Welcome back to Anatomy and Physiology on Catalyst University. My name is Kevin Tokoff. Please make sure to like this video and subscribe to my channel for future videos and notifications. At the end of the previous video, I left you with some thoughts on males versus females in terms of simplicity.
And I think we can all agree that men are simple creatures. We are. And even down to the biological processes like spermatogenesis, which is male gametogenesis, and what we're going to talk about here which is oogenesis or female gematogenesis. And over here on the left, this side of the diagram, this is spermatogenesis, what we talked about.
And we went from a spermatogonium to a primary spermatocyte and then meiosis I to secondary spermatocytes, meiosis II to spermatids, and then maturation to mature sperm cells or spermatozoa. And the other thing that you should know about this is this process takes place in one fell swoop. There's no stops. Now, it does take a little bit of time, but there's no significant periods of time where this process just stops or halts altogether. It's kind of like if you're late to school and you got an exam and you're driving from home, you're not going to stop on the way to get some breakfast, okay?
You're probably just going to get in your car and speed all the way to school and not make one stop until you get in there to take your test, okay? That's spermatogenesis. In contrast, oogenesis is kind of like if you...
had a few days to spare on a vacation, and you were just driving around seeing some scenic routes, kind of like how they did it in the movie Vacation with Chevy Chase. You know, they're going from Chicago all the way to Wally World, but they stop in various places like Dodge City and so on and so forth. It's very scenic.
There's all sorts of points where they stop and take their time. But the other thing about oogenesis is that, notice, we go from an oogonium down to just one ovum, okay? We don't get four of them. Actually, what we're going to see is that some of these cells are viable, that is the ovum, and others are not. And these ones that are not, we're going to see are termed polar bodies.
So make sure you understand that other than obvious differences, there are some very subtle ones between these two types of gametogenesis. In females, not all of the daughter cells are viable. Some of them, like polar bodies, are not used, and they're essentially degraded now. The other thing is that there are going to be significant periods of time where we halt the cell cycle during some point in meiosis. And it can remain that way for years, sometimes even decades.
So this is a very slow process relative to spermatogenesis. And another big difference between spermatogenesis and oogenesis is in the strategy for gamete making. In men, men just kind of...
make a bunch of sperm cells and then just like okay here's a billion of them right here and let's hope one of them works okay and presumably you know the weaker ones will be weeded out by natural selection inside the female reproductive tract but one of them hopefully works but we're just going to fling a billion at once okay the female strategy is very different rather than making a billion eggs the female is going to focus all its attention on making one all the others are going to die. They're not even going to be used. And the only one strongest egg is going to be the one that has the potential to be fertilized. That's the one that can be ovulated. Now the other thing about oogenesis that's also important to understand is we often throw around this term the egg.
And depending on what source you're looking at, the egg can mean different things. In some cases, the egg can mean the ovum. In some cases, it's the ootid.
In some cases, it's a secondary oocyte. There's not really a clear idea what the egg is. And there's also a lot of confusion on the stages.
If you look this up on any Google images or go to different websites, there's a lot of very misleading things online. So hopefully I can clear up a lot of that stuff in this video. So without further ado, let's go and look at oogenesis.
Oogenesis is going to start off very similar to spermatogenesis. In spermatogenesis, we start with a spermatogonium. Here we're going to start with an oogonium and that oogonium is going to divide. Now this is what we call an asymmetric division.
This is the same thing that happened in spermatogenesis. It's going to divide asymmetrically. What that means is one of the daughter cells is going to become what we call a primary oocyte.
The other one is going to replenish the stock of oogonium. So we still have some oogoniums to make more primary oocytes. So the one that becomes the primary oocyte after it divides, it's going to go through some differentiation events and maturation events, which ultimately get it into this primary oocyte.
Now, this primary oocyte is going to begin meiosis 1. Emphasis on begin. It's not going to complete meiosis 1. In fact, it's not even going to get very far at all. It's only going to get to prophase 1. Remember, out of all the stages of meiosis, prophase, metaphase, anaphase, and telophase, prophase is the first of the four.
So it's not getting very far at all. This primary oocyte is going to be stuck somewhere in here in prophase 1. And this event occurs. occurs well before puberty. So by the time the individual, the female that is, is at the pubescent age where they are fertile and they can begin menstruating and they can get pregnant, they already have a lot of these primary oocytes that are all arrested in prophase 1. And that's what we call meiotic arrest.
