Question

Describe the structure of blastula.

Answer 1

Blastulation is the process of formation of the hollow and multicellular blastocyst. The process of blastulation can be summarized as follows:

1. The embryo (blastocyst) that enters the uterus remains floating in uterine cavity for 2-4 days after its entry i.e. till the end of 7th day after fertilization.
2. The outer layer of cells seen in the morula now forms the layer called the trophoblast.
3. Cells from the trophoblast begin to absorb the glycogen rich uterine milk
4. The blastocyst doubles in size from 0.15 mm to 0.30 mm.
5. With more fluid entering inside the blastocyst cavity is formed.
6. These outer cells become flat and are called trophoblast cells (since they help only in absorbing nutrition for the developing embryo).
7. The larger inner cells form inner cell mass or embryoblast (the embryo proper develops from the embryoblasts).
8. These remain attached to the trophoblasts on only one side.
9. The trophoblast cells in contact with the embryonal knob are called cells of Rauber.
10. At this stage, the blastocyst shows polarity i.e. the side with inner cell mass is called the embryonal end and the side opposite to it is the abembryonic end.
11. By the end of the 7th day, the blastocyst is fully formed and ready for implantation and gastrulation.
12. The function of zona pellucida is to prevent the implantation of the embryo at an abnormal site. It does not expose the sticky and phagocytic trophoblast cells till it reaches the implantation site i.e. within the uterus, after which the zona pellucida ruptures.