What is a human embryo model made from stem cells?

About a week after fertilization, a human embryo burrows into the wall of the uterus (implantation) and then starts building the beginnings of a body. Studying what happens in embryos at this stage gives clues to the causes of infertility, miscarriage and conditions present from birth. But the use of real human embryos in research is strictly limited, both by the view that the start of life is to be treated with care (ethics) and for medical reasons. Researchers can also study the embryos of animals such as mice, but mice and humans differ in how implantation happens, how the embryo develops and how important genes work, so animals are said to be a poor substitute.

Because human embryo models are made from human stem cells, no fertilized egg has to be destroyed. Since many can be made, the same experiment can be repeated again and again to confirm results, and researchers can follow over time what happens when a gene’s action is changed, or how cells signal to each other. Japan’s guidelines, too, allow embryo models to be made only for basic research on how the human body forms or for developing new ways to diagnose, prevent or treat disease, and new medicines.

What they copy is the embryo before implantation (the blastocyst). A blastocyst is the embryo at day 5 to 7 after fertilization. It is a ball in which a thin layer of cells wraps a hollow, with a clump of cells inside that will later become the body. A blastocyst gives rise not only to the body but also to parts outside it, such as the placenta.

In 2018, researchers showed that when two kinds of mouse stem cells are mixed and grown (ES cells, which give rise to the body, and stem cells that give rise to the placenta), the cells come together into clumps resembling blastocysts. These clumps were named “blastoids.”

For humans, two research groups each made them in 2021. One group made blastoids from human ES and iPS cells that resemble blastocysts in shape, size, cell number and mix of cell types. The other reprogrammed human body cells into blastocyst-like clumps.

Late the same year, a method was shown that made human blastoids form in more than 70% of attempts. The beginnings of all three kinds of cells that make up a blastocyst were also present. Placed on cells from the inner lining of the uterus wall that had been prepared with hormones, they attached facing a set direction, as embryos do at implantation. Besides blastoids, there are other embryo models, such as gastruloids, that copy part of a stage after implantation.

The biggest difference is how they come about. A real embryo grows from a fertilized egg formed when a sperm meets an egg. A human embryo model is made from stem cells, using no fertilized egg, sperm or egg. ES cells, though, were originally made by taking cells from a blastocyst. Research uses ES cells that have already been made, so making an embryo model does not mean using a fertilized egg each time.

Their structure and workings are not the same either. Today’s human embryo models are considered groups of cells that show only some features of blastocysts or of embryos from around implantation onward (shape, structure, gene activity, cell types and so on), and they cannot be called the same as fertilized embryos. The international society of stem cell researchers also advises against the term “synthetic embryo”: in its view, embryo models that also contain cells such as those that give rise to the placenta (integrated embryo models) are neither synthetic nor embryos.

They are also different from “organoids,” clumps that resemble organs. An organoid copies a particular organ, such as the liver or brain. A human embryo model copies the way an embryo itself develops, with many parts linking up as it grows.

Today’s embryo models are considered unable to become a person. In a 2018 study, when mouse blastoids were placed in a mouse uterus, the inner lining of the uterus wall changed as it does at implantation. But they did not grow into embryos. As of 2024, there were no reports of offspring born from an embryo model, even in mice or other animals.

Still, the stages that can be copied are moving forward bit by bit. There is a report of mouse ES cells reproducing a stage equal to a mouse embryo about 8.5 days after fertilization (when the tube that becomes the brain and spinal cord has formed and a heartbeat can be seen).

What will happen next is not known. Embryo models may someday come closer to fertilized embryos and become able to be born as a person, but some researchers think this is impossible in principle, and it cannot be judged at this point. Noting that cloning took more than 30 years to go from a cloned frog (1962) to a mammal, a sheep (1996), a Japanese government panel of experts expects many difficulties before embryo models could reach the point of giving birth to an animal. And it says that if embryo models reach the stage where a mammal is born from one, Japan will consider treating them like fertilized embryos as the “budding of human life,” or regulating them like the law that prevents human cloning.

Research on human embryos grown from fertilized eggs has long had rules. Under principles Japan set out in 2004, fertilized embryos are to be treated with special respect as the “budding of human life,” and even in research they are not returned to a uterus and may be handled only until a single line (the primitive streak) appears in the embryo and organs start to form. This is set at 14 days at most. Because a human embryo model is not a fertilized embryo, these principles did not directly apply, and there were no clear rules on how to handle one.

In November 2024, a Japanese government panel of experts set out its view on how to handle human embryo models. Its view: today’s human embryo models are not treated the same as fertilized embryos, but because they might come closer to them, and because improper research such as placing them in a uterus cannot be ruled out, some rules are needed. In response, Japan’s education and health ministries revised the guidelines for research using ES cells, iPS cells and other cells, which took effect on April 1, 2026.

The main rules are these. Placing an embryo model in a human or animal uterus, and making an individual from it by other means such as an artificial uterus, are banned. Research is limited to basic research that helps explain how the human body forms or develop new ways to diagnose and treat disease and new medicines. Before starting, a study must be reviewed by the ethics committee of the institution doing it and reported to the minister of education and science. The plan must state how long the embryo models will be grown and why that is the minimum needed for its goal. Researchers are to share information about their research and work to spread public understanding.

Unlike fertilized embryos, there is no fixed limit on the number of days embryo models may be grown. A fixed limit was judged not needed for now, partly because today’s models look like clumps of cells, not like a fetus, and growing them for a long time is still hard with today’s technology. But because growing them until they look like a fetus at about 8 weeks after fertilization could cause public unease, the matter is to be reconsidered if needed as research advances.

Also, in July 2026, Japan passed a new law regulating genome-edited embryos and similar material. To match it, a draft plan was presented at a government panel meeting in October 2026: removing the rules on germ cells made from stem cells (sperm and eggs and the cells that lead to them) from the guidelines for iPS cells and other cells, so that they cover only research that makes human embryo models (still only a draft).

As of 2024, how embryo models are treated in national regulation was still unsettled in other countries too. One example of rules that cross borders is the guidelines of the international society of stem cell researchers (the International Society for Stem Cell Research, ISSCR). As a society’s guidelines, they are not law.

In its 2021 update, the guidelines split embryo models into two kinds. One kind contains both cells that become the body and cells that become parts outside the body, such as the placenta (such as blastoids; “integrated embryo models”); it was placed among research done only after specialized review. The other kind lacks the cells for parts outside the body and copies only part of an embryo (such as gastruloids; “non-integrated embryo models”); it was placed among research that is reported but not usually given specialized review. For both, placing a human embryo model in a human or animal uterus is not allowed.

The August 2025 update (which changed only the embryo-model parts) dropped this two-way split and now calls them all “stem cell-based embryo models (SCBEMs).” Every embryo model with a three-dimensional structure should have a clear scientific reason, a set end point and suitable review. The guidelines also say growing one with no end point is never appropriate. On top of banning transfer to a uterus, they newly ban growing embryo models, without a uterus, in an artificial device to the point where they could live on their own.