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Egg, sperm, and fertilization

Sperm and egg cells are special sex cells for reproduction. Sperm cells are small, have a tail for movement, and carry male genetic material. The egg is large, round, and has a protective layer called the zona pellucida. During fertilization, sperm meets egg, and their genetic material combines. Created by Jeff Otjen.

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Video transcript

- [Voiceover] Organisms that reproduce sexually have got to get their genes together somehow. To do this, they package their genetic material into specialized cells called sex cells. This is the sperm cell. It's the sex cell of the male. The sole purpose of the sperm, the entire reason for its existence, is to transfer the male's genetic material into the female sex cell or the egg. So the sperm cell is packed with features that allow it to fulfill its job. It's basically a little torpedo. You can see, just like a torpedo, it has a pointed head which allows it to travel in the forward direction. At the back end it has the tail, and the tail is just a flagellum, and as the flagellum spins it acts like a little propeller. And then it has a middle section. Now within that middle section, wrapped around the base of the flagellar tail, are all of these little organelles called mitochondria. You can see I'm drawing these mitochondria wrapped around the flagellar base here. And mitochondria are organelles that are responsible for giving energy to a cell. And the sperm cell has a whole bunch packed right into the base of the flagellum here. Probably 75 to 100, and they're quite large mitochondria. In fact, these mitochondria are often fused together to create big organelles. And the reason is because in order to propel the sperm torpedo towards the egg, it needs a lot of energy. And that all comes from these mitochondria. Now the payload of the torpedo is here in the head, and that's the genetic material, our DNA within the nucleus. I'll draw it kind of all coiled back here. And here's the nuclear envelope here. And just like any regular torpedo, the sperm torpedo has a warhead right in the front. And that warhead is a little collection of enzymes called the acrosome. And the acrosome is gonna be important later on for fertilization. But that's pretty much it. There's a tail at the back to provide propulsion, some mitochondria in the middle section to give energy to the tail, a head which contains the nuclear material, and the acrosome. This is a pretty bare bones cell. It's designed to move fast and to get to the egg. There are no bells and whistles here. Now that's in contrast to the egg cell. Now the first thing you'll notice is that the egg cell is round, unlike the torpedo shape of the sperm cell. This is not a cell that's made for active mobility. The second thing is that the egg cell is huge compared to the sperm cell. It's so big, in fact, that it's almost visible. In fact, sometimes it is visible to the human eye. Now compared to the sperm cell, which I'll draw in here, the egg cell is about 10,000 times more massive. And similar to the sperm cell, the egg cell has its share of genetic material ready to combine during fertilization. You can see it here within the nucleus. And so you may have noticed this thick outer coating on the egg cell here, that's a very important structure called the zona pellucida. And the zona pellucida is a thick layer of glycoproteins that sit on the outside of the egg. And glycoproteins are basically a protein, I'll draw a protein here in green, with a whole bunch of branching sugar chains that are coming off of them. And so what this looks like is basically a little tree or a long branching thing that's growing out of the edge of the egg cell. And there's a whole bunch of them, and they form this very thick kind of protective layer that the sperm has to get through. And the edge of the egg cell is the actual plasma membrane. And once the sperm can deliver its genetic material beyond that, fertilization has occurred. Now there's a whole bunch of other structures within the egg cytoplasm as well. And remember this thing is huge. And I'm gonna draw in a few here in green. Now what I'm drawing in are actually more mitochondria. Now remember the egg cell had 75 to 100 big mitochondria right at the base of the flagellum to provide energy to drive locomotion. Well, the egg cell has mitochondria too. It's got a lot of other different organelles as well. But the egg cell is so large, it's got somewhere between 100 and 200 thousand mitochondria present. So keep those mitochondria in mind, we'll talk about them a little bit later in the next section. So now that you've met the two major players here, the sperm and the egg, or the male and female sex cells respectively, we can talk about what happens when they meet. And that process is called fertilization. So we'll go ahead and label our egg down here. And we'll put a quick label on this as well. This is the zona pellucida that we talked about earlier. So we'll draw the sperm here coming in to meet the egg. We'll draw its tail. We'll draw its middle section here. And we'll draw its torpedo-like head here. Get rid of all this zona pellucida glycoprotein in the middle here. Now here are the mitochondria in the middle section here, and we have the genetic material payload of our sperm