Transcription
Hello students. In today's lecture, I will talk to you about fertilization. Now, whenever we talk about development in the first week of life, what will happen? As soon as there is an ejaculation, the sperm is going to meet with the secondary oocyte. And we know that the secondary oocyte and sperm meet in the ampullary part of the fallopian tube, and that process is known as fertilization.
So, in the first seven days, there are three things which are always there, and you have questions on them. The first is fertilization. Second is known as cleavage. Now, cleavage is a process of mitotic activities. And the third and the last step is implantation. That means when there is a first seven-day life, if you are talking about from the fertilization, the fertilization is on the first day, and the first week ends with the implantation in the uterine cavity.
So, let's discuss about fertilization. How to write it in your university exam? So, my dear students, we always require two gametes for fertilization: the female gamete, which is in the form of a secondary oocyte, and we need a male gamete that is known as spermatozoa. Now, these both gametes meet in the fallopian tube. And before meeting, you need the priming of these two gametes.
So, when we are talking about fertilization, the first thing that comes is the definition of fertilization. So, the definition of fertilization means when there is a fusion of male and female gametes, and these gametes, when they fuse, you are having a product, and that product is known as a zygote. And here, if you will see, you will realize that the sperm and the oocyte are both haploid in nature. And when we are talking about the zygote, it is diploid in nature. So, what is the definition of fertilization? Fertilization is a process of the fusion of the male and female gamete to form a zygote. And by the process of fertilization, the two haploid cells fuse to form a diploid cell. Clear?
So, where does fertilization take place? Fertilization takes place in the ampulla of the fallopian tube. So, as I told you, there is a prerequisite which is always there for the male gamete to become or to achieve the capability to fertilize an ovum. What does it mean? That suppose if you are having a collection of freshly ejaculated semen outside the female body, what I am saying, outside the female body, if you are collecting some semen into a vessel, now the sperms which are present into this semen are right now not having the capability to fertilize the ovum. Why? Because these sperms have to go under priming. The process is known as priming. And that priming can take place in two ways: either you will place these spermatozoa inside the female genital tract, or you may do it artificially inside the lab with the help of some enzymes.
So, my dear students, these spermatozoa have to go under some priming, and that priming is the prerequisite. And after that, the spermatozoa will get or will achieve the capability to fertilize the ovum. So, what is that prerequisite? The first process is known as capacitation. What is that capacitation? Now, in capacitation, we are doing a process that is known as conditioning. That is known as conditioning of your spermatozoa. And the conditioning is also known as maturation of spermatozoa. So, by the conditioning or maturation of spermatozoa, the spermatozoon is going to achieve the ability to fertilize.
Now, C for capacitation, C for conditioning. And this conditioning will take place in the female genital tract when there is a contact of the sperm with the female mucosa. And female mucosa produces some enzymes, and those enzymes will change the property of the head of the sperm. And what are these two things that will change? There are two things which change. That is known as the conditioning. First thing is known as there is a removal of glycoprotein coat. There is a removal of glycoprotein coat from the head of the sperm. And second thing is there is a removal of semen plasma protein. So, these two things have to be removed from the head of the sperm to make or to allow or to mature the spermatozoa.
So, my dear students, whenever you are reading or whenever you are writing fertilization in your exam, this is my sincere suggestion that always write capacitation in bold letters because it is a very important prerequisite which causes priming or maturation of the spermatozoa. And it is defined as C for capacitation, C for conditioning. So, capacitation is defined as a conditioning of the spermatozoa. And in this process, when the spermatozoa are passing through the female genital tract, and there is a contact of female mucosa, the female mucosa releases enzymes which will remove the glycoprotein coat and the seminal plasma proteins from the head of the sperm. Clear? So, this is the first process which is known as capacitation.
Now, there is a second prerequisite which is known as acrosomal reaction. Acrosomal reaction. Now, what is the acrosomal reaction? Now, acrosomal reaction means that the head of the sperm starts to release the enzymes. Now, what are these enzymes? These are hyaluronidase and acrosin. These enzymes are released from the head of the sperm once they will contact the secondary oocyte.
