Biology: Cleavage, Gastrulation, and Organogenesis

This lesson has not been reviewed.

Please purchase the lesson to review.

Once you have a fertilized egg, what happens next? Well, now comes events that books have been written about. We'll condense it down just a little bit. We're going to talk about the first thing that begins to happen and that is a series of mitotic divisions called "cleavage." Cleavage is a very rapid series of mitotic divisions. The cells absolutely fly past G1, go into S and G2. In other words, what happens is you don't get cell growth during cleavage. The cells start out as one egg cell, split into two, split into four, split into eight, split into 16, and rapidly move into a ball of cells. So it ends up something like this.
Cleavage, therefore, starts out as one cell. We're going to get two cells. Two will give four, four will give eight, and you'll notice that the cells are getting smaller at each division. And so what's going to eventually happen is you're going to look something like this. You take a look at this. Here's the first cleavage. Now, one of the things I want you to understand is we're going to be talking later about cleavage plates, and that's what's meant right there. So there's the one cell that just became two. And then two cells will become four. Again, these are elongated and you see the cleavage plates right there.
Well, eventually you start to get this ball-shaped looking thing, and that is the very first stage beyond the zygote that we can put a name on, and that's called the morula, which is a solid ball of cells, and eventually this solid ball of cells, because of the division patterns that are going to occur in there, and the pressure is put on by the cells pushing against each other, is eventually going to form into a hollow ball of cells, which is going to be called the blastula.
Now, let's take a look at a diagrammatic representation of what's going on here. In our first series of divisions--and you can see what's happening. Now let's take a look at what it looks like inside of there, because inside of the blastula it's hollow. In fact, there's a structure in there called the "blastocoel." Coel means cavity. We're going to see in a minute what this is all about, but I want to concentrate on this for a second. Does this happen in all developing embryos? Absolutely. Does it happen the same way? No. And that's where you get into the reams of information that have been produced in looking at the way different organisms develop.
For example, if you have an organism that's going to have a lot of yolk--and yolk starts out as--it's an egg--it's cytoplasm. But then what ends up happening is after your first division the yolk gets concentrated in one of those two cells and then those cells, because they have a lot of yolk mass in them, will divide a little bit slower and so they'll end up as bigger cells than the cells that are going to end up turning into the creature itself. So some cells, like say a frog's egg, will end up with literally a polarity to the egg with an animal pole, which will give you the animal, and the vegetal pole, which stores all of the yolk. And some don't cleave all the way through, so you get something like the chicken egg that has these special kinds of cleavages that don't go all the way through. Like I said, there have been reams of information written about this, but let's understand the process.
And so now we have this thing called the "blastocoel." And now we have to start thinking about, okay, now that we have all of these cells, how are we going to generate organs from this? It's just a ball of cells. And so now we come to a process called "organogenesis" and "morphogenesis." In other words, the formation of organs and changes--morphogenesis. Hang on for this. This is great stuff.
As we take a look at say something like a blastocoel and a blastula, I want you to take a look at this top picture right here. Now, this is a cell very much--actually, this is a frog. So you've got a large yolk mass down at the bottom, and this is going to be a fairly complex one. But the bottom line is you still have a blastocoel right here. We've mapped this in what we call the fates. In other words, the different colors are going to represent the different layers and what they're going to turn out to be. Remember, we're going to be talking about a triploblastic embryo here. Now, that's a complex one. Let me show you one that might be just a little bit simpler, and we'll use that as our model.
Let me show you how, for example, a sea urchin might develop. In a sea urchin blastula, which would look something like that, the first thing that's going to happen is you're going to develop a pore right here called the blastopore. Well, guess what? Blastopore--you and I and frogs and chickens, same kind of thing. We're going to develop a blastopore. I'll show you that situation later. What happens in the blastopore is you start to get what is called--now this is where it gets very cool because the cells start to interact with each other. Some of them elongate, they start to move along each other because of cell surface interactions, because of cytoskeleton changes, there's a whole complex series of reactions that cause these cells at the blastopore to start to get longer and literally just start what is called an "invagination." So what we're going to get is literally a sucking in, if you will, of the blastopore, and in addition, you're going to get some cells migrating in there.
