Biology: Protostomes and Deuterostomes

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What's our final division of our monophyletic arm of the animal kingdom? Where's it going to lead us. Is there anything else that we can really talk about when it comes to animals? We've talked about the fact that there are some without tissues and some with, that some have diploblastic and others have triploblastic germ layers. And along with the diploblastic goes radial symmetry and here goes bilateral symmetry. And then we said, "Well, some of them form body cavities and some of them don't." And then we said, "And the ones that form body cavities, some of them form pseudo body cavities called `pseudocoelomes,' and some form true coelomes." And, oh my gosh, we still have another division point, don't we?
This is a major division point, and it's really going to split these things up, and there are some complexities to this, so hang on to your biological caps. Now, one of the things, you see, that is going to happen here, is we're going to separate--let's go to the top. You've got to be careful. You're taking arthropods, things like bugs, and mollusks, and worms, and you're putting them all in the same group. Now, I've got to tell you there are some dotted lines on this tree. You're just going to have to bear with we biologists on that one for a while. There are still some pieces to that puzzle we've got to fill in. But you can see what's happening here. There are some forks there, so we still have time to go on those. But what I want to tell you about now is this next division that takes things like mollusks, like a snail, and a worm, and puts them together, and then goes on over here and takes things like echinoderms and me, and puts us on the same branch. Why is that? How can that be?
Well, it has to do with a lot of the materials in terms of embryonic development. This is where development and the migration of tissues, and the fate of tissues, comes into play--right there, big time, let's talk. Now, here's the thing. I'll give you some names. On the left we're going to have what are called the "protostomes." I'll explain why we call them protostomes in the end, then the deuterostomes. There's a little biological joke you guys didn't get when I said, "I'll explain it in the end," but you'll get it in the end. There's another one.
So protostomes and deuterostomes. Let's talk about those. What makes the protostomes and deuterostomes or what, even more importantly, sends them on either branch? Here we go. Cleavage. Cleavage and fate is the first thing I want to talk to you about. You guys know about cleavage. Cleavage is the series of cell divisions that happens immediately after an egg is fertilized. The first one I want to talk to you about is something called "spiral cleavage." Now spiral cleavage occurs in the protostomes, so let me tell you about the protostomes. They have what is called spiral and determinate cleavage.
Deuterostomes have what is called radial and indeterminate cleavage. Let me talk about the determinate and indeterminate part first. That's a little bit easier to picture. Determinate simply means that these lose their totipotency very quickly. You remember what totipotency means? Totipotency means that you literally have the ability to turn into anything and the protostomes are determinate, meaning that they've lost their totipotency, that the protostomes have absolutely no chance to turn into different types of cells. They're committed right after their first division. On the other hand, the deuterostomes, their cells, are not triggered that you're going to be a brain cell, and you're going to be a nervous system cell, and you're going to be an egg cell until much later, so they're indeterminate.
Let's talk about physical type of cleavage. Let's talk about spiral cleavage and radial cleavage. I think radial cleavage would probably be a better way to start. What happens in radial cleavage is that when the cells divide, they are going to form a neat stack, and so this cell is going to form so that its cleavage plane is right above the next one. So imagine that this thing is growing in this direction. The cleavage planes, they line up. On the other hand, spiral cleavage would not go that way. If this were going to spiral cleave, it would cleave in a different direction than the one below it. What that's going to do is if this were--which it's not--but if this were going to cleave in that direction this time, then literally, when your cell divides, you would have a situation like this one right here, where the cleavage plane of the one above it is offset from the one below it, and so you get this spiraling formation, and then again, and then again, and the again, and what ends up happening is as your cells divide and the embryo grows, its cells are offset from one another and they form this spiral pattern. Now, why is that important? It's important because it happens. This is genetically programmed, and therefore it's going to lead to the further differences as we move our way down and take a look at some of the development issues that we're going to deal with here. So number one--cleavage type.
Let's talk about number two. The second thing that's going to separate the protostomes from the deuterostomes have to do with how the coelome develops. Remember, these are both coelomates--protostomes and deuterostomes. We have established that. But now the next one is coelome development. Here's what happens in the protostomes. In the protostomes we have what is called schizocoelous, as in split. I'm going to show you schizocoelous development here. If you have this gastrula like so, what you're going to do is you are going to form mesoderm sections right here, and these mesoderm sections are eventually going to end up becoming the coelome. And so the coelome, and you can see it's starting to form right in here, the coelome is going to form from this idea of the split pockets of mesodermal tissue eventually that are going to give rise--and I'm going to get ahead of myself here--to a coelome--there's the coelome right in here, but they form from a split situation.
Look at this guy right here. Now, the deuterostome is going to do it in a completely different way. The deuterostome is going to form his coelome like so. Now what's going to happen is this is going to grow outward and you're going to form your coelome from pockets. And when you form your coelome eventually from here, it's going to form like that from outgrowths of pockets, literally, from the mesoderm right here. Now, look at them in comparison to each other. Split development and one continuous layer with the pocket development coming around. The product is the same--coelomes. The way they form it is different. That's the key there. So we have a schizocoelous and an enterocoelus--entero--inside.
Last, but not least, we come to where does the protostome and deuterostome get its name from? If you know any of the languages like Latin and Greek, you know that protostomes means first mouth. Deuterostome means second mouth. What does that mean? Well, that means this. What is the first mouth that an embryo develops? What is that first hole? Do you remember? It's called the blastopore. In protostomes, the first hole becomes the mouth. So in other words, if we were drawing this thing, and remember my egg here, and we have the blastopore, and the cells were pouring down the blastopore. In a protostome that becomes the mouth. In the deuterostome things were a little bit backwards. The anus develops from the blastopore. So protostomes, mouth develops from the blastopore; deuterostomes, anus develops from the blastopore. What do you know, they seem to have completely reversed their mechanism of development. If you think about that, that makes perfect sense if you think of some of the stuff that Hans Spemann found out. Remember what Spemann did? When he took an early cell fro that blastopore, what did he do? When he took that early cell, right by the mouth there and he transplanted it somewhere else and it became a head. Why? Because it was an early cell. And then when he took the later cell that was down there that was going to stay there anyway and transplanted in somewhere else, it became a tail. So therefore, that later cell was as the blastopore later, and at that point it was ready to become a tail. Think about it. What organism was Spemann using? Spemann was using the salamanders. He was using deuterostomes, and therefore, that early cell, that first cell to go in there, that is going to be the head, that's going to be the mouth region.
So it's all sorts of cool stuff that goes on here. Where are we going with this? Very simple. We now have an organism that we can put together, and we finally, at long last can say, "We know how we established our phylogenetic tree from animals."
The Evolution of Life on Earth
Evolution of the Animal Kingdom
Protostomes and Deuterostomes Page [2 of 2]