DUBAI, United Arab Emirates (CNN) — The way a human cell divides and reproduces to become the human body of more than 30 trillion cells remains one of science’s greatest mysteries.

From the moment the sperm unites with the egg, the human embryo begins to develop according to complex and poorly understood processes. Much of what is known about embryonic development comes from animals such as mice, rabbits, chickens, and frogs, and research on human embryos is subject to strict oversight and regulation in most countries.

But animal studies have little to reveal to researchers. What happens during human fetal development, especially during the critical first month, is largely unknown.

“The basic narrative is in the first month, and the remaining eight months of pregnancy are a lot of growth, basically,” said Jacob Hanna, a professor of stem cell biology and embryology at the Weizmann Institute of Science in Israel. “But that first month is still a black box.”

The ability to unlock the secrets of this black box could open up a world of biomedical possibilities, allowing scientists to unravel an even more mysterious episode of fetal development. If scientists can discover the secret of the first month, it could lead to a better understanding of miscarriages, birth defects and the side effects of drugs taken during pregnancy. Some researchers believe they have found a way to do this, bypassing the need for eggs or sperm.

Using advances in stem cells, labs around the world are working to create embryo-like tissue that can act like an embryo but cannot develop into an embryo.

Recent achievements in this field, the culmination of years of painstaking laboratory work, have given hope and some concern. The ethical status of these models has raised urgent questions about the extent to which they should be treated like human embryos, and whether they can be misused.

What exactly did the scientists achieve?

Embryonic tissue is a cluster of cells grown in a lab, smaller than a grain of rice and represents the earliest stages of human development, before any organs form. She has no beating heart or mind.

The more advanced models, released in September by an Israeli team of which Hanna was a part, show all the cell types necessary for fetal development — the placenta, yolk sac, chorionic sac (outer membrane) and other tissues. To develop.

The structures were left to form for eight days, reaching a stage of development similar to day 14 of a human embryo in the embryo, the critical moment when normal embryos acquire the internal structures that help them move on to the next stage: the development of body organ progenitors.

Hanna says they are the most accurate models ever made, and unlike models made by other groups, no genetic modification has been done to activate the genes needed to create different types of cells, only chemical blocks have been used. .

“It’s not just putting the cells together, they’re there, but when you look at the architecture, you start to see the finer details,” he noted.

How to grow embryos in the lab?

Hanna’s team did not use fertilized eggs. They started with human cells called pluripotent stem cells, which are programmed into multiple cell types and are widely used in biomedical research. Some are derived from adult human skin cells.

The team reprogrammed these cells to what they call a “naïve stage,” which corresponds to day seven of normal human embryo development, when it implants itself in the uterus. These “naïve” cells were divided into three groups.

A group that was supposed to become embryos was left untouched. The other two groups are “stimulated” using certain chemicals that turn on specific genes and make the tissues needed to support the fetus, such as the placenta. After two days, the three groups were brought together, Hanna said.

Hanna explained, “In the first three days, you don’t see much, you see a group of cells growing. But on the fourth day, you start to see… Now you know there’s tissue, you know. To tell where the embryo will develop… and where the yolk will be.” can.” .

At the equivalent day 7 stage, the artificial human embryo samples were clumps of about 120 cells that together were about 0.01 mm in diameter. On day 14, it contained about 2,500 cells and measured half a millimeter.

Hanna and his team noted that these models faithfully reflect the way an early embryo acquires all the structures it needs to become an embryo. The internal system matched the images in uterus atlases produced in the 1960s, and when they used secretions from the cells for a commercial pregnancy test, the result was again positive.

However, only 1% of aggregated cells continued to self-organize to form a nucleus-like structure. A higher percentage would be needed to turn embryo samples into a useful tool for scientists, Hanna said, which is possible but will take years to perfect.

He continued: “I think we can learn a lot from embryonic models based on stem cells. Currently there are some drawbacks. Their production is very inefficient, so obviously efficiency needs to be increased.” We can learn from these models.” Rog-Khan is a team leader and chair of the Public Engagement Group at the Abraham Institute, which focuses on life sciences research.

An ethical alternative?

Many scientists argue that human embryo samples, especially if they can be produced in large numbers, offer an ethical alternative to research on the rare and valuable human embryos that are typically obtained as a byproduct of the IVF process.

“Because of the stem cell base, we can measure everything (human) embryos. It changes the types of experiments we can do,” says Naomi Morris, team leader at the Francis Crick Institute’s Developmental Modeling Laboratory in London. “And we can answer the questions.”