Biology: Animal Development: Fertilization Events

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Let's take a look at the events that occur leading up to an embryo. In other words, let's take a look at the events of fertilization and the first initial steps towards embryogenesis. This is going to give us insight into animals and the entire system that they're going to eventually develop.
Now, it's very interesting, and we could spend hours, literally, looking at the events in different organisms. We could spend a lot of time on one group and then move to the mammals. We could just do the mammals. But you want to know something very interesting? A lot of our knowledge of development is based on sea urchins. In fact, some of the events I'm about to describe to you happen in sea urchin eggs. We're not even going to spend time on what happens in mammal eggs at this point, because the events are in many ways so similar. Why would that be? Why would your fertilization events be similar to a sea urchin's? Well, remember that phylogenetic tree. Remember the fact that echinoderms and chordata are both on that last branch of the tree. They share a lot of evolutionary ancestry. And since they share a lot of evolutionary ancestry, they share a lot of common traits. So much of our knowledge of fertilization and development comes from studying echinoderms and other chordates, like frogs.
So let's take a look at the fertilization events in the sea urchin. First of all, let's talk about the egg. Now, the egg of a sea urchin on the outside has kind of a gelatinous layer. That is covering the egg, which, of course, you know is a cell. Since the egg is a cell it must have a plasma and a membrane, and it does. Right outside of that plasma membrane is a membrane called the "vitelline membrane." One other thing I want to show you before we move on. Below the plasma membrane are all these little--let's use some cell biology here--our own little Golgi derived vesicles called the cortical granules.
Now, one of the questions my students always ask me is how come more than one sperm can't fertilize an egg. That's one of the first things you learn when you're studying embryology. In other words, what prevents polyspermy? We'll get to that. I want you to keep that in mind.
Let's take a look at the vents that occur when a sperm fertilizes the egg. The first thing that's going to happen, of course, is the sperm are going to approach the eggs. As the sperm approached the egg, and literally, I don't know if you've ever seen video of this, but the egg ends up looking like it's being parasitized by all these worms. I've got these drawn way out of proportion--sperm are much smaller than the egg. So you have all these sperm trying to enter the gelatinous layer of the egg.
Now, let's review sperm structure. I know you know about sperm structure, but let's take a quick look at it. You remember that sperm have a flagellum, and that flagellum is just a continuation of the cytoplasm, and it's loaded with mitochondria for burning energy so it can swim. What I want to talk to you about right now is a structure in the head of the sperm that is not the nucleus, but contains hydrolytic enzymes, digestive enzymes, called the acrosome. The acrosome is literally the warhead of the sperm. It's the way the sperm is going to penetrate and get into that egg.
Now we can talk about the events of fertilization. This diagram I'm going to show you is kind of a time lapse diagram, so let's go through it one step at a time. Here's the egg. There are the cortical granules, just to give you an idea. This would be the plasma membrane. This line with the yellow or the orange spots on it would be the vitelline membrane, and here's the gelatinous coat. Let's go to step one. Step one--sperm attacks egg. The warhead is released. The sperm acrosome bursts open and starts to cause a burrowing in to the jelly. So in other words, what happens is the sperms literally start to compete for entrance into the vitelline membrane, eventually hoping to get to the plasma membrane. Only one will be successful, and then after that we're going to have some trips to keep the other sperm out. What's going to happen here is the acrosomal enzyme, because they're hydrolytic to dissolve this gelatinous matrix on the outside of the egg. And then what happens is you get these microfilaments called the "acrosomal process," and like a little drill it almost looks like. They're really not drilling through, but these would have receptors on them, and they're actually made of out actin that will touch and attach, hook on, to the vitelline membrane.
Now, the sperm is not there yet. Fertilization is not going to happen, but that's the first step. These are the acrosomal events. So number one, the sperm approaches the egg. Number two, the acrosome releases hydrolytic enzymes. Three--the hydrolytic enzymes work their way through the gelatinous coat. Four--once the jelly coat starts to dissolve we have the actin filaments that are going to get in there, and we have the acrosomal process developing, which attaches to the vitelline membrane.
Now the process is literally going to start digging its way through the coat, through the vitelline membrane, and get into the plasma membrane. Now, once the plasma membrane is affected by the sperm, major events begin to happen. Once there is sperm, egg membrane contact, thing start to really occur here. The first thing that happens is the sperm releases its nucleus. So upon that sperm process reaching the plasma membrane through the vitelline membrane, the nucleus is released. Right now the whole object of this from the egg's perspective is going to be keep other sperm out--polyspermy prevention. Some major events are going to happen to keep that from happening here. We have what is now called a sodium ion depolarization. Let me explain what that means.
You guys have heard of sodium ions before. You know that they are charged particles. What's going to happen is as soon as that nucleus is released from the sperm and starts to go into the egg, we are going to have a major sodium ion change, and we're going to get a depolarization there. Literally, the sodium ions are going to rush into the egg, and that's going to prevent polyspermy. We're going to open ion channels and sodium ions through ion channels are going to rush in here. Because of this depolarization nobody is exactly sure what happens here, but membrane characteristics change and now sperm can no longer properly recognize the vitelline membrane and make their way through the plasma membrane. That's one to three seconds after contact. That's practically immediate. So one to three seconds after the membranes fuse, we have the sodium influx, the number one way to keep polyspermy from happening. The number two way of keeping polyspermy from happening is what we're going to call the cortical reaction. Remember those cortical bodies.
So we've seen the acrosomal reaction. Let's look at the cortical reactions. In the cortical reactions... Again, after fusing here's what's going to occur. The ER. Remember the ER? The endoplasmic reticulum of the egg releases more ions, calcium ions, and in releasing calcium ions, this causes the cortical granules to fuse with the plasma membrane. See, they were inside the egg, and now they're fusing. And since these were Golgi vesicles they must contain stuff, you know that. What do they contain? Well, they contain enzymes. Now we're going to get the cortical granules fused with the plasma membrane, and the cortical granules are going to give off a few things. What are they going to give off? They're going to give off enzymes and they're going to give off mucopolysaccharides. So what is this going to do? Where is this? This is happening in this space right here, which we're going to call the perivitelline space, the space between the plasma membrane and the vitelline membrane. In this perivitelline space the enzymes are going to change the nature of this membrane, the mucopolysaccharides are going to set up a concentration gradient. Here comes osmosis. And what's going to happen then is because--here's the plasma membrane. Here's the perivitelline space. Because we have all these mucopolysaccharides--water is going to rush in here and cause a further separation of these two membranes, and now this membrane is going to be a big time protector of polyspermy and the vitelline membrane has now become what is called the fertilization membrane. And guess what? No sperm can now enter that egg because we have number one--we've changed our membrane nature. Number two, we've put a water barrier in here. No sperm can get into that egg.
So what do we have? We now have a sperm nucleus plus an egg nucleus ready to form a 2N zygote and begin development.
The Evolution of Life on Earth
Chordate Development
Animal Development: A Close-up Look at Fertilization Events Page [2 of 2]