So here's my blastopore, my opening to the blastocoel, and we're going to start getting some cells moving in there. So I'm going to draw these cells in here as having moved in there, and I'm going to draw them in red because they are going to end up as--just so you know where we're going--as mesoderm, the middle layer. Now, I said that it starts to do an invagination. By the way, the movement of these cells in there is called "involution." So number one, we have movement of cells, and two, we have invagination, which means that we are literally getting a depression in there. So let's draw that. And so in the sea urchin what's going to happen is it starts to bend inward, literally. So we're pulling this. Cellularly, literally there are going to be fibers that are going to pull this in that direction.
Now, watch what's going to happen next. As we continue to build up our mesoderm, as cells continue to go in here, this is going to continue in this direction. Watch what's going to happen next. Eventually, you're going to get something that's going to look like this. Get it? Look you have an opening here and the potential for an opening right there. You see? Because look, that used to be the top of this embryo. Isn't that cool? Remember our mesoderm in here. Now, look you guys, what did we just form? We just formed our third layer of cells. Because if you think about it, here's my ectoderm, here's my mesoderm, and here is my endoderm, my inner layer. And eventually we get something that's going to form an opening there with what can become a digestive tube. You see? And you can have the mouth here and the anus there. So you have the digestive tube, you've put in your mesoderm. This is all cells; let's not forget that. When I pull this in I'm talking cells; I'm not talking single cell. So all of this is cells, cells, cells. Those are endodermal cells, and all of this out here will be ectodermal cells. How cool is that? This is so great.
Well, guess what? It just looks harder in the frog egg. In the frog egg we're going to get a blastopore forming, too, and we're going to get migration of cells, and we're going to get invagination, and that's going to happen right here. Right in there is where we're going to start our invagination and our involution. And look what's happening. Now, here's the blastocoel. Look what's happening to the blastocoel. As the cells move in this direction and they start to move into the blastopore, what's starting to happen is you're getting layering of cells. So you're getting layering of cells in here where I'm coloring, and that's going to be your ectoderm. You're literally making the blastocoel disappear. It's closing up as more cells build in there, and you're literally making a brand new cavity. And that brand new cavity is going to be called the "archenteron." Lots of vocabulary here, but that's okay, we've got to refer to it as something. What we've done is once again we have formed a situation where we have an opening here and the potential for an opening over here at the other end of the archenteron, and this diagram doesn't show it, so once again, we now have formed--what have we formed? Three cell layers. We have the outside cell layer, the ectoderm. We have the middle cell layer, the mesoderm, and we have this layer here lining the archenteron, the endoderm.
So how does that turn into an embryo and you? Well, you know what? The rest of it is just sheer old logic. How do cells differentiate? Three hundred books written on that. But, in general, what's going to happen next? The three layers are going to differentiate, and if you think about it, all you need to do is realize if you understand what ectoderm means, you're going to know what the ectoderm turns into. It's going to turn into the skin, isn't it, because it's on the outside. It's going to turn into the nerves in the skin. It's going to turn into your nervous system and the outsides of your eyes. So your nervous system, your cornea, and your lenses, anything literally that's towards the outside of your body is going to be ectodermal in origin.
On the other hand, mesodermal stuff is going to be the stuff in the middle. So one of the things that happens is it forms something called the notochord, which is literally a cartilaginous rod that causes the embryo to stretch. More on that at another time. The notochord actually ends up for many organisms, ends up becoming the basis for the vertebrae. It's going to form your muscles. It's going to form your skeleton. It's going to form all those things that are unique to something with guts. It's going to form the circulatory system, the respiratory system, the organs, the reproductive system.
Well, what about the endoderm? The endoderm, the innermost thing. So it's going to form the inner linings, the inner linings of many of these organs, like the digestive tract. I showed you how it forms the inner linings of the digestive tract right there. So it's going to form inner linings.
You know, it's a brief overview, but the bottom line is this--we've gone from one cell to two cells to four cells to thousands of cells, and have begun to form an organism.
The Evolution of Life on Earth
Chordate Development
Cleavage, Gastrulation, and Organogenesis: A Closer Look Page [2 of 2]