It is a discovery that sheds new light on the very earliest times of human life. Researchers have just highlighted the mechanisms by which our first cells combine into a whole: the embryo. This is “the first study on the mechanics of morphogenesis of the human embryo”, summarizes this work published Wednesday in the journal Nature.
We place ourselves a few days after fertilization. The meeting between the sperm and the egg has already given rise to a stem cell, which has divided into around ten other cells. Then comes the moment when these cells come together and clump together to form a single whole. The embryo, in its earliest stage, is there. It is only then that the cells will differentiate to gradually reveal organs and then, little by little, a human form.
Contraction of cells
This very first step, called “compaction”, is therefore crucial. It is the subject of this study, mainly carried out by researcher Julie Firmin and involving the CNRS, Inserm and the Institut Curie. Its conclusions call into question the way we have seen the formation of an embryo for several decades. The main mechanism was considered to be that by which cells stick to each other, via the adhesion of their walls.
However, according to this study, this factor only plays a secondary role. Most crucial is each cell’s ability to contract, a mechanism by which they pull toward each other. “You have to imagine a circle of people holding hands” and which gradually closes, explains researcher Jean-Léon Maître, who led the study.
To reach this conclusion, the researchers examined the cells of several embryos unused during in vitro fertilization, and frozen at different stages between three and five days. The more advanced their stage, the stronger their cells are able to contract. No change, however, for the degree of adhesion of the walls, this remaining stable. The researchers conclude that it is the first mechanism, and not the second, which plays a central role in the rapprochement of cells and the formation of the embryo.
“Not a surprise”
“What makes the cells stick to each other is not the quantity of glue, but these contraction efforts,” insists Jean-Léon Maître. “It’s not a surprise at all,” he says. Over the last twenty years, studies have successively shown similar mechanisms in flies, then mammals such as mice. However, if all these animals and humans have in common the predominance of the contraction mechanism, the details vary: it is not distributed, for example, in the same way within the cell.
It is therefore the human embryo that the study published on Wednesday allows us to better understand, without having to immediately expect very concrete consequences. We can, of course, imagine that one day, thanks to this knowledge, the formation of embryos intended for in vitro fertilization will be facilitated. But, currently, we choose anyway to implant embryos that have successfully passed this training stage.
Major breakthrough
If this study marks a major advance, it is above all in the knowledge of the very beginning of human life, a field of research that has been gaining momentum in recent years. We can also include the recent manufacturing in the laboratory, by several research teams, of structures close to the embryo. Sometimes referred to as “synthetic embryos” even if this term is controversial, these structures should make it possible to study how cells and then organs differentiate during the first weeks of gestation.
