Gastrulation

Gastrula is the stage of the embryo after blastula. The gastrula stage is an important stage in embryonic development. During this stage, the blastula is reorganized into gastrula. Gastrulation meaning is that it takes place after the blastulation and gastrulation process, the embryonic layers or the germ layers are formed. These germ layers are further responsible for the formation of the organs. 

To thoroughly understand gastrulation meaning we need to understand the process of blastulation and how the embryo is formed and embedded into the uterus. By understanding these, we can definitely know the meaning of what is gastrula. 

Embryonic Development and Cleavage

The development of the embryo after the process of fertilization is known as embryonic development. Cleavage, blastulation, implantation, gastrula stage and organogenesis are the processes that take place for the development of gametes. 

1. Cleavage is known as the division of cells when the zygote is formed. It is also called an internal zygote division. After 30 hours of fertilization, the first cleavage is completed. There is a furrow formed that is known as cleavage furrow. It passes from the animal-vegetal axis and also from the centre of the zygote. 

2. Two blastomeres are formed after this first division. This type of cleavage is known as holoblastic cleavage. In 60 hours, the second cleavage is completed. This cleavage is at a right angle to the first one. This cleavage is also meridional in nature. This forms a 3-celled stage. 

3. 8 blastomeres are formed in the third cleavage. This cleavage is horizontal in nature. This division is slightly unequal in nature. And then thereafter the rate and pattern of the cleavage are non-specific in nature. 

The humans show the slowest cleavage division. There is an asynchronous type of division in humans. When the cleavage divides, the blastomeres are increased in an arithmetic division. The cleavages show mitotic division and the daughter cells that are formed are known as blastomeres. When cleavage takes place then at that time no growth is seen in the blastomeres. Here, the total size and volume of the embryo remain the same. This is because there is no growth phase in the interphase stage. 

There is a decrease in the size of blastomeres at the time of cleavage. This is because there is no growth in the blastomeres. At the time of cleavage divisions, the zona pellucida remains intact. At the time of cleavage, there is no increase in the mass of the cytoplasm. But, the DNA content and the chromosomal amount keeps on increasing. On the amount and distribution of yolk, the rate of cleavage depends. 

Morula and Blastula

A solid ball of cells is formed as a result of cleavage. This is known as Morula. It is an 8-16 celled structure. The outer cover is formed by the zona pellucida. There is a process of compaction that takes place in the morula. The outer cells of the morula are smaller in size. They are also flat. They are present with tight junctions with the inner cell mass. The inner cell mass has slightly large cells. They are also round in nature with the presence of gap junctions. As to progress for the process of implantation, the morula starts descending towards the uterus. When this process happens, then the corona radiata is detached from the structure. 

On one side of the embryonal knob, the inner cell mass starts to lie. When the blastocoel is formed then, the morula is converted to the blastula. In mammals, it is called a blastocyst. This is because it has a different nature of the surface layer and the inner cell mass is eccentric in nature. 

As the blastocyst grows, there is an increase in pressure and due to this a small hole is produced in the zona pellucida. Through this hole, the blastocyst squeezes out. So while coming out sometimes this blastocyst can be broken down into two pieces. When such an event happens then, there are two identical blastocysts present and this results in the formation of identical twins. These identical twins are also known as maternal twins or monozygotic twins. The trophoblast cells that are in direct contact with the embryonal knob are known as cells of Rauber. The animal pole is the area of an embryonal knob. 

Just opposite the animal pole is the abembryonic pole. The embryonic disc is formed when the embryonal knob starts to show rearrangement. Periclinal division takes place in the cells of the trophoblast layer. Syncytiotrophoblast and cytotrophoblast are the two layers that are formed. The syncytiotrophoblast in the outer layer and cytotrophoblast in the inner layer. Further, these two layers give rise to the chorion, amnion and the foetal part of the placenta.

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Gastrula Stage and Gastrulation

We understood the gastrulation definition in the above paragraph. We will understand the structure of the gastrula and the process of gastrulation briefly. 

Primary germ layers are formed in the process of gastrulation by the movement of cells in small masses or sheets. The three primary germ layers are the ectoderm, endoderm and mesoderm. Morphogenetic movements is the name given to movements that happen at the time of gastrulation. These movements then lead to the initiation of morphogenesis. Gastrula is formed as the product of the gastrulation process. The amniotic cavity is the space that appears between the ectoderm and the trophoblast. It is filled with amniotic fluid. Amniogenic cells form the roof of this cavity. These cells are derived from the trophoblast cells. 

Formation of Primary Germ Layer

A germinal disc is formed by the rearrangement of the cells of the inner cell mass or the embryonic knob. The germinal disc then further differentiates into two layers that are the epiblast and the hypoblast. The epiblast is the outer layer and the hypoblast is the inner layer. The process of gastrulation begins with the formation of a primitive streak on the surface of epiblast cells. The hypoblast cells are the first cells to move inwards. They help in creating the endoderm layer. After the endoderm, the mesoderm layer is formed by the inward moving of the epiblast cells. The cells that remain in the epiblast from the ectoderm. So, the epiblast is responsible for the formation of all the germ layers in the body.