Transcription
Hello students, in the continuation of the development of the first week, today we are dealing with the second part of the development, that is cleavage. So again, whenever you are writing in the exam on this question, cleavage, the first thing that comes is the definition of cleavage.
Now, my dear students, you have to understand this concept in your mind that we are dealing with a very big cell of the human body, which is your oocyte. And we know that the oocyte is the largest cell. So, the normal somatic cells of our body are not this much in size. So, you have to decrease the size of this oocyte to a normal somatic cell of the human body. Now, how is it possible? And it is there is only one thing which is known as fast, rapid, repeated mitosis. So, by the help of these repeated rapid mitosis, we will decrease the size of the cell of this oocyte, or the initial formation of your zygote, immediately after fertilization, to a normal somatic cell. Clear?
So, the need of cleavage is to decrease the cell size to regain the normal nuclear-cytoplasmic ratio of the human somatic cell. So, otherwise, what will happen? If there is a large amount of cytoplasm is present and the nucleus is small, there is a lack of control of the nucleus on the cellular activity. So, that's why the cleavage is required. So, whenever we are talking about the definition of cleavage, it is defined as a rapid and repeated mitotic divisions. And these mitotic divisions are taking place in the developing zygote to gain the smaller sizes of the cell. And these are known as blastomeres. These are known as blastomeres. So, these are the two things why the cleavage is necessary and what is the definition of cleavage. So, I hope it is clear that as soon as there is fertilization occurs, we have to decrease the sizes of the cells to a normal somatic human cell. And that's why there is a necessity of the cleavage. And that's why the cleavage is defined as a repeated rapid mitotic activities taking place in the developing zygote to decrease the cell size. And those cells are known as blastomeres.
Now, what is the difference between the cleavage and mitosis? Now, we are using the word cleavage, we are not using the word mitosis. So, there is one difference that when the mitosis will take place, the cell size will not decrease. Whenever there is a mitosis, the cell size will remain same. You will have the two cells, then you will have further cells. So, mitosis will not decrease the size. But in the cleavage, we are decreasing the cytoplasm in each and every division to regain the normal nuclear-cytoplasmic ratio. Clear?
Now, what time the cleavage begins? So, the cleavage starts immediately after fertilization. As soon as the male and female pronucleus fuses with each other, and splitting starts at the same time, the process of cleavage starts. So, what will happen? You will have the two cells, then three cells, four cells, six cells, eight cells, like so on.
Now, what is the duration of cleavage? Now, cleavage starts from fertilization and its duration is till implantation. Till implantation. And we know that implantation is taking place at around your seventh day after fertilization. So, the cleavage is in the whole first week of your ah development.
Now, what is the role of zona pellucida in cleavage? Now, why we are talking about this zona pellucida here? Because when you will have the uterine cavity, you know that the zygote forms inside the fallopian tube. Now, this is your ampulla part where the zygote forms. Now, it starts moving towards your uterine cavity. And with the transportation from fallopian tube into this uterine cavity, simultaneously the cleavage is going on. But this layer of zona pellucida avoids the abnormal implantation of this developing conceptus into the fallopian tube. So, if in other terms, you can say that the stages which are all covered by zona pellucida are under cleavage. Clear? Because as soon as the zona pellucida disappears, there is implantation. And I am saying that what is the duration of cleavage? Till implantation. So, when the zona pellucida disappears, there is implantation. That means there is a stoppage of cleavage. Clear?
Now, what are the stages of your cleavage? So, first, there is a two-cell stage. Now, my dear students, you have to understand one thing in the cleavage that there is one big cell. Now, when this big cell will divide, you will have another unequal distribution. Again, this will give rise to the two cells. One is larger in size with more cytoplasm, one is smaller in size. Now, when the next step will occur, the larger cell will show the mitosis first. So, this will again divide into one bigger cell, one smaller cell. So, at one stage, first you will have two cells, then you will have one, two, three, three cells, then it will divide, then you will have one, two, three, four, four cells. So, some books say it is two-cell stage, then three-cell stage, then four. Clear? So, why three cells? Three cells is because there is an unequal mitosis occurs in the cleavage because one cell is having a larger amount of cytoplasm, another is having a smaller amount of cytoplasm. And the larger cell then will show first mitosis in the next step. Clear? So, this is how the cleavage will take place. So, here when we are talking about the steps of the cleavage, we are going like this: 2, 4, 8, 16, 32, 64, like this.
