Blastogenesis

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Eight-cell stage embryo

The blastogenesis designates the first section of prenatal development, or more precisely the period of the fertilization to implantation in the uterus (in humans 1.-2. Embryonic). This period is primarily characterized by the development of the blastocyst . This article describes human blastogenesis. For the most part, it can be transferred to other placenta animals, but only applies to a limited extent when it comes to exact times.

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After fertilization of the oocyte ( egg cell ) by the sperm , the resulting zygote becomes inaccessible to other sperm. The zona pellucida becomes compact. After the fusion of the cell nuclei of the egg and sperm, the zygote begins to divide and reaches the 2-cell stage after about 30 hours, the 4-cell stage after 40 and the 16-cell stage after about 3 days. Other intermediate stages can also occur, such as the 3-, 7- or 9-cell stage. From 16–32 cells, which are also called blastomeres , one speaks of the morula due to the mulberry-like appearance of the blastomeric accumulation . Despite these cell divisions , the zygote's volume does not increase. It remains limited by the compact zona pellucida and only becomes larger during hatching (separation from the zona pellucida). Even before hatching, the blastocyst cavity forms as a result of the inflowing fluid. The blastocyst cavity is a cell-free germinal vesicle that pushes the morula to the side. From this point on, one can speak of a blastocyst . The first differentiation takes place between external cells ( trophoblast ) and the inner cell mass ( embryoblast ), which is surrounded by the outer cells . The trophoblast will invade the lining of the uterus, while the embryoblast will largely form the growing embryo.

In mammals , hatching is followed by implantation (nidation) of the blastocyst in the uterus. Adhesion molecules in the area where the embryoblast is located bind the trophoblasts lying on its side in the direction of the uterine lining. These trophoblast cells invade the endometrium , where they develop into syncytiotrophoblasts . The syncytiotrophoblast has the property that it contains many cell nuclei in a plasma membrane and can penetrate the endometrium very quickly. This creates cracks (lacunae), which later form the basis for the utero-placental circulation. During this phase, so-called nidation bleeding can occur, which can be confused with menstrual bleeding .

During this trophoblast invasion, the embryoblast differentiates into a two-leaved stage. In the direction of the blastocyst cavity, the hypoblasts form a single layer of cells, while the other cells ( epiblasts ) form the primary amniotic cavity , bulging in the direction of the hypoblasts .

After blastogenesis comes embryogenesis .

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