Embryo Development Overview: Weeks 1-2

Hi, I'm Nishner. So we're going to continue talking about the development of the embryo. We already finished it. If you guys haven't already seen it, go watch our first video, which is the development up until week one, right? So we talked about fertilization, we talked about cleavage, and we talked about the formation of the blastocyst, right? And then how we got to the end of that, we're going to now go into the development throughout the embryo up until week two. We're going to mainly talk about, in this video, gastrulation. We'll have a tiny little peek into implantation, but I don't want to get into too much detail on that. Reason why is we'll go more into that when we talk about the development of the placenta, okay? But let's see where we left off. We left off talking about how the trophoblast, right, which is the outer cell mass, used to be the outer cell mass, we said it's going to get derived into a cytotrophoblast and a syncytiotrophoblast, and we'll talk about their significance. We also talked about how the inner cell mass differentiated, become more specialized, and it became the embryo blast. And how it starts to differentiate and become a little bit more specialized, and it forms what's called the bilaminar disc, right? It's going to become the embryo. Trophoblast is important because it becomes a part of the chorion, and it becomes a part of the placenta, which is very important for being able to provide oxygen, nutrients, get rid of waste, all right? Between the mother and the fetus. So we finished off here with our... Kind of like the end part here where we had our trophoblast, which we said we're going to differentiate into these two, and the embryoblast. Remember, what did I say? Embryoblast turns into a bilimiter disc. We're going to talk about this a little bit more in just a second. I want to focus on the outer layer now. That's where it can get a little bit tough. When you focus on these things, you want to focus on one part of the time. What I want to do first is we're going to talk about that outer layer first, just a little bit. Again, we'll get into it more when we talk about the development of the placenta. I want to talk about... what the significance of the trophoblast is. Then after that, we'll come back and we'll talk about how the embryoblast develops into the trilaminar disc via the process of gastrulation. All right, so what I want to do is let's go back into our diagram and talk about what's happening from this point, okay? All right, so let's go ahead and take a look at our diagram again. So here we have the uterus, right? And we already talked about what had happened to this point. We already said about how the anterior pituitary releases the LH, right? LH triggers the ovulation process where we pop out that secondary oocyte, right? And it's frozen imidaphase 2 in the ampulla. Then we said that the sperm is going to meet the actual oocyte frozen imidaphase 2 at the level of the ampulla. We're going to get fertilization and then that whole process of cleavage is occurring, right? And we talked about how cleavage is basically going from the zygote to the 4-cell stage to the 8-cell stage to the 16-cell stage and then to the blastocyst. Well, what happens is when all this cleavage process is occurring, this actual zygote or these different cleavage processes are actually, they're moving towards the uterine cavity. So then what happens is that little blastocyst, it kind of just pops off here into the actual uterine cavity. And then what happens is it has specific types of proteins on it, certain types of selectins or integrins that allow for it to hook up with the endometrium and start implanting itself. Right? So what we're going to do now is we're going to take a deeper look, looking at how that actual, that blastocyst is going to implant and what are the processes with that outer trophoblastic layer. Okay? So what we have over here, again, don't worry yet about the bilimiter disc. We'll talk about this a little bit later. But remember we said that there's an outer cell layer, this outer blue lining. We call that the cytotrophoblast. Well, the trophoblast which divides into a cytotrophoblast and syncytiotrophoblast. What happens is, imagine here for a second, you have these well-defined cells, okay? And here they are, these kind of like baby blue cells right there. We're going to have these well-defined cell margins. These are called your cytotrophoblast. So you have the well-defined cell margins with the nucleus inside of the cell, right? What happens is, some of the other cells start proliferating. And as they start proliferating, their cell membranes actually start disintegrating. So they start proliferating outside of that zona pellucida, right? So remember how we had the zona pellucida all the way around? It starts releasing certain types of enzymes that allows for it to kind of break outside of that. And as it does that, these cells start proliferating outside of the cell, and then their cell membranes start disintegrating. When they start disintegrating, it forms kind of a fluid, if you will, kind of a cytoplasm. which is going to be consisting of the nuclei of these cells, right? So what happens is all the cell membranes of the... These