It's stuck there in prophase 1. Now at some point, that secondary oocyte, which is still in prophase 1, is going to get the signal to complete meiosis 1. Now, when that primary oocyte completes meiosis one, you're going to get two daughter cells. However, this is another example of an asymmetric division. One of those cells is going to be the one that continues, and that's the secondary oocyte, which is now a haploid cell. And the other cell, which is kind of a piece of garbage, so to speak, is called the polar body. okay the female strategy is to put all the energy into one cell and to get that cell up for fertilization.
So that means that when this primary oocyte divides via meiosis 1, all the cytoplasm is going to go toward the secondary oocyte. All the goodies are going to be put in the secondary oocyte and then very little is going to go into the polar body. Now the only thing that's going to end up in the polar body is one copy of every chromosome.
Okay, so that's how this process reduces the chromosome number by half. So some of the DNA ends up in here in the polar body. But all the goodies, all that stuff winds up here in the secondary oocyte, okay? And then that secondary oocyte is going to begin meiosis II, but it's going to be arrested again, okay?
So this is actually going to be meiotic arrest II. I'm actually going to put that up here so we can see it. Meiotic arrest II, and it's going to be arrested in metaphase II this time. So this secondary oocyte will actually go through prophase II this time.
and then it will start metaphase two and then it will just get stuck there. So this is the second meiotic arrest. So what we have here is a secondary oocyte that is stuck in meiosis two. It's arrested in meiosis two.
Now that is significant because contrary to popular belief, it's actually not a mature ovum that's ovulated. It's actually only a secondary oocyte that has not actually finished meiosis II. It's stuck in metaphase II in the second meiotic arrest. That secondary oocyte is ovulated.
And again, we haven't really talked about where it gets ovulated to where fertilization occurs. That's not important now. What is important is that if a sperm cell manages to make its way through the reproductive tract, it can then fertilize that secondary oocyte.
Now, the next few steps are going to occur very quickly. Okay. So whenever this sperm cell penetrates the secondary oocyte.
That process is called fertilization. That's all it is. It's not dumping the DNA or the contents of the sperm into the secondary oocyte. It's literally just penetrating the wall of the secondary oocyte. That's all fertilization is.
It has not yet dumped in its DNA. That comes a little bit later. So then, of course, after fertilization, Yes, there is a time.
very soon after that when the sperm cell injects, so to speak, its DNA into the secondary oocyte. Once that occurs, only then the secondary oocyte completes meiosis II. So that's a little bit unintuitive. It's not actually until the sperm cell dumps its DNA into the secondary oocyte that the secondary oocyte completes meiosis II.
Remember, it's still arrested, but once the DNA enters it from the sperm cell then it completes meiosis too. And we can see here that we produce a polar body. Again, the polar body doesn't get any of the goodies, but it does get one half of the DNA. And then again, we get this structure, which is called an ootid. Now the ootid, all it is, is the equivalent of the spermatid.
Remember the spermatids and spermatogenesis came before the spermatozoa. The ootid exists for an extremely short period of time. In fact, some sources don't even include it because it's such a short period of time.
But basically, when the secondary oocyte completes meiosis II, it's very briefly this one structure called an ootid, but it will then undergo rapid maturation into the ovum. Now, here's the kicker. When we look at an ootid or an ovum, doesn't matter what we're talking about, they have separate DNA, and these DNA are present as what we call a pronucleus. So the male DNA is contained in what we call a male pronucleus, and the female DNA is in the female pronucleus.
The reason they're not true nuclei yet, and they're called pronuclei, is because the male pronucleus contains only the male DNA, and the female pronucleus contains only a female DNA, okay? But there's other maturation processes that convert the ootid into an ovum. But here's the thing about the ovum. Even in that form, the pronuclei have not yet fused. So that's the key point about the ovum.
The male and female pronuclei are separate. They have not yet fused. However, at some point, there is the event in which the pronuclei do fuse and they form a mature nucleus. And when that occurs, you now have the zygote, which will then grow and undergo cleavage events and all that good stuff that we can talk about later. The zygote is the cell from which all of your cells came from, okay?
Not the ovum. The ovum, these pronuclei have not yet fused. And to give you a good understanding of what that actually looks like.
This right here is actually an ovum. Notice here there's two pronuclei. Now I obviously can't tell which one's which. One of them would be male, the other would be female.
But notice they're about to fuse. They haven't yet fused, but they're right on the verge of that. So this right here is not a zygote yet.
Even though it has all the DNA of the male and female, that DNA is technically separate in separate pronuclei. So it is not yet mature, it is still an ovum. However, once they fuse into a mature nucleus, then you have a zygote and then the zygote can form all the other cells. Okay, but hopefully I cleared up for you all the different pieces of oogenesis and it makes sense to you.
Please make sure to like this video and subscribe to my channel for future videos and notifications. Thank you very much.