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In biology, human development, from the fertilization of an egg to the formation of the complete baby, has a large area called ‘black box’because we don’t know what happens there. We have a lot of data about what happens in the first days after fertilization and also during the last months thanks to ultrasounds. Worse, there is an area between the second and fourth weeks of development that is terra incognita. The ethics. It is without a doubt the great wall of developmental biology right now, since to see what happens to an embryo in these weeks we would have to have it in a culture dish for more than 14 days. But this is something that ethics does not allow, since after those days the embryo must be inside a uterus or destroyed. The change. Now science is working to find exactly how to see the embryo in this time window, and the Chinese Academy of Sciences and the Pompeu Fabra University of Barcelona achieved it. Specifically, it has achieved cultivate macaque embryoids which are embryo models derived from stem cells until day 25. In this way, processes that until now were hidden have emerged. A team of researchers from the Chinese Academy of Sciences and the Pompeu Fabra University of Barcelona has managed to cultivate macaque embryoids (embryo models derived from stem cells) until day 25, revealing processes that until now remained hidden. And this has already given us data about how our body is formed. Gastrulation. Lewis Wolpert, a famous biologistused to say that “the most important moment in your life is not birth, nor marriage, nor death, but gastrulation.” And he was right. To understand this importance, you must know that during gastrulation the embryo stops being a simple sphere of equal cells and transforms into a complex structure with three different layers that will give rise to all the organs of the body. But it is also essential to be able to define the axes of the body, that is, knowing where the head will be, the tail and what is right and left. Something that until now was impossible to see because previous models of primate embryos ended up collapsing after day 17. The solution. The research has therefore used a new system of 3D suspension culture which has enabled macaque embryonic stem cells to self-organize and develop complex structures until day 25 outside the uterus. What they have seen. What researchers have observed in these “embryoids” is fascinating because of its similarity to natural embryos. As detailed in the paper from Nature, these models have recapitulated key events of the late gastrulation. Among this, the formation of the central nervous system stands out, the precursor of the digestive system, the first blood cells or even those that in the future will give rise to eggs and sperm. The most surprising thing is that the transcriptomic analysis (the study of which genes are active cell by cell) revealed that differentiation trajectories were similar to those found in natural monkey embryos during this stage. At last. This means that we have, for the first time, a reliable simulator to study human development. Since we share a large part of our biology with macaques, this model allows us to investigate the causes of abortions early spontaneous and congenital malformations without crossing the ethical red lines of experimentation with human embryos. They are not real embryos. This is something fundamental for the limits that ethics imposes on us that we mentioned at the beginning. What has been cultured in this case are not real embryos, but models derived from stem cells where neither eggs nor sperm were involved. This has the aspect that it can never become a viable living being if it is implanted in a uterus and its usefulness is limited only to laboratories to analyze how we are developing and revealing critical points where an embryo can be aborted. In Xataka | There are more and more men obsessed with one thing: donating their semen