During fertilization, the development of the embryo also begins, in which a simple embryo having very less diverse cell types is gradually converted to a complex organism with diverse cell types. The development includes a series of processes −
Fertilization is of three types −
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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After the sperm entry meiosis is completed and a day after fertilization, 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 cilia 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 division (Longitudinal division) and in the next cleavage, one of the blastomeres undergoes meridional division and the other blastomere undergoes equatorial division (vertical division). It is called a Rotational division. 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 division 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 mammalian 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 facilitates 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.
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.
| Blastocyst | Embryo |
|---|---|
| It is a blastula stage in the case of mammals that arises from the morula stage | Embryo is that stage in which blastocyst gets implanted on the uterine walls. |
| It has a cavity called blastocoel formed due to cavitation. | No such cavity is formed |
| This stage is present only in mammals | Embryo is formed after fertilization in both plants and animals. |
| Development of blastocyst occurs due to cleavage of morula stage | An Embryo starts its development after it gets implanted on the uterine wall. |
| Time period − 5 days to 2 weeks after fertilization | Time period − 2 weeks to 11 weeks after fertilization. |
According to some researchers there is a strong correlation between the Blastocyst dimension and the rate of clinical pregnancy. Based on morphometric measurements, the chance of clinical 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 clinical pregnancy and implantation.
Fertilization 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 mammalian 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.
Q1. Which signaling is involved in deciding the fate of ICM?
Ans. Hippo signaling.
Q2. What is the main function of the placenta?
Ans. Gaseous exchange, uptake of nutrients, and elimination 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 fertilization 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 multiplies to form morula, blastocyst, and then the blastocyst implant in the uterine wall, which eventually forms an embryo and later a fetus.
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