Blastulation

As Blastulation is the term used to describe the formation of a fluid-filled cell sphere, the blastocyst or blastula (lat. germinal vesicle) during embryonic development. The implantation of the blastocyst in the uterine lining marks the actual start of pregnancy.

What is blastulation?

After the female egg cell has been fertilized, embryonic cell division begins. The egg cell divides symmetrically, whereby the number of cells steadily doubles until 128 cells are reached. The cell ball created by cell division is called morula (Latin for mulberry).

In the final stages of cell division, the morula begins to fill with tissue fluid, thereby developing into a 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 layer of cells that is directly adjacent to the zona pellucida (Latin for egg skin). The cells of the trophoblast are connected by strong connecting proteins called tight junctions. The structures of the placenta later develop from the trophoblast.

Within the single-layer cell sphere there is an accumulation of cells, the embryoblast. In the next step, many important structures of the embryo will form from this small cluster of cells. The fluid-filled cavity in the blastocyst is called the blastocoel.

Like the egg cell, the blastocyst is surrounded by the protective zona pellucida. Before the blastocyst can implant itself, it “slips” out of this egg membrane. The fully developed blastocyst begins to implant itself in the uterine lining during nidation, thus triggering the actual pregnancy.

During implantation, some cells of the trophoblast (the outer blastocyst envelope) differentiate into polynuclear 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 & task

The fluid-filled cell ball is the starting point for embryonic development in all animal living beings. In the course of development, this ball stretches and forms the internal organs inwards and the extremities and sensory organs outwards. Therefore, the formation of the blastocyst is an important step in the development of the new living being. The liquid-filled cavity of the Blastocoel enables cell layers to be inversed.

In the next stage of embryogenesis, gastrulation (large stomach), the tissue of the blastocyst, known as the embryoblast, will multiply and fill the blastocyst, then called gastrula, from the inside to a smaller cavity.

In this step, all body axes are determined and each cell is assigned its future cell fate. This allocation takes place through asymmetrical distribution of cell components and asymmetrical DNA expression.

Another task of the blastula is the creation of the embryonic envelope 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 shed again after birth (afterbirth). In terms of cell biology, the placenta arises from the unicellular blastocyte envelope, the trophoblast.

Like all early embryonic stages, the formation of the blastocyst is essential for the development and maintenance of a pregnancy. Malformed blastocysts are flushed out during menstruation without any sign of pregnancy. For nidation problems (implantation problems) intact blastocysts are also removed by menstruation.

The blastocyst is of technical importance in medicine and biology as a source of stem cells. The embryoblast consists of pluripotent stem cells, which can be differentiated into any type of cell and tissue by the administration of appropriate transcription factors.

However, pluripotent stem cells cannot develop into a complete embryo on their own.When harvesting stem cells, the blastocyst is completely removed and destroyed, which has raised ethical concerns. Therefore, the removal of these cells from humans is subject to strict legal regulation in every country.

Illnesses & ailments

The formation of the blastocyst is an essential step in embryonic development and any malformation normally leads to a complete termination of embryogenesis and removal of the blastocyst during the following menstruation.

Only the implanting blastocyst secretes an increasing amount of human chorionic gonadotropin (hCG), the increasing concentration of which in the blood marks the beginning of pregnancy and suppresses the occurrence of new menstrual bleeding.

Since the success of the blastulation is critical, this stage is very sensitive to external disruptive factors such as environmental toxins, alcohol, heat, infectious diseases, physical stress and the like. The occurrence of such factors can delay or stop the maturation of the blastocyst.

Another critical process is the implantation of the blastocyst. This process can also be prevented by the above factors. In cases of female infertility, however, the uterus often does not have the necessary capacity, which prevents implantation. The causes for this are varied and require hormonal treatment. In rare cases, the blastocyst itself is not able to produce sufficient hCG and thus to maintain further embryonic development. In these cases, too, hormonal therapies can help.

The blastocyst stage is also of interest for modern in vitro fertilization, since the implantation of fertilized egg cells in women with fertility problems has little chance of success. Thanks to modern techniques, the fertilized egg cells can now be used in the test tube up to the blastocyst stage and then implanted. Combined with a corresponding hormone therapy, the chances of success of this method are significantly higher.