So, my dear students, these two things, capacitation and the acrosomal reaction, are compulsory, or you can say the prerequisite for a spermatozoa to penetrate the different barriers of the secondary oocyte and to fertilize it. Clear? So, when we are starting the writing in the exam, first we will define fertilization, then we will define the process of priming, where you have to detail about capacitation and acrosomal reaction.
Now, we will move to the steps of fertilization. So, my dear students, what will happen? That when we are talking about the female genital tract, now suppose this is your female genital tract. Here is our fallopian tube. Now, the spermatozoa is entering till the ampulla. Now, in this ampulla, what will happen? The secondary oocyte, which has been released from the ovary by ovulation, also approaches at this point. Now, in this temporary region, both the male and female gametes meet.
So, for fertilization, what will happen? This male gamete pierces the barriers of the female gamete. So, what are the barriers that have been pierced by the male gamete? So, for that, you should have an idea about the structure of the secondary oocyte. So, when you will see the secondary oocyte, there is an outermost layer, and this outermost layer is known as the corona radiata, where you will have two or three layers of cells, and these are known as corona radiata cells. Clear?
Now, inside the corona radiata, you will have a very important layer which is known as the zona pellucida. What is that? Zona pellucida. So, this is your zona pellucida. Now, inside the zona pellucida, you will have a third layer which is known as the vitelline membrane. What is that? Vitelline membrane. Vitelline membrane is another name of the plasma membrane of the secondary oocyte. So, from outside to inside, when you are seeing the secondary oocyte, you will find three layers. This is the outermost layer which is known as corona radiata. Then this is zona pellucida. And this is vitelline membrane. And this is your cytoplasm of the oocyte, which is known as ooplasm. And inside the cytoplasm, you will have the nuclear material of the female gamete.
So, my dear students, what will happen when the spermatozoa will approach the secondary oocyte? It will first pierce this layer. So, what is the first step of fertilization? Penetration of corona radiata. Then it will pierce this layer. That is the second step, that means penetration of zona pellucida. Then there is a third step, penetration of vitelline membrane. Then there is a fourth step, fusion of male and female nuclear material. Clear?
So, when you are writing the steps of fertilization, you have the idea of the three layers of the secondary oocyte: corona radiata, zona pellucida, and vitelline membrane. So, suppose if I am writing it in an exam, so what I will start? There are four steps: one, penetration of the outer layer, that is corona radiata; second, penetration of zona pellucida; third, penetration of vitelline membrane; and the fourth step is fusion of male and female nuclear material. So, these are the four steps of fertilization.
So, once you will go through these four steps, which I am not detailing here, I am just explaining how to write in an exam, there is one very important thing that occurs. As soon as our male gamete contacts with the corona radiata, the acrosomal reaction starts. What starts? Acrosomal reaction. And with the help of this acrosomal reaction, it releases the enzymes, and those enzymes will start to penetrate these three layers. Clear? So, if acrosomal reaction will not happen, the spermatozoa will not be able to pierce any of these three layers. So, these three layer penetration is possible just because of the acrosomal reaction which occurs at the head of your spermatozoa.
So, now what will happen? Now, what is happening here? As soon as you will see that this is your vitelline membrane. After piercing the two layers, that means your this sperm has already pierced the two layers, this is your corona radiata, this is our zona pellucida, and now the sperm has pierced the third layer, that is vitelline membrane. Now, once the head of the sperm comes in contact with the vitelline membrane, what will happen? That this plasma membrane of your head fuses with the vitelline membrane. The plasma membrane of the head fuses with this vitelline membrane of the oocyte. And this membrane on the interior surface of the head of the spermatozoa already disappears while it is going deep and deep. So, we will remove this from here.
