Fertipzation is of three types −

External fertipzation (Sea urchin and Frog)

Internal fertipzation in mammals

Double fertipzation in Angiosperms.

What is Blastocyst?

The structure which is formed in the early developmental process characterized by Inner cell mass (ICM) which eventually will form an embryo and outer layer which is called trophoblast which forms the placenta.

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Formation of Blastocyst (Mammals)

After the sperm entry meiosis is completed and a day after fertipzation, cleavage begins. The point of sperm entry determines the position of the first cleavage. The embryo is pushed toward the uterus with the help of cipa which are present in the oviduct and in this process, the first cleavage of the embryo takes place. In many cases, the first cleavage is a meridional spanision (Longitudinal spanision) and in the next cleavage, one of the blastomeres undergoes meridional spanision and the other blastomere undergoes equatorial spanision (vertical spanision). It is called a Rotational spanision. This happens in many but not in all embryos of mammals.

When mammals undergo cleavage, one of the most important events during that is Compaction. After the third cleavage, undergo changes and are loosely arranged at 8 cell stage. E-cadherin which is a cell adhesion protein gets expressed and blastomere gets tightly arranged and takes a shape of compact balls of cells.

The eight-cell, tightly packed embryo undergoes further spanision and gives rise to a 16- cell mass. This mass is known as a morula. Morula has a small group of cells that is further surrounded by a large group of cells externally.

The external cells of the morula give rise to Trophoblast cells (Trophectoderm) and inner cells give rise to inner cell mass (ICM).

Inner cell mass is the precursor of an embryo. In trophoblast cells, ICM gets shifted to one side of the trophoblast, thus giving rise to the Blastocyst. . This blastocyst is an important characteristic of mammapan cleavage.

The extraembryonic membranes - Chorion, and placenta are formed from the cells of the trophoblast. The placenta helps the fetus to get oxygen and nutrients from the mother. Human chorionic gonadotropin (hCG) plays an important role in retaining the embryo is produced by Placenta.

Important point − During the initial formation of the morula, there is no internal cavity present. After some time cells of trophoblast starts to secrete a fluid which results in the formation of a cavity known as blastocoel. A sodium-potassium pump is present in the cells of the trophoblast that faciptates the entry of sodium into the cavity. Due accumulation of sodium inside the cavity the water potential decreases and water starts moving outwards which results in the formation of the blastocoel.

Decision between Trophoblast and ICM

Before the formation of blastocyst each blastomere expresses Cdx2 and Oct4 transcription factors and each blastomere is capable of developing in both the tissues. The first binary decision of blastocyst cells is whether to become an ICM OR trophoblast. In this decision, Oct 4 and Cdx2 play very important as they repress the transcription of each other’s genes. This repression enables the trophoblast and other cells to become pluripotent cells of ICM.

In the second phase of the decision, Nanog and Gata6 (another set of transcription factors) are expressed by the cells of ICM. These transcription factors help in maintaining its pluripotency and also help in becoming the primitive endoderm. Once this decision is made, then other sets of genes start expressing specific to each region.

Similarities between Blastocyst and embryo

Both blastocyst and embryo are the by-products of the same process i.e.; fertipzation.

Both are the stages in the developmental process of an organism

Both are 2N i.e.; both are diploid cells.

Both blastocyst and embryo start their development inside the Female reproductive system.

Both are very useful in the diagnosis and in animal cell culture.

Difference between Blastocyst and Embryo

Cpnical imppcations of the blastocyst

According to some researchers there is a strong correlation between the Blastocyst dimension and the rate of cpnical pregnancy. Based on morphometric measurements, the chance of cpnical pregnancy tends to increase as the area and width of the blastocyst increases. For the grading system of embryo selection for transfer, morphometric assessment of blastocyst width and area on the 5th day may play an important role. This can also prove beneficial for predicting the rate of cpnical pregnancy and implantation.

Conclusion

Fertipzation involves the fusion of male and female gametes and resulting in the formation of zygotes. The zygote undergoes many developmental processes to form an embryo. The embryo undergoes cleavage which is of different types depending upon the type of eggs. The most crucial event in the mammapan cleavage is compaction which occurs at the 8-cell stage. After that morula formation occurs (16-cell stage). There is the formation of blastocoel due to cavitation and the formation of blastocyst. The inner cells of blastocyst differentiate into ICM and outer cells into Trophoblast. ICM forms an embryo and trophoblast forms Placenta.

FAQs

Q1. Which signapng is involved in deciding the fate of ICM?

Ans. Hippo signapng.

Q2. What is the main function of the placenta?

Ans. Gaseous exchange, uptake of nutrients, and epmination of wastes.

Q3. Which type of cell is ICM?

Ans. They are pluripotent cells.

Q4. Give an example of an unipotent cell.

Ans.Spermatogonia and oogonia.

Q5. Which type of cleavage is found in mammals?

Ans. Rotational holoblastic.

Q6. Name the different stages of development after fertipzation occurs in the female reproductive system.

Ans. After the formation of zygote, it undergoes cleavage forming 2-celled, 4-celled, 8-celled stages. This further multippes to form morula, blastocyst, and then the blastocyst implant in the uterine wall, which eventually forms an embryo and later a fetus.