Development of Animal Gametes: Study with Video Lessons, Practice Problems & Examples

Gametogenesis involves the formation of male and female gametes through spermatogenesis and oogenesis, respectively. Spermatogenesis produces four haploid sperm from diploid primordial germ cells via mitosis and meiosis, culminating in mature sperm capable of fertilization. In contrast, oogenesis results in one large haploid ovum and polar bodies, with primary oocytes formed before birth and completing meiosis only upon fertilization. This highlights the energy investment in female gamete production compared to males, emphasizing the complexity and differences in reproductive strategies.

Gamete Development

Hello, everyone. In this lesson, we are going to be talking about the development of animal gametes. So we're talking about gametogenesis. And specifically, we're going to be talking about the creation of sperm, spermatogenesis, and the creation of eggs, oogenesis. First off, we're going to start with the production of sperm, spermatogenesis. But I want you guys to know that in animals there are very specific developmental systems that create sperm and egg. And it's actually a pretty complex process. So let's start off with Spermatogenesis, which is going to be the process that creates the male gametes. So creates the male gametes, and we all know that the male gametes are going to be sperm. And how does this all begin? Well, I have bolded the cells that I want you guys to know. The first cell that becomes important in this step is going to be the diploid primordial germ cells. The diploid primordial germ cells, you can pretty much think of these as like stem cells that are able to continuously divide to give cells to the process of spermatogenesis, and they are going to be found in the seminiferous tubules of the testes. Testes. In fact, this entire process is going to happen in the seminiferous tubules of the testes and the epididymis of the testes. Okay? So the Diploid Primordial Germ Cells is going to be the beginning of this entire process, and it is going to undergo mitosis to create the spermatogonia. Now, the diploid primordial germ cells have the 2 ploidy because they are diploid. They have 2 sets of chromosomes. And if they create these spermatogonia via mitosis, that means that the spermatogonia are also diploid. Because mitosis makes cells that are exactly the same as the parent cell. So spermatogonia are formed via the mitotic division of these primordial germ cells. Now, these spermatogonia are then going to go on to create these primary spermatocytes. And these Primary Spermatocytes are also going to be made from these Spermatogonia via the process of Mitosis. So since they are made via the process of Mitosis, Primary Spermatocytes are also going to be diploid cells. Now, this is where the process of meiosis is going to begin. The Primary Spermatocytes are going to undergo one round of meiosis, so they're going to go through meiosis 1 to create these secondary Spermatocytes. So since the primary Spermatocytes underwent Meiosis 1 to create the secondary Spermatocytes, what's going to be the ploidy of the secondary Spermatocytes? They are going to be haploid because all of the cells that are products of meiosis 1 only have one set of chromosomes, so they are going to be haploid cells. Okay? Alright. So the secondary spermatocytes are 2 haploid cells formed via meiosis 1. And then the secondary Spermatocytes are gonna undergo Meiosis 2, and they are going to create these cells called Spermatids. Now, these Spermatid cells are also gonna be haploid because they are a product of meiosis. Now, each of those 2 secondary spermatocytes are going to make 2 spermatids. Now, you may be thinking, okay, we've gotten to spermatids. Or is that another word for sperm? Actually, no, it's not. These are going to be immature sperm. So they are non motile, they cannot swim yet, and they have no acrosome. So these are the immature sperm. They're not ready to be utilized to fertilize an egg quite yet. There's actually another step that they have to go through. So these spermatids are non motile. They don't have their tails. They cannot swim, and they don't have an acrosome, which is going to be the cap on the head of the sperm. And this cap on the head of the sperm is utilized to actually penetrate into the egg. So those are very important qualifications, and they are going to get these qualifications once they go through the process of Spermiogenesis. Spermiogenesis is basically like a maturation stage. These Spermatids are going to undergo maturation, and they are going to turn into Sperm cells. And these are going to be fully developed Sperm cells. And that means that they are going to be motile and have an acrosome. So once the sperm have reached this point they are completely ready to go and they can now fertilize an egg if need be. And that is going to be the process of Spermatogenesis. It's not as simple as we may think. And remember, guys, that sperm is haploid. Okay? All right, so now let's go down to a figure that might be helpful, might be easier to understand than all of those words that I said right there. Okay? So let's just start again and then we'll go through this diagram so it may help you guys remember this information. Okay, so you're going to have the Spermatogonium. And the Spermatogonium is going to be a diploid cell that was created from the Primordial Germ cells. We're going to have the Primordial Germ Cell, and the Primordial Germ Cell is going to create the Spermatogonia via the process of Mitosis. And remember the Primordial Germ Cell is Diploid. Okay? So the Spermatogonium is going to be created via the process of Mitosis, and the Spermatogonium is going to divide and it's going to create the Primary Spermatocyte. The Primary Spermatocytes are going to be diploid as well because they are also made via the process of mitosis from the spermatogonium, and since the process