Animal Development

Hi. In this video we'll be talking about the specifics of animal development. Now, development begins with fertilization, which is the proverbial fusion of sperm and egg to form a zygote. This occurs in the fallopian tube and the egg is actually swept into the tube. Bythe cilia fertilization actually has a very short window in which it can occur. It has to occur within 24 hours of ovulation, which is the rupturing of the Corpus Ludie, Um, and releasing of the, uh, EG. Now, when the sperm and egg actually get together, you have a special reaction occur. You see, the sperm has this tip scene right here called the Ark A zone. And the ark is, um, contains these special enzymes that break down the protective coat around the egg so the sperm fuses with the egg releases those ARCA zones which allow it thio actually get through the egg and, uh, also signal the egg to not let any more sperm in. And this is accomplished by something called the cortical reaction, which releases a bunch of calcium which deep polarizes the membrane, causing a change in membrane potential. And this basically signals the completion of the second biotic division of the egg. All of that is to say that the sperm and egg fuse and there's a bunch of stuff that happens. And it prevents multiple sperm from fusing with the egg and also allows thes sperm and egg form the zygote and begin development. All right, let's flip the page.

after fertilization, the zygote has to travel to the uterus. During this time, cleavage occurs, which are these rapid my ta tick divisions, and they result in something called a more ULA. So the zygote turns into the more ula as cells divide into smaller cells. So this is an important thing to note. These cell divisions don't result in, UH, let's say, sells the size of the zygote. A zygote is actually dividing, but each time it divides the individual cells air, getting smaller and smaller. Now there are actually two different types of cleavage. There's what we call indeterminant cleavage, where the cells that arise from the cleavage are able to develop into a whole organism. This is how so called fraternal twins can develop. You have indeterminate cleavage. These cells, um, somehow separate, and they the two cells or clumps develop into separate organisms, separate complete organisms. Now you can also have determinate cleavage where the cells that arise are committed to differentiation and differentiation. We know eyes based on differences in transcription, mainly but gene expression in general. Now, something we briefly mentioned in the lesson on development but didn't really get into was the idea of induction, and that is that differentiated cells can influence the cell fate of nearby cells of their neighbors. So this is related to those social control mechanisms. Right cells can actually induce other cells to differentiate a certain way, and this is very important part of development. And the inducers are chemical signals generally, and they are released from one cell, diffuse out and stimulate a response in some nearby cell that has the proper receptor for that. Now these cells that air created during cleavage thes individual cells like we see right here these air called blast Amir's. It's simply a cell created during cleavage. The more ULA what we talked about these I go forming is the early stage embryo that consists of a solid ball of cells, the key being here solid. We'll see why, because in fact, during the course of development, that solid ball of cells is going to hollow itself out. Now we should also mention cytoplasmic determinants. These are regulatory molecules located in specific regions of the egg, so that during the course of these divisions they're not evenly distributed to the resulting cells. So basically imagine that we have this egg here. And it has Cem cytoplasmic determinants here that I'm representing with blue dots and then say down here it's got these other different cytoplasmic determinants that I'm representing with red dots. And then this cell goes through cleavage and divides into four unique sells. Well, look at those cytoplasmic determinants. They were not evenly distributed amongst those cells. This is important because this uneven distribution results in the formation of specialized cells. So these cells right here they didn't get the red or the blue cytoplasmic determinants. So they're going to go down a different path of development than this cell which got these blue determinants and this cell that got these red ones. So the the end game of this is that Onley certain blast Amir's Onley certain of these cells that result from the cleavage will have regulatory cascades triggered by the determinants meaning they will have certain genes expressed or not expressed as a result. And that will differentiate them from, say, their neighbor who maybe didn't get the same cytoplasmic determinants. Now, once we have the more ula that's solid ball of cells. The next phase of development is called blast elation and this is where the more illa develops into a blastocyst. Now the blast assist is basically theme the mammalian form of the blast Ula. So blast Ula is just a hollow ball of cells that forms during development in mammals. We call this blast Ula a blastocyst. Just kind of a lot of terminology being thrown around here trying to keep it simple. So again, blast Youlus just a hollow ball of cells in mammals. We specifically call it a blast. Assist that inner space which we conceal me Take myself out of the shot here this inner space that you can see in here that is the blast A coal which is a fluid filled cavity inside the blast assist. We also have something called the trophy blast that air the cells that surround the blast a call and give rise to the, uh, Korean and placenta which are structures that are not part of the developing organism but air support structures. Right? Um they help keep that developing organism alive. They're very important. The organism couldn't live and develop properly without it, but they don't actually become part of that organism's body. And you can see those trophoblastic cells labeled here in the image. And they are on the periphery there, around the outside of that blast ical. Last thing to point out is the inner cell mass which again is been labeled here. And these ourselves inside of the blast Ula that actually give rise to the organism. So those outer sells the trophy blast give rise to the support structures. The inner cells give rise to the actual organism itself. All right, let's turn the page.

