In 2002, an article was published about the establishment of the mammalian body plan. It was argued that it is already configured at the moment of conception and takes shape in the first 24 hours. That is, hours after the fertilization of the oocyte, the place where the head and feet will appear, as well as the belly and back, is defined.
A few years before it was thought that, in the first days of the embryo, its cells did not present characteristics that distinguished them from each other, until, during implantation, they were oriented towards different purposes and this determined their location in the organism.
The researchers decided to mark specific spots on the zygote, just after fertilization, and showed that they appear in predictable places on the embryo. It was even stated that the place where the sperm enters established where the first division occurs. And those first two cells are already oriented toward different functions.
Richard Gardner, an embryologist at the University of Oxford, UK, suggested that the top and bottom of the zygote align with the left and right sides of the blastocyst and can establish them.
On the other hand, Magdalena Zernicka-Goetz, from the Wellcome Institute/Cancer Research in the United Kingdom, discovered that the embryo maintains the pattern after implantation. He demonstrated this by labeling mouse blastocysts with fluorescent proteins and transferring them into female mice. In subsequent work, this researcher stained the first two cells of the embryo with a different color and followed the trail of its offspring until the blastocyst stage. The conclusion was that one of the cells gave rise to the internal cell mass (ICM) and the other to the placenta and supporting tissues. Zernicka-Goetz asserts: “There is a memory of the first division of our life.” That is, the first division of the zygote into two blastomeres influences the fate of each of them and, therefore, that of each tissue in the body.
Very recently, the same researcher published a work on the unequal contribution of the first two blastomeres in the human embryo. It shows that the majority of epiblastic cells (those that will give rise to the future human body) have their origin in one of the 2 blastomeres, the product of the first cell division. Although the first two cells may look the same, this study shows that most of the human body is formed from one of these cells. That is, the fate of the first two cells is different. “This is a big step forward,” says developmental biologist Ali Brivanlou of Rockefeller University in New York, adding: “It warms my heart to see that we are now getting to a point where we can ask about traits specific to humans in our own development rather than generalizing from model organisms.
As we noted at the beginning of this article, researchers thought for a long time that the cells resulting from the first divisions of the embryo of a mammal with fewer than 16 cells began their differentiation later. However, it was found that this was not the case, but that, from the first division, each cell had a different path. Zernicka-Goetz wanted to know if it happens the same way in human beings. “My dream was to understand how cells specify their fate and how the complexity of life begins to evolve,” she says. After searching different in vitro fertilization clinics, she managed to get one of them to provide her with 54 zygotes that had not completed the first division. Once completed, they marked one of the resulting blastomeres with a fluorescent protein, so they could follow the trajectory of these cells. After about four or five days, they verified that most of the cells that formed what would later become the fetus came from the blastomere that divided more rapidly. The offspring of the other cell gave rise to the yolk sac.
This research, published in the journal Cell, confirms what was already being studied and makes us verify that, from the fertilization of the oocyte, we are faced with a unicellular human organism that will later be bicellular, tricellular (since one of the first two cells becomes divides before), tetra cellular… It will not be a small group of cells, but a living human being that we must respect.
Therefore, from bioethics, we must point out that it is not appropriate to investigate human embryos, using and eliminating them to expand our knowledge about them. Certainly, knowing more about the development of the human embryo in its early stages is positive, but the end does not justify the means. We are happy to know better how the first division of the zygote occurs and how the progeny of each blastomere gives rise to different things, but it would be desirable that these scientific advances do not come at the cost of the life of small human embryos.