torpedo here in the back, and our acrosome here in the front. Now the first thing that happens during fertilization is that the sperm comes into contact with the zona pellucida. And the zona pellucida actually binds to the outside of the sperm, and that's called sperm binding. And it's step number one. Now what happens when the sperm gets bound to the zona pellucida is that that sets up a reaction called the acrosomal reaction. So step number two is called the acrosome reaction. And that little warhead tip of the sperm torpedo gets released, and so we have all of the acrosomal enzymes that were sitting in the head that just kind of leak out into that zona pellucida. And as those enzymes leak out, they actually start to digest away the zona pellucida. Here you can see I'm kind of eating away here at those glycoprotein, and that allows the sperm head to dive in deeper towards our plasma membrane. Now as the sperm gets closer to the plasma membrane of the egg, and it comes in contact right here, it starts up a process of binding. The two touch and they come together. And as they start together, it causes another reaction, and that third reaction is called cortical reaction. And what I haven't drawn here is another structure in the egg, and those structures are right underneath the plasma membrane, and they just sit there waiting. And they wait and their entire job is to wait for a sperm to bind. And as soon as a single sperm binds, they get ejected out into that zone pellucida as well. Just like the acrosomal enzymes, these enzymes that are contained within the cortical granules also start eating away at the zona pellucida. These enzymes eat away, and they dissolve and chew up these glycoproteins, but specifically they chew up the glycoprotein that allows sperm to bind. So at this point we have a single sperm that's bound, set off the cortical reaction, and these cortical granules are released that chew up all the other places that more sperm can bind. So as our other sperm torpedoes are coming in, they're just bouncing off. They hit the glycoproteins, but the one that they need to bind to isn't there because it's all been chewed up and degraded by these cortical granules. So that's actually called a block to polyspermy. Now that's a very important concept. Polyspermy is a term that just means multiple sperm. And what we don't want is for more than one sperm to inject its nuclear material, its DNA into this egg. What you'd end up with is an egg with a single contribution from mom and then one or two or three or 100 contributions or genetic material from dad. And that would never work, you'd end up with all sorts of problems as the egg started to divide. Occasionally, that does happen, and it can result in a zygote that fails. But for the most part, as these cortical granules dissolve away all the sperm binding glycoproteins of the zona pellucida, other sperm just can't get in and they bounce off as they arrive. So now we have a sperm that's made its way all the way to the plasma membrane of our egg cell. It's started to bind to the plasma membrane, the acrosome is gone, I'll erase that here. It's done its job. The cortical granules have been released, and they're preventing other sperm from getting in. And we start to actually fuse our plasma membranes of our sperm cell and our egg cell. And that allows for this entire structure to come in. All of the genetic material here within the, all of the genetic material within the nucleus of the sperm cell can start to come out and get released here into the egg. And once we have fusion of genetic material, that is fertilization. So just to recap, we'll go back to look at our close-up of our sperm. We can see that it's a very mobile structure evolved basically to get genetic material from the male to the female egg cell. It's got a tail that propulses it, it's got mitochondria that feed it energy, it's got a head with a payload of nuclear material and an acrosome warhead on the tip. The egg cell is a giant cell by comparison. It's got a specialized layer of glycoproteins on the outside that have a bunch of specialized features and then a bunch of cytoplasm, including mitochondria. And then the process of egg meeting sperm itself is called fertilization. Sperm binds to the zona pellucida, the glycoproteins, you have an acrosomal reaction, and then a cortical reaction prevents more than one sperm getting in. And then material, the genetic material of the sperm is transferred. Now you'll notice here that I drew the genetic material from the nucleus coming in. Now some of you may be wondering, "Well don't mitochondria have genetic material as well?" Well, that's true. Mitochondria do have mitochondrial DNA. And potentially, some of these mitochondria can get sucked in during that genetic transfer process as well. But remember, our egg cell had 100 to 200 thousand mitochondrial copies, and our sperm cell only had 75 to 100. Now there's a little bit of debate, but in the end the male contributes essentially no mitochondria to the zygote that's formed after the egg and sperm fuse. Now, it could be that some of those mitochondria actually do make their way in and then are degraded. We're not really sure. But given just the numbers, statistically with one to two hundred thousand versus only 75 to 100, nearly all of the genetic material from the mitochondria is gonna be from the mother anyway.