What is Blastula?
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.
Blastula Formation
After understanding what blastula is, 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 fertilization. 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. The ts of the blastula stands for the transverse section of the blastula.
Embryonic Development
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.
Cleavage
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 center 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 keep on increasing. On the amount and distribution of yolk, the rate of cleavage depends.
Morula
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 blastula is 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 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 the 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 fetal part of the placenta. The blastula diagram or the ts of the blastula is shown below.
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Implantation
This is the process when the blastocyst is embedded into 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 is 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.
About Blastula Provided by Vedantu
Blastula, its formation and development is an extremely complex concept that requires a critical understanding of various terminologies that are discussed in biology, certain concepts related to blastulation are discussed in chapter 3 human reproduction of the NCERT book of the biology of class 12th. The chapter - human reproduction is prescribed by the Central Board of Secondary Education, according to the curriculum set by the CBSE, it carries significant weightage in the board examination and is, therefore, extremely important in the study of biology.
The study notes provided by Vedantu on the topic of blastula are extremely important as they are curated in a way that simplifies the complex concept of blastula, it is mainly for students who may find it difficult to grasp the concept of blastula that is not critically explained in the NCERT book, therefor, Vedantu has done extensive research and analyzed previous year question papers to give you the best experience at learning biology and the various concepts associated with it.
The formation of a blastula is extremely important In understanding the study of the anatomy and physiology of a female reproductive system. During the development of an embryo, blastula or blastomeres or hollow spheres of cells are produced by repeated cleavage of a fertilized egg. The blastocoel also known as a fluid-filled cavity is enclosed by a blastoderm which is an epithelial covering or layer formed by the cells of the blastula. The next step after the development of a blastula is called gastrulation in which the blastula goes under a transition to the gastrula. A blastula should not be confused by a blastocyst. A blastocyst is developed in organisms such as mammals -the earlier morula which is a berry-like cluster of cells is developed into a different form of blastula, known as a blastocyst.
Key Concepts Important in the Understanding of the Blastula are as Follows-
The Female Reproductive System
Gametogenesis
Menstrual Cycle
Fertilization and Implantation
Pregnancy and Embryonic Development
Parturition and Lactation
FAQs on Blastula
1. What is a blastula and what is its primary function in embryonic development?
A blastula is an early-stage embryo typically consisting of a hollow sphere of cells, called blastomeres, that surround a fluid-filled cavity known as the blastocoel. Its primary function is to organize the cells from the preceding morula stage, preparing for the complex cell movements of gastrulation. In mammals, this structure is more specifically called a blastocyst, which has distinct cell types that form the embryo and placenta.
2. What is the main difference between a morula and a blastula?
The key difference between a morula and a blastula lies in their structure and cellular arrangement. A morula is a solid, compact ball of 16-32 cells formed through cleavage, with no internal cavity. A blastula (or blastocyst in humans) develops from the morula and is a hollow structure, featuring an internal fluid-filled cavity called the blastocoel, which separates the cells into an outer layer and an inner cell mass.
3. What are the essential parts of a human blastocyst as per the CBSE Class 12 syllabus?
According to the NCERT syllabus for Class 12 Biology, the human blastocyst is composed of three essential parts:
- Trophoblast: The outer layer of cells that plays a crucial role in implantation and later develops into the fetal part of the placenta.
- Inner Cell Mass (ICM): A cluster of cells located inside the trophoblast, which is destined to develop into the embryo itself.
- Blastocoel: The fluid-filled central cavity that nourishes the embryo and allows for cell migration during further development.
4. How is a blastula different from a gastrula?
A blastula and a gastrula represent two sequential and structurally different stages in embryonic development. The blastula is a hollow sphere of cells with a single cell layer (or in mammals, an outer layer and inner cell mass). The gastrula is a more complex, multi-layered structure that is formed from the blastula through a process called gastrulation. During gastrulation, cells migrate inward to form the three primary germ layers: the ectoderm, mesoderm, and endoderm.
5. What is the process of implantation in relation to the blastocyst stage?
Implantation is the process where the blastocyst attaches to and embeds itself into the endometrium (the inner lining of the uterus), which typically occurs about 6-12 days after fertilization. The process begins after the blastocyst hatches from the zona pellucida. The trophoblast cells then adhere to the uterine wall and release enzymes that allow the blastocyst to burrow deep into the tissue, establishing the pregnancy and initiating the formation of the placenta.
6. Why is the formation of the blastocoel a critical step in development?
The formation of the blastocoel is a critical milestone for two main reasons. Firstly, it facilitates the first major instance of cell differentiation by segregating the blastomeres into two distinct populations: the outer trophoblast and the inner cell mass. Secondly, the fluid-filled cavity provides the necessary space for the complex cellular movements and rearrangements that occur during gastrulation, a process that would be impossible within the solid structure of the morula.
7. Why is the term ‘blastocyst’ used for humans instead of the more general term ‘blastula’?
The term ‘blastocyst’ is used specifically for mammals, including humans, because of its unique and more complex structure compared to the blastula of other animals. The key distinction is the early differentiation into two specific cell types: the trophoblast (which forms the placenta) and the inner cell mass (which forms the embryo). This early specialization is a hallmark of mammalian development and is not present in the simpler, more uniform blastulas found in organisms like sea urchins or frogs.
8. How does the structure of the blastocyst's inner cell mass relate to the formation of identical twins?
The inner cell mass (ICM) is composed of pluripotent stem cells, which have the ability to develop into any part of the fetus. Identical (monozygotic) twins are formed when this single inner cell mass splits into two separate groups of cells during the blastocyst stage. Each of these cell groups then develops into a complete, separate embryo. Because they originate from the same ICM, the resulting twins are genetically identical.
