Tokyo (AFP)

Scientists have generated early-stage human embryonic structures, which could help lift the veil of the "black box" of early stages of human development and advance research into miscarriages and physical malformations in fetuses.

Two teams of researchers, whose work is published Wednesday in the journal Nature, have found different methods of making a blastocyst - the first stage of the embryo, in the laboratory - about five days after the egg is fertilized by a sperm.

This sphere of about 200 cells already has a complex cellular structure (a layer of outer cells - the future placenta - surrounding a fluid-filled cavity that contains a mass of embryonic cells).

These models called "blastoids", which cannot continue their development like natural embryos, could help to understand events which occur at the beginning of embryonic development such as spontaneous termination of pregnancy or malformations favored by certain drugs or pollutants.

"Studying human development is very difficult, especially at this stage of development, it is largely a black box," Jun Wu, a researcher at the University of Texas, said at a press conference. directed one of the two studies.

This work comes at a time when new ethical recommendations concerning research on embryos are being drawn up.

Currently, research into the early stages of development depends on donated embryos conceived through IVF treatments.

But these donations are few in number and subject to many restrictions.

Being able to produce these embryo models "on a large scale" will "revolutionize our understanding of the early stages of human development," said José Polo, professor at Monash University (Australia), who led the second study.

- Cells that "talk to each other" -

Until now, the creation of blastocysts in the laboratory had only been carried out in animals: in 2018, Dutch researchers succeeded in producing them from mouse stem cells.

To do the same in humans, the two teams used different methods.

Jun Wu's used stem cells derived from human embryos and induced pluripotent cells (stem cells produced in the laboratory from adult cells).

José Polo's team started with adult skin cells.

Both achieved the same result: the cells gradually organized themselves to reproduce the three structures that make up human blastocysts.

"What surprised us is that when we put them together, they self-organize, they seem to talk to each other, in a certain way ... then they come together", detailed José Polo.

However, the blastoids obtained by the two teams differ from natural blastocysts: they contain cells of undetermined type and do not include certain elements that come specifically from the interaction between the egg and the sperm.

In addition, only 20% of the tests on average worked.

- Ethics debate -

The researchers insist that these laboratory models are not embryos per se and are not capable of further development.

As a precaution, they nevertheless ended the experiment four days after culturing the blastoids, the equivalent of about 10 days of development in the case of normal fertilization.

Research on the human embryo is limited to 14 days after fertilization.

While these studies represent "an exciting step forward", more work is needed to "better control the process" and improve the success rate, says Peter Rugg-Gunn, of the Babraham Institute (Cambridge, UK). , cited by the UK Science Media Center (SMC).

In addition, these embryo models "can help formulate hypotheses which will have to be validated on human embryos", and therefore will not replace research on embryos resulting from donations, believes Teresa Rayon, biologist at the Francis Crick Institute (London ).

They also raise ethical questions, because some "could see in research on human blastoids a step towards the manufacture of human embryos", observe Yi Zheng and Jianping Fu, specialists in biomedical engineering at the University of Michigan (States -Unis), in an article also published in Nature.

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