Now, there are two important terms which are important from your exam point of view. One is morula and second is blastocyst. So, what is happening? How is the morula different from the eight-cell stage? And how is the morula different from the blastocyst? Now, my dear students, what is happening here? When there is an eight-cell stage, there is. Now, this is the eight-cell stage. You are having the eight cells. Now, these all eight cells are of similar properties. And these all eight cells are in this stage known as totipotent cells. Why are they totipotent? Because these all cells can form your intraembryonic as well as extraembryonic tissues. Now, we have to change the attitude of these cells into a specified attitude. So, for that, there is a process that occurs which is known as compaction. What is that compaction? Now, after this process of compaction, these cells will rearrange themselves in the next cleavage. So, what is the next stage? These are the eight cells. In the next stage, we will have 16 cells. But these 16 cells will arrange themselves into two parts. One is known as outer cell mass. One is known as outer cell mass. And another is known as inner cell mass. Another is known as inner cell mass. So, in the eight-cell stage, there is nothing like outer and inner cell mass. But in the 16-cell stage, you will have arrangement of the cells into an outer cell mass and inner cell mass. And these cells are not totipotent. Clear? Why not totipotent? Because the inner cell mass is now only responsible to form the derivatives of your ectoderm, endoderm, and mesoderm. That means they can form your body cells. So, these inner cell mass behave like pluripotent cells. They are pluripotent cells.
So, here you have to understand this concept that when we are talking about the cleavage, at the eight-cell stage, all cells up to this are totipotent and they can form intra as well as extraembryonic tissue. But after the eight-cell stage, there is a process that occurs, compaction. And in the next cleavage, when you will have the 16-cell stage, the cells will arrange themselves into an inner and outer cell mass. And the inner cell mass cells can produce only and only derivatives of your ectoderm, endoderm, mesoderm. They cannot form the placenta. They cannot form the extraembryonic tissue. So, my dear students, now this 16-cell stage is known as morula. Why? Because it appears like a mulberry shape. So, now the question comes is, what are the features of morula? So, the first feature is that it is a 16-cell stage. Second feature is morula is formed at the third day of fertilization. The third character of the morula is that the cells are arranged into the inner and outer cell mass. They are not totipotent. They are having the inner cell mass which is known as pluripotent. So, the inner cell mass is going to form the future embryonic, intraembryonic tissue. Future intraembryonic tissue. And the outer cell mass is going to form your future trophoblast cells, which are going to form the placenta. And there is one more characteristic that I told you, the morula is also covered or lined by zona pellucida. Still, the zona pellucida has not disappeared. Clear? Means that morula is not going to implant. Implantation has not occurred at the morula stage. So, these are the four features of morula.
Now, the next step comes is blastocyst. Now, how is the blastocyst differentiated from the morula? Now, when you are talking about the migration of your developing zygote, now this is your morula. Now, on the third day, this morula is formed. And now it is approaching towards your this uterine cavity. Now, in the uterine cavity, you are having the tissue fluid or you can say uterine fluid. Now, this uterine fluid is also known as uterine milk. Why? Because this fluid is full of nutrients. And this fluid will start to enter inside the morula. Now, this fluid starts to enter inside the morula. And on the fourth day, on the fourth day of fertilization, what will happen? Now, the scenario will change. So, let's see what will happen next.