of mitosis creates genetically identical daughter cells, these Spermatocytes are also diploid. Now, this is where it gets interesting. The Spermatocytes, the primary Spermatocytes or Spermatocyte number 1 are going to undergo Meiosis 1. The first stage in Meiosis. So these primary Spermatocytes are going to then go through Meiosis 1 to create these cells called Secondary Spermatocytes that are haploid because the products of meiosis 1 are haploid. So now, they only put this section because it's going to get repetitive. Basically, all of or the secondary Spermatocytes are going to divide like this. So the secondary Spermatocytes are then going to undergo Meiosis 2, and they are going to create, just so you guys know this says Spermatoid. Spermatids means the same exact thing. So they are going to create these Spermatids which are also Haploids. These are Spermatids. Now, remember that technically we're done with the cell divisions, but we're not done with the creation of sperm itself. And, in fact, you're going to have to go through the maturation stage to generate the sperm that are motile and actually able to fertilize an egg. So these guys right here are going to be our fully mature sperm, and they are going to be created via the process of Spermiogenesis. And this is going to happen, and this is going to give the acrosomal cap to the end of the head of the sperm and it's going to create the tail of the sperm so that the sperm are able to move and swim around and they are able to fertilize the eggs. So this is just a diagram of what I said above. Hopefully, this is helpful. I like visual representations. They make it easier for me to understand what's going on. Okay, guys. So now let's go down and let's talk about oogenesis. So oogenesis is going to be the process of the creation of the female gametes, and those are going to be eggs. Now, oogenesis is very similar to spermatogenesis in the different cell types that it has, and the names of the cells that we have given them, but it is far, far, far more complicated, and it is a little confusing, I think. And oogenesis is going to be different from spermatogenesis in 1, it's pretty complicated. 2, it is going to have periods of rest or halt. The process stalls a lot, and this is going to be dealing with the coming to age of puberty, this is going to be dealing with ovulation, this is going to be dealing with pregnancy, this is going to be dealing with menstruation. You're going to see that the creation of these eggs and the development of these eggs is going to start then stop, start then stop, start then stop. And, guys, oogenesis is going to be really interesting because all of the oocytes, the primary oocytes that we will talk about in a second, for a female are going to be created even before she is born. But they don't finish developing until they are fertilized by a sperm, which is really crazy. So and then females don't continue to make more eggs. Once they're done with the amount of eggs that they have, once they've used them all up, they will no longer create gametes. This is different than males. Spermatogenesis, after puberty begins, is going to continue for the rest of his life. So there are some major differences between these two forms of gametogenesis. But, guys, let's get into talking about oogenesis, and I will try to make this as easy as I can. But don't feel bad, it can be kind of confusing. Okay. So, again, we're going to start off with a Diploid Primordial Germ Cell, just as we did in Spermatogenesis. And these are going to be diploid, so they're going to be 2 in. They're going to have 2 sets of chromosomes. Again, guys, think of these kind of like stem cells that are able to divide to create the cells for this process. And these are going to be found in the ovaries, which are the female gonads. Now, the Diploid Primordial Germ cells are going to undergo mitosis to create the oogonia. The oogonia are going to be important cells that are going to then create the Primary oocyte. But because the oogonia are made via Mitosis they are also Diploid cells. Okay? Now, the oogonia are then going to create the Primary oocytes, also via the process of mitosis. So the Primary oocytes are going to be Diploid as well. Now, these are going to be the cells that begin the process of meiosis. These cells, the primary oocytes, are going to undergo the first round of meiosis. Now, just so you guys know, all of the primary oocytes for a female have been made before she is born. So every single one of her egg cells that every single one of these cells that will become an egg has been turned into a primary oocyte at this time before she is born, which is pretty crazy. And then it is going to go into prophase 1. So the primary oocyte is going to begin meiosis 1, but it is not going to finish meiosis 1 until the day of ovulation, which can be years after the female is born. I believe the first age of ovulation for most females is between the ages of 12 14. So those primary oocytes are waiting in the process of meiosis 1 for years at a time. So the Primary oocytes are gonna go through Meiosis 1. Now, they are going to halt in Meiosis 1 until ovulation. And then, the secondary oocyte in the 1st polar body are going to be made via Meiosis 1 from the Primary oocyte. Okay? And this is going to happen at ovulation. Let me write this down for you guys. So this happens at ovulation. And let me write this down for you guys. This happens, the creation of primary oocytes happens before birth, generally, or very very soon after birth. Okay? So now that the the follicle has decided that this particular egg or this particular cell is going to develop and be ovulated, that primary oocyte, when it is going to be ovulated, it will finish meiosis 1 and it's going to create this secondary oocyte and this first polar body. Now, because they are made via meiosis 1, the secondary oocyte and the 1st polar body are going to be haploid. Okay? Because