So, if you will remove this, what will happen? Now, this is in contact. Now, here you will have the female nuclear material. Here we will have the male nuclear material in the head. You know, the head contains the nucleus which is covered by the acrosomal cap. So, once you will approach here, what is happening? That the posterior plasma membrane of the head now fuses with this vitelline membrane. The anterior surface of the acrosomal cap as well as the plasma membrane has already been disappeared.
Now, what will happen? As soon as this process of fusion of plasma membrane takes place and male material starts to go into the female cytoplasm, it produces a kind of calcium wave. And what is the effect of the calcium wave? Now, there is formation of granules. And this calcium wave shifts these granules to the outer side, and now they are known as cortical granules. Now, these cortical granules contain some enzymes. These enzymes will change the property of the vitelline membrane and they will harden the vitelline membrane.
So, what is the first effect of the calcium wave? Calcium wave first produces or shifts the granules of cytoplasm towards the cortex. And these granules release enzymes. And these enzymes change the property of the vitelline membrane as well as the zona pellucida. So, that is known as vitelline depolarization and it is also known as zona reaction. So, what are the two things happening here? What is happening? As soon as the plasma membrane of the male gamete fuses with the vitelline membrane, there is a production of calcium wave. And this calcium wave is having multiple effects. One of the effects is that the granules which are present in the cytoplasm now shifted to the periphery. Now they are known as cortical granules. And these cortical granules secrete some enzymes. These enzymes will change the property of the vitelline membrane as well as the zona pellucida. This is known as vitelline depolarization and zona reaction. So, by these two processes, the property of the vitelline membrane and zona pellucida has been changed. So, they are now become impermeable. They are now become impermeable for other sperms. And it avoids polyspermy. It avoids polyspermy.
So, this is one impact of the calcium wave. What is the second effect of the calcium wave? The second effect of the calcium wave is that, you know, there is a presence of nuclear material in the female which is still in the arrest of second meiotic division. So, this nuclear material completes its second meiotic division and it will form a pure haploid nuclear material and one polar body. And this polar body will go into the perivitelline space.
So, the calcium wave is first creating a kind of granules, and these granules are responsible to avoid polyspermy by changing the property of the vitelline membrane and zona pellucida. Second, this calcium wave is responsible to complete the second meiotic division of this oocyte nuclear material and it will produce a definitive haploid secondary oocyte. And this definitive haploid number of the nuclei now going to merge with this pronucleus or the male nuclear material.
The third effect is that, as we know, after the formation of the zygote, there is a very high-speed division or cleavage that is going to take place. So, for that, there is a metabolic activation occurs. Clear? So, there are three things in your exam: what are the effects of calcium wave? So, the first effect is that it produces the cortical granules, and these cortical granules prevent polyspermy by changing the property of the vitelline membrane and zona pellucida. Second effect is that it is able to initiate the arrested second meiotic division of female nuclei to complete the division, and so that you will receive a pure haploid definitive oocyte. And the third effect is that this calcium wave is responsible for metabolic activation of your ooplasm.
Now, the next comes is after fertilization, what are the results of fertilization? So, my dear students, when you are talking about the results of fertilization, the first result is that it is responsible to regain the normal diploid number of chromosomes because the two haploid cells are joining each other to form a diploid cell. So, the number of that species is regained. Second, there is a determination of the sex at the level of chromosome. How? If 22X of the female fuses with 22X of the male, then you will have a female child. But if this egg will join with 22Y of the sperm, then you will have a male child. Third, the fertilization process, of fertilization, there is a completion of the second meiotic division of your oocyte, which has been arrested for a long. And the fourth effect is that there is a metabolic activation occurs in the ooplasm, which makes this oocyte ready for cleavage. Clear? So, these are the four effects of fertilization.