So, in the developing your zygote, now this is your morula, which is having outer cells. Now, these outer cells are known as future trophoblast. And you will have inner cells. Now, the uterine fluid, which will pierce this outer zona pellucida of morula, enters into these intercellular spaces of inner cell mass. Clear? So, here you will have the collection of your uterine milk or uterine fluid into the area between the inner cell mass. Now, these spaces gradually enlarge and fuse with each other. Now, if these spaces will enlarge and fuse with each other, what will happen? They shift all these cells at one end of the developing zygote. So, what will happen? If you will remove all these inner cell mass and if you will shift them to one end, so they will come at one end. And now a large cavity will appear inside this developing zygote. Now, this stage is now known as blastocyst. Cyst means any swelling which contains the fluid. So, now here you can see that there is a cavity formed. Now, this cavity is known as blastocele. This cavity is known as blastocele. So, how is the blastocele formed? The blastocele is formed by the diffusion of uterine milk through the zona pellucida into the morula, where the fluid will go into between the inner cell mass. And the spaces will join together to form a large cavity. And this fluid pushes this fluid pushes all inner cells on one end. And a large blastocele will appear. Now, this is the stage after that the implantation will start. So, at what day the blastocyst will form? So, on the fourth day or after fertilization, the blastocyst stage is forming. And the cells are around 32. Then you will have the next cleavage is 64. Clear? So, the cleavage is still going on. You have seen that 16-cell stage is morula, then you will have 32, then you will have 64. So, this is what how the blastocyst is formed.
But now there are some important terminologies that come into the picture. What terminologies? Now, one is this outer cell mass or outer cell layer is known as trophoblast. It is now no more known as outer wall. It is now known as trophoblast. The inner cell mass is now known as embryoblast. So, this is the first thing that once the blastocyst will form, there are three names you can see: blastocele, this outer layer which is now known as trophoblastic cell layer, this inner cell mass which is now known as embryoblast. There is one more thing that these trophoblasts divide into two parts. How? The trophoblasts which are covering this area of embryoblast, the trophoblasts which are covering this area of embryoblast are known as polar trophoblast. So, it is divided into two parts: polar trophoblast and mural trophoblast. So, what do you mean by polar trophoblast? Polar trophoblast are the trophoblasts which are just related to the embryoblast inside the blastocyst. And the remaining layer of trophoblast which is not related with embryoblast cells is known as mural trophoblast. Clear?
Now, there is one more nomenclature that comes. Now, what is that? This embryoblast and formation of embryoblast divide this ah blastocyst into two parts. Now, this area is known as embryonic pole of the blastocyst. It is known as embryonic pole. Embryonic pole is also known as animal pole. So, this is known as embryonic pole. Now, this embryonic pole is the area from where the embryo is going to implant first with the maternal endometrium. This remaining part is known as abembryonic pole. What is that? Abembryonic pole. So, here now you can see there are few terminologies which are important when you are writing an exam. First, there is a two-cell stage, four-cell stage, then eight-cell stage. After eight-cell stage, you will have a process of compaction. Then you will have 16-cell stage. Now, the compaction creates the outer and inner cell mass, which is known as morula. Then in morula, there is a formation of a cavity. And this stage is known as blastocyst. And in blastocyst, the ah cavity pushes the inner cell mass on one end. Now, these cells are now known as embryoblast. The outer lining is known as trophoblast. Trophoblast divided into two: the trophoblast covering the embryoblast are known as polar trophoblast. The trophoblast on the remaining part are known as mural trophoblast. Now, the area of blastocyst which is having the embryoblast inside is known as embryonic pole. And the remaining part of the area is known as abembryonic pole.
So, what are the fates of these three parts? So, what is the fate of hemoroblast? Embryoblast, I told you that these are the pluripotent cells. And embryoblast cells are going to form your tissues of embryo. What is the fate of trophoblast? The trophoblast is going to form extraembryonic tissue. That means tropho means nutrition. So, they are going to form placenta. And what is the fate of the blastocele? The blastocele is going to form your yolk sac. So, these are some terminologies which are always there in your mind when you are writing a short note on cleavage. What is blastocele? What is uterine milk? What is trophoblast? What is polar trophoblast? What is mural trophoblast? What is your embryonic pole? What is abembryonic pole? And what is the fate of embryoblast? Clear?
So, now at the end of this session, you will have the idea that the cleavage starts immediately after fertilization. And the purpose of cleavage is to receive the normal size of the somatic cell. And also to receive the normal nuclear-cytoplasmic ratio. So, there is a rapid mitosis will take place. Then you have to write down the two, then four, then eight-cell stages. After the eight-cell stages, there is a compaction, which is going to form a morula, a 16-cell stage. After the morula, there is an invasion of the uterine milk. So, there is a formation of a cavity. And ultimately, the cavity will enlarge. You will have a blastocyst. And the blastocyst will have a well-defined trophoblastic layer. And inside, you will have embryoblast. So, this is all for the lecture. Thank you.