cells start actually breaking open and releasing out the nuclei and the cytoplasm into kind of just a little area. And what happens is all of the cytoplasm starts kind of fusing with one another, forming a syncytium, right? And that's going to be our syncytiotrophoblast. So this part here, which is going to be kind of like these finger liposomes that are kind of extending out into the uterus. So this is our uterine lining right here. This is going to be called the syncytio. trophoblast. Okay, and again it's important to remember just so I give you guys a good idea here. Imagine what there's all the cells, right? Here's the finger process. All of the actual nuclei are just kind of sitting out here in the cytoplasm. There's no well-defined cell margins. Okay, that's our syncytiotrophoblast. At the base, you're going to have these well-defined cells. which have their cell membrane and the nucleus with inside of that. Eventually, what will start happening is that these cells will start actually forming kind of a defined cell, kind of like finger process or villi that extends through the center of this, up into the center of that actual villus, right? But again, for right now, I want you to remember all this cytoplasm on the side with the nuclei kind of just within that fluid is called this incisio-trophoblast. And then the cells with the well-defined margins or membranes with the nuclei inside of them are called the cytotrophoblast. So again, these are your cytotrophoblasts. And again, the syncytiotrophoblast is going to be just the actual syncytium of all the cell cytoplasm and nuclei kind of just sitting in this kind of pool of fluid. All right? Why is that important? Well, eventually what happens is... The syncytiotrophoblast will continue to kind of release certain types of hydrolytic enzymes that will work its way through the actual uterine lining. And guess what is out here in the uterine lining? You have these maternal blood vessels. So let's just say here's going to be like a maternal blood vessel. Here's going to be a maternal blood vessel. And here's another maternal blood vessel. What's eventually going to happen is that this actual syncytiotrophoblast is going to continue to keep working its way out here to kind of become confluent with these maternal blood vessels. Because once this actually becomes confluent with the maternal blood vessels, now the actual embryo can receive oxygen, nutrients, hormones, everything it needs in order for it to grow. Because there's going to be a connection between the embryo and the maternal vascular system via that syncytiotrophoblast and cytotrophoblast. So that should make sense. Once it implants via the integrins and selectins to the uterine lining, It's going to burrow itself in a little bit, and then what happens is those cytotrophoblastic cells are going to start proliferating and working its way through the zona pellucida, making these finger-like processes. The cells inside of those finger-like processes start to disintegrate and break down. Their cytoplasm spills out of them, and their nuclei spills out of them. And they form a pool of what we call a syncytium, which is going to be this syncytiotrophoblast. Then eventually what happens is... The cytotrophoblast continues to proliferate these cells and they proliferate right into the center of that actual villus. And eventually what's going to happen is this actual structure is going to become confluent with the actual maternal blood vessels. And before you know it, we're going to have a placenta, a connection between the vascular system of the mother and then the embryo. And that's going to be the development of the placenta. We'll talk about that in more detail later. I just wanted to give you guys a little look into this. Now. A little bit later throughout the process, we're going to kind of jump a little bit forward. As this actual embryo continues and continues to develop, right, here's the important thing. Remember we said that the corpus luteum ejected out the secondary oocyte. And LH right now is what's driving the production of progesterone, right? And we were up to about, we said that it happened around day 14, day 15. We've already gone through about a week, all right? So we've already done about seven to eight days by now. from the point of implantation. As we start getting closer to day 28, what starts happening? You start shedding the endometrial lining. How do we prevent the endometrial lining from getting shed? Well, guess what happens a little bit later towards the, you know, around day 24, maybe day 21 to 24. The syncytiotrophoblast, it starts actually making a specific type of hormone. So cool. It starts releasing what's called beta. HCG, okay, which is human chorionic gonadotropin. Now remember, I told you that progesterone levels start to drop later on in the luteal phase. When that syncytiotrophoblast releases the beta HCG, That's what it does to the corpus luteum of the ovary. It has it continue to produce what? Progesterone. And so the progesterone is going to remain elevated. The reason why that's important is once progesterone levels start dropping, what happens to the vessels inside of the uterine lining? They start spasming. And as they start spasming, what happens? They rupture. And as