once the blast assistance formed, it has to implant in the uterus. This is called implantation, getting really creative with the names here. Now HCG is a chemical secreted by the implanted blastocyst, and it kind of lets the body know that pregnancy is underway. In fact, most early pregnancy tests actually test for the presence of HCG. Now, HCG is not the end all be all hormone involved in this and eventually it will sort of be replaced by secretion of estrogen progesterone. But all of this is a little beyond the scope of what you need to know for by a one. So what you should know is that those trophoblastic cells will develop into the placenta and they will actually take over the estrogen and progesterone production, and they will continue to produce it throughout pregnancy. Those hormones air very important thio pregnancy and to supporting the developing organism. Now, once we have our blast assist and planted, it can start to undergo what's called gassed relation, which is the formation of these three germ layers which forms a gas strolla. So as you can see here we have our blast assist. Right? Um has cells around the outside blast, Ecole, that fluid filled cavity inside. And what happens is it starts to kind of poke inward like we see happening right here. So it pokes inward. This is gassed relation. And the result is what we see here. The gas Strolla. And let me just highlight in red. This is an what I'm highlighting in red is like the mass of cells. There's space inside. However, there's hollow cavity. Now let's go over a little terminology real quick. We have our blast. Oh, poor. That's the opening to this space. Uh, we have also our three germ layers, right? So those three germ layers three germ layers are the acted ERM which are the cells that give rise thio nerves. The adrenal medulla, which is a gland in the body, Uh, skin, brain eyes, inner ear or your vestibular system, if you will. You also have the miso durm on the meso Derm. You can see there's actually, uh, one little patch right here. This is also me, Zod. ERM over here, you take myself out of shots. So it's easier to see all of this. So we have miso Durham on both sides, and these are internal cells that will give rise to organs the adrenal cortex, which is also part of the adrenal gland. Different part, though, um, the blood, bone, gonads and soft tissues. And then, lastly, we have the endo derm. In the end, O derm is the layer of those innermost cells that you can see kind of in this mustard color. I'm going to circle it in blue right now. So this is Art Endo Durham. Those innermost cells form the epithelial linings of the digestive tract of liver, pancreas and lungs. So in a large part they formed. The inner cells of the what's called the Elementary Canal are basically the cavity that runs through our bodies. All right, let's flip the page.

Now, after those germ layers have formed, we can begin organo Genesis or the process of organ and tissue development. And you might remember that those me the method ERM was made up of those two pockets. What we call those so mites those air pairs of miss a dermal tissue and he sells position in the, um in the so so might determines what it will turn into. So let's imagine that, uh, here is our so might and it's made up of a bunch of little individual cells again, this is just for the purposes of example. So depending on where the cell is located in the so might, it'll turn into different things. It'll actually break away in groups and migrate and form different things. So this cell might form, Let's say, cardiac muscle like we see here and maybe these cells the cell breaks away informs, Ah, skeletal muscle. So it's the actual physical position of the cell in the so mate that determines what it will develop into. And I'm sure you have some pretty good guesses as to how and why that can happen. Based on what we've already learned about development, concentration, radiance, and chemical signals. Now the museum layer develops into many different things we can see here, including cardiac muscle, skeletal muscle tube, you'll cells of the kidney, um, red blood cells and smooth muscle cells like those around the intestine. Now, in addition, to organize Genesis, there's another really important process that occurs an animal development. And this is no relation, which is the actual formation of nervous tissue from the primary germ layers. So a couple points of terminology here we have the note accord, which you can see here in the picture. This structure, um, on a fire scroll down a little so you can see the other images. The Noto cord is also present here and here, and it is labeled in this last image here. So what is the note accord? The note accord is kind of like a primitive backbone that develops in core dates, which is a a big class of types of animals, and in some animals it actually develops into the vertebrae of the spine. But in many animals, it's actually just a transient structure. It forms during development, and then it goes away at some point, and that's totally normal, part of the process, and it's because of that link to evolution. That close link that development has toe evolution. Now, in addition to the note, accord should be aware of the neural tube, which is a hollow structure that eventually will form the brain and spinal cord. The neural tube forms in kind of an interesting way. We have this neural plate, as it's called, and it basically, as you can see here it folds in on itself and seals the Ecto Durm. There's actor Derma on this side. This is also Ecto Durham. It seals the Ecto Durham together. So let me get rid of some of these eras just so this becomes a little more clear so you can see the ectodermal are the neural plate folds in on itself and then eventually boom. We have the actor Durham fuse. And again this neural tube structure that you see here will form the brain and spinal cord. In fact, it swells in certain places to form the embryonic brains. It'll swell and form little bulges that actually become embryonic brain structures. So the Meso Durham cells, uh, that formed the node accord actually are the same cells that induce. Remember, induction. Induce the ectodermal cells to furrow or form this, uh, form this fold and then fused together. Thio form the neural tube, the neural folds that surround the neural groove, um, actually give rise to the central nervous system. Now the last thing we need to talk about is sell determination, which is that irreversible commitment of a cell to a pick particular developmental path, which results in a specific cell type. We've talked about this. It happens through the process of differentiation. But what we haven't talked about is how differentiation is actually a gradual process. Sell a can give rise to Selby or sell C. Selby is capable of giving rise to sell D and sell E and sell. C can give rise to sell e or F, so differentiation occurs in stages to get from sell a to, let's say, sell F one. The cell has to go through that intermediary phase of being cell C, and at that point it's fate. As becoming cell F is still not sealed, It could also become sell e. It has options, so differentiation is a gradual process. However, once the cell is committed to a particular path. It's Fate is sealed. It's destiny. ISS sewed in the stars, and it is going to become differentiated to that particular type of cell. All right, that's all I have for this lesson. See you guys next time.