Blastulation is the formation of a fluid-filled ball of cells, the blastocyst or blastula (Latin for germinal vesicle) during embryonic development. The implantation of the blastocyst in the uterine mucosa marks the actual
actual beginning of a pregnancy.
What is blastulation?
Blastulation is the formation of a fluid-filled ball of cells, the blastocyst during embryonic development. After fertilization of the female egg, embryonic cell division begins. The egg divides symmetrically, steadily doubling the number of cells until 128 cells are reached. The cell sphere formed by cell division is called a morula (Latin for mulberry). In the final stages of cell division, the morula begins to fill with tissue fluid and thus develops into the blastocyst . Morphologically, the blastocyst is a fluid-filled cell sphere. The outer layer of the blastocyst, the so-called trophoblast, is formed by a single-layered cell association that lies directly against the zona pellucida (Latin for egg skin). The cells of the trophoblast are connected by strong connecting proteins, the tight junctions. Later, the structures of the placenta also form from the trophoblast. Within the single-layered cell sphere is a collection of cells, the embryoblast. From this small cluster of cells, many important structures of the embryo will form in the next step. The fluid-filled cavity located within the blastocyst is called the blastocoel. Like the egg, the blastocyst is surrounded by the protective zona pellucida. Before the blastocyst can implant, it “hatches” from this egg membrane. The fully developed blastocyst begins to settle into the endometrium during nidation, initiating the actual pregnancy. During implantation, some cells of the trophoblast (the outer blastocyst shell) differentiate into multinucleated syncytiotrophoblasts. These fused cells produce the hormone human chorionic gonadotropin (hCG). The appearance of this substance in the blood hormonally marks the beginning of pregnancy.
Function and task
The fluid-filled cell sphere is the starting point of embryonic development in all animal life forms. During development, this sphere stretches and forms the internal organs inwardly and the extremities and sensory organs outwardly. Therefore, the formation of the blastocyst is an important step in the development of the new living being. The fluid-filled cavity of the blastocoel makes the invagination of cell layers possible. In the next stage of embryogenesis, gastrulation (gr. stomach), the tissue of the blastocyst, called embryoblast, will proliferate and fill the blastocyst, then called gastrula, from the inside except for a smaller cavity. At this step, all body axes are then established and each cell is assigned its future cell fate. This assignment is done by asymmetric distribution of cell components and asymmetric DNA expression. Another task of the blastula is the formation of the embryonic membrane or placenta in which the embryo matures, protected and surrounded by fluid. The placenta grows together with the uterus, but is not formed by it and is rejected after birth (afterbirth). Cell biologically, the placenta arises from the unicellular blastocyst envelope, the trophoblast. The formation of the blastocyst, like all early embryonic stages, is essential for the establishment and maintenance of pregnancy. Malformed blastocysts are flushed out during menstruation without any signs of pregnancy. In the case of nidation problems (implantation problems)
intact blastocysts are also removed by menstruation. The blastocyst has a technical importance in medicine and biology as a source of stem cells. The embryoblast consists of pluripotent stem cells that can be differentiated into any type of cell or tissue by the administration of appropriate transcription factors. However, pluripotent stem cells cannot develop independently into a complete embryo. When stem cells are harvested, the blastocyst is completely removed and destroyed, which has raised ethical concerns. Therefore, the harvesting of these cells from humans is subject to strict legal regulation in each country.
Diseases and ailments
The formation of the blastocyst is an essential step in embryonic development, and any malformation usually results in complete abortion of embryogenesis and removal of the blastocyst during the following menstrual period. Only the implanting blastocyst secretes increasing levels of human chorionic gonadotropin (hCG), whose increasing concentration in the blood marks the onset of pregnancy and suppresses the occurrence of new menstrual bleeding. Since the success of blastulation is critical, this stage is very sensitive to external interfering factors such as environmental toxins, alcohol, heat, infectious diseases, physical stress and the like. The occurrence of such factors may delay or abort the maturation of the blastocyst. Another critical process is the implantation of the blastocyst. This process can also be prevented by above factors. However, in cases of female infertility, often the uterus does not have the necessary receptive capacity, which prevents implantation. The reasons for this are many and require hormonal treatment. In rarer cases, the blastocyst itself is unable to produce sufficient hCG and thus maintain further embryonic development. Hormonal therapies can also help in these cases. The blastocyst stage is also of interest for modern in vitro fertilization, since implantation of fertilized eggs in women with fertility problems has low chances of success. Thanks to modern techniques, fertilized eggs can now be grown in the test tube to the blastocyst stage and then implanted. Combined with appropriate hormone therapy, the chances of success with this method are much higher.