they are the products of meiosis 1. Now, this is where it becomes kind of interesting. In Spermatogenesis, the cells are evenly made, but in oogenesis, they're going to be unevenly made. Whenever the Primary Oocyte divides into the Secondary Oocyte and the 1st polar body, the Secondary Oocyte is going to be so much larger than the First Polar Body. And that is going to be because the First Polar Body is going to be discarded. It is not going to be used. It will be recycled inside of the Female, and the only usable cell is gonna be the secondary oocyte. The secondary oocyte is going to go through meiosis 2. So, the secondary oocyte is going to be ovulated, Okay? So Secondary Oocyte is going to be ovulated, and then it is going to enter the Fallopian Tubes. Now, it's going to undergo Meiosis 2 to finish the production and create an egg, but only if it is fertilized, which is crazy. So, technically, the cell that is fertilized by the egg is the secondary oocyte, And the secondary oocyte only finishes Meiosis 2 after the sperm has already engaged and penetrated into the egg. So, Meiosis 2 occurs in the secondary oocyte to create another unequal division of cytoplasm to create the ovum and the secondary polar bodies. So the ovum is gonna be what we think of as the egg, and the secondary polar body is going to be another cell that is going to be discarded. Basically, we don't use the polar bodies whatsoever. They're going to be discarded. Now, the Ovum is going to be the Mature Female Gamite that is haploid. So both of those cells are going to be haploid. Now, the ovum is going to be made and it's going to be a gigantic cell. And this cell is the largest cell in the human body, male or female. It is the largest cell in the human body. I believe, if I am correct, it's 10000 times the size of a sperm, and sometimes we can even see that particular cell without a microscope. It is so big, which is insane. But it has to be so big to have all of the nutrients, the cytoplasm, the organelles, the cytoplasmic determinants for the zygote. So that one cell basically holds everything except the father's genetic material to create a new being. So it's going to be quite large. It's going to need all of those things. So then the ovum, which is fertilized, will then develop into the zygote, and into the baby eventually. Okay? So, I'm going to go through this again, like I did with Spermatogenesis, but in a figure because sometimes it's just easier to see it, in the figure. But I wanted to write for you guys, just so you guys know, meiosis 2 only finishes after fertilization. So, meiosis 2 only finishes after fertilization. Do you guys know why this might happen? Why doesn't the secondary oocyte just go through meiosis 2 to create the egg, and then the egg can wait around to be fertilized? Well, they think that it's because why go through the trouble if the cell isn't gonna be fertilized anyways? Then there's no point in going through the second round of meiosis. There's just no point. But if the sperm makes contact with the secondary oocyte and begins to fertilize it, then it will finish the process of Meiosis II, and it will create the egg, which within the egg's genetic material will fuse with the Sperm's genetic material and a Zygote will be made. Okay, guys? Alright. So, let's have a look at this figure, which is going to be another representation of what I already told you guys. So we're gonna have the Primary oocyte. The Primary oocyte is going to go through some maturation processes, similar to the maturation that the sperm had to go through. And remember that the Primary oocyte is going to undergo meiosis 1 to create the secondary oocytes. We have, let me write this, we have Meiosis 1. Now, remember the primary oocyte is going to be Diploid, and whenever it goes through Meiosis 1, it is going to create the secondary oocyte and the 1st polar body, both of which are going to be haploid. You guys can see that the secondary oocyte is a lot bigger than the 1st polar body. And the secondary oocyte is kept, and generally the first polar body is discarded. You may see that it goes through meiosis 2 as well. Sometimes this happens, sometimes it doesn't happen. All I want you guys to know is that all of the polar bodies will be discarded. They're not going to be utilized to create an egg or a zygote. They're just not utilized. So the secondary oocyte is going to go through Meiosis 2. And once it goes through Meiosis 2, it is going to create a haploid mature egg, or ovum, and it's going to create another polar body. So all of these polar bodies are going to be discarded. So all of these guys are going to be discarded, and they will not be utilized. So, what's another difference here that we can see between Spermatogenesis and oogenesis? What do you get from spermatogenesis? You should get 4 sperm. Right? Because you had 4 cells that were made at the end of gametogenesis or Meiosis 2. But what do you get out of oogenesis? You get one cell, and this is it. One cell is all you create via the whole process of gametogenesis, you get one egg cell. So, obviously, you can see that whenever they talk about reproductive energy, the female puts so much more energy into each of her gametes in comparison to the male, because she only makes one with each round of oogenesis, while males make 4 with each round of spermatogenesis. And, I mean, she makes the largest egg she makes the largest cell in the entire human body, that's pretty impressive as well. But this is going to be the basic understanding that I want you guys to know about the process of oogenesis. If you guys want to know when those steps happen, I mean, like, at ovulation or at fertilization, you guys can learn that as well, but I don't think that it is going to be needed. What I think they want you to know is how these cells are going to divide, what types of cells are they going to create, which ones are gonna be discarded, which ones are gonna be kept

