Almost real blastocysts
The mouse embryo was first grown from a single somatic cell
Polina Loseva, N+1
Spanish scientists have come up with a new way to simulate the early development of mammals: they have grown structures resembling blastocysts from a single stem cell. They also demonstrated that it is possible to reprogram a somatic cell into a stem cell and grow a similar embryo from it. The newly formed embryos were implanted into the uterus of mice, and they even began to develop, although full-fledged tissues did not grow in them. The study is published in the journal Cell (Li et al., Generation of Blastocyst-like Structures from Mouse Embryonic and Adult Cell Cultures).
During the first week of development, the mammalian embryo goes through several stages. First – crushing, a lump of identical blastomere cells is formed. Then - compactification, blastomeres are compressed, forming a morula. Then a cavity forms in the middle of them and a blastocyst forms. This is a cellular ball, inside of which there is a cavity with a liquid and a lump of cells adjacent to it. The surface of the ball is called a trophoblast, it will subsequently give extra-germ shells (including the placenta). The inner cell mass turns partly into shells, and partly into the actual tissues of the embryo. After both cell layers have formed in the blastocyst, it is implanted into the uterine wall, where cells begin to specialize further and body axes (anterior-posterior and upper-lower) arise.
But since all these processes occur entirely in the womb, they are quite difficult to study in the laboratory. For this, as a rule, stem cell cultures are used, from which scientists are trying to assemble (sometimes literally, in layers) a model of the embryo at one stage or another of development. Recently, we talked about the fact that microfluidics was used for the first time to create an imitation of the post-implantation development of a mouse.
Nevertheless, all the methods of constructing artificial embryos known so far make them completely unviable. The experiment of Dutch scientists, who collected a blastoid (blastocyst-like structure) from two types of cells: embryonic stem cells and trophoblast stem cells, has progressed the furthest so far. These embryos even managed to be implanted into the uterus, but it is impossible to study the early stages of development before implantation on them.
The next step in this direction was taken by an international group of scientists led by Professor Juan Carlos Izpisúa Belmonte of the Catholic University of Murcia (Spain) and the Salk Institute (USA). They decided to build an embryo based on "advanced" pluripotent cells (expanded pluripotent stem cells). These cells are similar to blastomeres, that is, they can give rise not only to the embryo, but also to extra-embryonic tissues. To get them, the researchers took embryonic stem cells and cultured them under the influence of a cocktail of signaling substances, "restoring" their lost functions.
The researchers found that if you act on such pluripotent cells with substances that cause differentiation of both the trophoblast and the internal cell mass, then full-fledged blastoids grow out of them in about 15 percent of cases. But with embryonic stem cells, it was not possible to repeat the experiment – blastoids were not formed from them.
When blastomer-like cells were grown singly, they first died. Then the scientists took a line of cells resistant to puromycin and mixed it with other, unstable cells. They were grown together for a while, and then treated with puromycin and the "helper cells" died. Thus, with the help of temporary support of other cells, it was possible to grow an embryo from a single stem cell - however, in only 2.7 percent of cases.
The researchers confirmed that their blastoids are similar to ordinary blastocysts: in cellular composition, staining for different markers and gene expression. They even checked that the processes characteristic of mammalian development take place in blastoids – for example, the inactivation of the X chromosome. In the inner cell mass, both copies of it are active at the beginning of development, and in trophoblast cells, as a rule, the paternal X chromosome "falls silent" - and this is exactly what happened in most of the outer cells of the blastoid.
Then the authors of the work demonstrated that their blastoids are similar to blastocysts in vitro. For example, a culture of embryonic stem cells was isolated from them and shown that they can be embedded in the embryos of ordinary mice and give chimeric animals. In addition, blastoids can be cultured, and their cells begin to form different post-implantation tissues.
Blastoids were implanted into the uterus of mice, and about 7 percent of them were implanted. Artificial embryos caused the formation of decidual tissue in the uterus – the precursor of the placenta – and established close contact with it, sufficient for the dye from the mother's body to enter the embryo. Embryos continued to develop at least until 7-8 days, tissues gradually grew in them, but they lagged significantly behind healthy embryos in development.
Finally, the scientists obtained blastomer-like cells from mouse somatic cells. They reprogrammed them according to the same principle as for creating induced pluripotent cells, and the resulting culture turned out to be similar in properties to "improved" pluripotent cells. They also managed to grow blastoids from it and implant them into the uterus.
Despite the fact that the new method has not yet led to the creation of fully viable embryos, the authors of the work expect that one day they will be able to grow bioengineered embryos with its help. If this really works out, then we can talk about the third method of cloning. Two are now known: the transfer of the somatic nucleus into the egg and the transfer of induced pluripotent cells into someone else's embryo. If the third cloning method works, it will be the first "real" cloning, since the embryo will fully develop from the somatic cell of an adult organism.
Juan Belmonte is known for his bold work in the field of embryology and stem cells. So, in 2016, a group under his leadership for the first time tested reprogramming – "rejuvenation" of the body's cells in mice in vivo to combat aging. And in July 2019, he and his colleagues created a chimeric embryo of a human and a monkey – however, for this he had to go to China.
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