cellular RNA profile is one of the tools used, but this leaves aside something as important in the embryo as the position of the cells. Others comment that it is important to know well the quality of the models to know their true usefulness. “It’s really crucial that we don’t waste time on bad models,” says Fredrik Lanner, a stem cell and developmental biologist at the Karolinska Institute in Stockholm.
One of the issues uncovered by these studies has been cell plasticity, which allows for unprecedented changes, as well as its organizational potential. Other new developments have also been discovered, such as the “hibernation” of embryos that have originated in winter and delay their development until the following spring.
“In the past there have been many debates, discussions, especially dramas,” along the lines of “my model is better than yours,” says Wu. But he believes that, although imperfect, all models are useful.
After reviewing the progress in this field of human embryonic models, it is worth considering a bioethical perspective that helps us clarify the path we should follow. First of all, we should know what we build these models from. If it is from embryonic stem cells we would be causing the death of a human embryo and that would not be correct. If it is from induced pluripotent stem cells, reprogramming adult cells, there would be no major objection, but we should know what we are developing because, the moment we find that a human embryo has been produced, it will deserve all our respect, in accordance with the dignity that the human being, the person, has.
As is evident throughout the article, scientists are trying to strike an impossible balance: understanding embryonic development without manipulating the embryo, but creating something that is increasingly identified with the human embryo, to the point that it can be indistinguishable from it. Here we see the contradiction between work that claims to respect ethics while at the same time approving the use of human embryos left over from assisted fertilisation clinics. It is certainly possible that great benefits for humanity could be obtained from their research, such as knowledge of fertility, the invention of new medicines or embryonic development, although at the moment we do not know the social value of these latest experiments. But, even if important advances were achieved, it is not in accordance with ethics to justify the means by the end.
