The embryo, blastula and morula are formed after the process of fertilization. When copulation or coitus takes place, the semen is secreted or released by the penis into the vagina of the female. This process is known as insemination. The blastula is formed from the morula. The morula is an 8-16 celled structure and when this morula receives the nutrition it is then converted into a blastula. This blastula then gets implanted into the uterus and further development takes place. This is the simple blastula definition.
After understanding what is blastula, we will learn about its formation. After the process of insemination, the human sperms can survive for 48-72 hours in the body of the females. These sperms move in a liquid medium. This liquid is secreted by the female genital tract. There is an organ in the sperm of a male that is known as prostaglandin. This prostaglandin helps in the movement of the sperms in the liquid medium. The sperms can move at a speed of 2-3mm/minute. When these sperms enter the female genital tract then there is a process known as capacitation of sperms. In this process, the cholesterol from the acrosome membrane of the sperm is removed. The decapacitation factors start to dilute and the membrane cover also starts weakening. The sperms show whip-lash movement with the help of their tails. They swim to the ampulla-isthmus junction in the females. The egg of the female is also brought towards this junction so that both the egg and sperm can meet. The meeting of the egg and sperm and then fusing them to form a zygote is known as fertilizations. When the sperm enters the ovum, then the ovum completes its meiotic division and both these haploid structures fuse to form a zygote that is diploid in nature. The ts of blastula stands for the transverse section of the blastula.
The development of the embryo after the process of fertilization is known as embryonic development. Cleavage, blastulation, implantation, gastrulation and organogenesis are the processes that take place for the development of gametes. We will learn about all of them one by one.
It 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. 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. 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.
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.
What is Blastulation?
We will understand what is blastula and the process of blastulation. This is the process of formation of the blastocyst. A nutrient fluid is secreted by the endometrium layer. Due to this, its mucosal cells are enlarged and they store nutrients. This is because when the morula starts entering the uterus, then it receives a rich supply of nutrients. There is also an enlargement in the outer peripheral cells and they start to become more flatter. The trophoblast is formed from them. It is also known as trophectoderm. The cavity inside it is known as blastocoel. Fluid is secreted by the trophoblast cells into this blastocoel cavity.
Structure of Blastula
After studying the blastula slide and understanding the blastula under microscope. 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. The blastula diagram or the ts of blastula is shown below.
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This is the process when the blastocyst is embedded onto the uterus. It is embedded in the endometrium layer of the uterus. In the region of the embryonal knob, the blastocyst comes in contact with the endometrium. The corrosion of the endometrium lining is caused by the surface cells of the trophoblast. These surface cells secrete lytic enzymes. They also help in giving rise to finger-like projections that are known as villi. The function of villi is to help in fixation and also absorption of the nutrients. When the process of implantation occurs then there is nutrient enrichment and cells are also enlarged. This results in the formation of the uterine part of the placenta and this part are known as decidua. The decidua is divided into three layers. They are:
Decidua Basalis: This layer lies below the chorionic villi and above the myometrium layer.
Decidua Capsularis: This layer lies between the embryo and the lumen of the uterus.
Decidua Parietalis: This layer helps to inline the uterus.
The covering layer of the trophoblast secretes the human chorionic gonadotropin hormone. This hormone can be detected in the urine. This is the basis of pregnancy tests. Pregnancy tests are also known as Gravindex tests. This HCG hormone helps in maintaining the corpus luteum and this corpus luteum secretes progesterone. This progesterone hormone prevents the process of menstruation. Another name of progesterone hormone is pregnancy hormone as this hormone is essential for the maintenance of pregnancy.