Horse diploid cells contain 64 chromosomes (2n = 64). How many chromosomes will be present in spermatogonium cells?

• A. 32
• B. 64
• C. 128
• D. 40

Horse diploid cells contain 64 chromosomes (2n=64). How many chromosomes will be present in primary oocyte cells?

• A. 32
• B. 64
• C. 128
• D. 40

Horse diploid cells contain 64 chromosomes (2n=64). How many chromosomes will be present in spermatids cells?

• A. 32
• B. 64
• C. 128
• D. 40

What is the correct term for the mature female gamete?

• A. Ovum
• B. Secondary polar body
• C. Secondary oocyte
• D. Oogonia

Here’s what students ask on this topic:

What is the process of spermatogenesis and how does it differ from oogenesis?

Spermatogenesis is the process by which male gametes, or sperm, are produced in the seminiferous tubules of the testes. It begins with diploid primordial germ cells that undergo mitosis to form spermatogonia. These spermatogonia then undergo mitosis to form primary spermatocytes, which are also diploid. The primary spermatocytes undergo meiosis I to form haploid secondary spermatocytes, which then undergo meiosis II to form spermatids. Spermatids mature into motile sperm through spermiogenesis. In contrast, oogenesis is the process of forming female gametes, or eggs, in the ovaries. It starts with diploid primordial germ cells that form oogonia via mitosis. Oogonia then form primary oocytes, which begin meiosis I but halt until ovulation. Upon ovulation, meiosis I completes, forming a secondary oocyte and a polar body. The secondary oocyte only completes meiosis II if fertilized, resulting in one large ovum and additional polar bodies. Unlike spermatogenesis, which produces four sperm, oogenesis results in one ovum and several discarded polar bodies.

What are the stages of spermatogenesis?

Spermatogenesis involves several stages:

1. **Primordial Germ Cells**: Diploid cells that undergo mitosis to form spermatogonia.

2. **Spermatogonia**: Diploid cells that divide by mitosis to produce primary spermatocytes.

3. **Primary Spermatocytes**: Diploid cells that undergo meiosis I to form haploid secondary spermatocytes.

4. **Secondary Spermatocytes**: Haploid cells that undergo meiosis II to produce spermatids.

5. **Spermatids**: Immature haploid cells that undergo spermiogenesis to become mature, motile sperm.

6. **Spermatozoa**: Fully mature sperm capable of fertilizing an egg.

How does the process of oogenesis differ from spermatogenesis in terms of timing and cell division?

Oogenesis and spermatogenesis differ significantly in timing and cell division. Oogenesis begins before birth, with primary oocytes formed during fetal development. These primary oocytes enter meiosis I but halt at prophase I until puberty. During each menstrual cycle, one primary oocyte completes meiosis I to form a secondary oocyte and a polar body. The secondary oocyte begins meiosis II but halts at metaphase II until fertilization. If fertilized, it completes meiosis II to form an ovum and another polar body. In contrast, spermatogenesis begins at puberty and continues throughout a male's life. It involves continuous mitotic and meiotic divisions, producing four sperm from each spermatogonium. Oogenesis results in one large ovum and polar bodies, while spermatogenesis produces four sperm.

What are the key differences between primary and secondary oocytes?

Primary oocytes are diploid cells formed from oogonia via mitosis before birth. They begin meiosis I but halt at prophase I until puberty. During ovulation, a primary oocyte completes meiosis I to form a haploid secondary oocyte and a polar body. Secondary oocytes are haploid cells that begin meiosis II but halt at metaphase II until fertilization. If fertilized, the secondary oocyte completes meiosis II to form an ovum and another polar body. The primary oocyte is larger and contains more cytoplasm than the secondary oocyte, which is smaller and destined to be either fertilized or discarded if not fertilized.

Why do females produce polar bodies during oogenesis?

Females produce polar bodies during oogenesis to ensure that the resulting ovum has sufficient cytoplasm and organelles to support early embryonic development. During meiosis I and II, the cytoplasm is unevenly divided, with most of it retained in the secondary oocyte and later the ovum. The smaller polar bodies, which contain minimal cytoplasm, are discarded. This asymmetrical division maximizes the resources available to the ovum, enhancing its chances of successful fertilization and subsequent development into a zygote. Polar bodies serve to discard the extra haploid sets of chromosomes, ensuring the ovum remains haploid.