But my dear students, there is one very important term, and that is known as fusion of the nuclei, which is the fourth step. Now, when we are talking about the fusion of the nuclei, there is a word that comes: pronucleus, male and female pronucleus. Now, what is this pronucleus? Now, what will happen once the male nuclear material starts to enter into the female cytoplasm, it will show some kind of swelling, and this swollen male nuclear material is known as pronucleus. Now, what is female pronucleus? Female pronucleus is formed after completion of the second meiotic division, where you will have a definitive haploid number of cells. So, male pronucleus is a swollen nucleus, and female pronucleus forms after the second meiotic division. And once the second meiotic division occurs, you will have a haploid number of nuclei in the nuclear material of the female in the ooplasm, which is labeled as a pronucleus of the female.
Then, my dear students, there is a very important concept. What will happen in the ooplasm when the haploid number of the male and haploid number of the female pronucleus join each other? There is a process which is known as DNA replication occurs. Now, this DNA replication is a compulsory step. Otherwise, what will happen? If without replication, this is having 23 chromosomes, this is having 23 chromosomes, and when these 23, 23 chromosomes arrange themselves in the center along with the spindles, and when there is splitting, what will happen? This is having a cell of 46. And when the vertical splitting without duplication occurs, again the cells will have 23, 23. So, this duplication or the DNA replication avoids this thing. So, whenever there is a DNA replication occurs after the mitosis, the cells always will have 46, 46. If this replication will not occur, as soon as the pronuclei are going to mix with each other, as soon as the male and female nuclear material and chromosomes align themselves on the spindles, there has to be a DNA replication. So, when there is a splitting of nuclear material is taking place, initially at the first part of the division, there has to be 46, 46 chromosomes in both the cells which are deriving by the first cleavage.
So, now, how to write down in the exam? First, you have to write down about the definition, then what is the site of implantation. These are the two steps of priming: capacitation and acrosomal reaction. Then there are four steps of fertilization: that means penetration of corona radiata, zona pellucida, vitelline membrane, and then fusion of the pronuclei. Then what is the effect of the calcium wave? So, calcium wave is producing three effects: first is the change in the property of your zona pellucida and vitelline membrane. Then the calcium wave is also responsible to complete the second meiotic division. And the last effect is metabolic action. What are the effects of fertilization? And lastly, a flowchart.
Now, how to make a flowchart? That you can see here. How to make a flowchart? I have started the flowchart from the entry of your semen into the female genital tract. So, once you will have the deposition of the spermatozoa inside the female genital tract, there is a reaction that occurs. So, the next step is the uterine contraction starts because of the release of oxytocin. And this uterine contraction aspirates the male gametes inside the uterine cavity. And there is a capacitation occurs, that means the conditioning of the spermatozoa will undergo. And once the capacitation is taking place, now the sperm is also having the property of acrosomal reaction. So, now it will reach to the ampulla of your fallopian tube, as soon as where you will have the initiation of acrosomal reaction. And this acrosomal reaction helps to penetrate all the three barriers. And after piercing the three barriers, what will happen? The head of the sperm plasma membrane merges or fuses with the vitelline membrane and it will stimulate the calcium wave. And the calcium wave will change the property of your vitelline membrane as well as zona pellucida to avoid polyspermy. And then there is a fusion of the male and female pronuclei, and you will have the formation of a diploid zygote. Clear?
So, my dear students, you have to draw this flowchart in your exam. Whenever you are writing about fertilization, you can draw this. One more diagram of your secondary oocyte, where you have to draw the three layers: one is corona radiata, zona pellucida, and vitelline membrane. And there is a perivitelline space which is having one polar body. This one polar body is before fertilization. And this is your nuclear material. Once the fertilization occurs, the number of polar bodies will increase because the fertilization occurs once the second meiotic division completes. So, the second meiotic division also produces a polar body. So, in this perivitelline space, you have more than one polar body. Clear?
So, my dear students, now at the end of this lecture, I hope you have the idea how to write down fertilization in the exam. You have to start with your definition. Then you should not forget to write some very important words in your short note, like capacitation, like acrosome reaction, like the steps of fertilization, what is the calcium wave, what are the effects of calcium wave. And I am suggesting you that always draw the flowchart in your exam, step by step, showing the fertilization. So, this is all for the lecture. Thank you.