they rupture, the cells inside that endometrial lining start doing what? They start actually becoming necrotic and they start getting shed. And if we shed the endometrial lining, what else will we shed? The embryo. And we want this to happen. We want them to be able to allow for the implantation. We don't want this to get shed out. So, if it releases beta-HCG, it triggers the corpus luteum to continue to keep making progesterone. If progesterone levels maintain a certain level, we're not going to prevent those vessels from spasming. And so we're not going to allow for shedding. We're going to allow for proper nutrition, glycogen. all the different types of lipids to be produced to nourish the baby. And so there's not going to be any shedding of the endometrial lining to ensure that there's proper implantation. So that's such a cool thing. Again, we'll go into more detail about implantation and the development of the placenta later. Let's come back here and talk a little bit more about how the development of the embryo is occurring. All right, so now we talked already about the process of the implantation. I just wanted you guys to not be like, wait, what happens to the trophoblast? Because we're primarily going to be focusing on the embryoblast and the whole... process of the bilaminar disc converting into the trilaminar disc. So again, a little bit of information on that. We'll go into more detail on it later. But what we're going to do now is, just so we don't have to focus on that trophoblast, we're just going to take out that bilaminar disc, okay, and the cavity above it and the cavity below it, and just see what happens specifically with that, okay? So take that out now, and here's the structure that we get. Let's make ourselves kind of understand what's going on here with this structure. So bilaminar. two discs, right? Two sheet-like discs. The top layer is an important layer, okay? The top layer is what we call the epiblast, okay? So the top layer is the epiblast. The bottom layer is actually going to be what's called the hypoblast. And again, this is actually going to be a part of the bilimiter disc. So this is the two components of the bilimiter disc. Now, below the bilimiter disc is going to be a little cavity. And what is that cavity called? That cavity is going to be called the yolk sac. So it's kind of like the primitive yolk sac. So we'll put here YS. This is just going to be the primitive yolk sac. Above it is going to be a fluid-filled cavity, and this is going to be called the amniotic cavity. So just so you guys have a little bit of an idea of what this actual microscopic structure is, is again, epiblast, hypoblast. Two layers sandwiched together. Above the epiblast is the amniotic cavity, and below the hypoblast is the primitive yolk sac. Okay? Eventually, this will help with red blood cell production in the actual embryo. Okay? But it provides a lot of nutrients as well. Now, what I want us to do is, and I want us to look at this in a way that helps us to kind of see what's going on, because that's kind of the biggest thing with embryology, is seeing the process, seeing the way that cells are moving, or the way things are migrating. It's the toughest part. All right, so what I want to do is I want to take a look at this actual bilimiter disc. Imagine we're looking at it from the top. Okay, so we're not going to see the hypoblast. We're only going to see the epiblast from the top. Okay, so imagine it's kind of like this. Okay, so we're looking at it from the top. Now, what we have here is on the top you're going to have this little kind of like little fused area. It looks kind of like a thickening or kind of like a depression maybe. You'll see it, it's called the procordial plate. And this is good because the reason why is this gives us kind of our orientation of anatomy. So here we're going to call this the procordial plate. And what the procordial plate is, is where the epiblast and the hypoblast are kind of just sandwiched together. But the procordial plate gives us orientation of cranial and caudal. So with respect to the actual procordial plate, the top part of it, near it, is going to be the cranial. end, okay, or the head end. Away from it down towards this side is going to be the caudal or the tail portion, okay. So here we're going to have that and then here we're going to have this. There's actually another membrane down here. If you really want to know, you'll have another membrane at the bottom here called the cloacal membrane and then you'll have a membrane right in front of the procordial plate which is called the buccopharyngeal membrane. Why that's important is buccal pharyngeal membrane, guess what that makes? The mouth. Guess what the cloacal plate makes? The butthole. Okay? So I'll show you the two ends. Okay? Procordial plate though, it gives you kind of an idea specifically of your positioning. So again, cranial end is where the procordial plate is, caudal end is going to be down here where the cloacal plate is, and then you'll have a membrane right in front of the actual procordial plate, which is called the buccal pharyngeal membrane. What I want us to do though is, is we're going to make a section, okay? I'm going to make a section, and we're going to look at it right here. So I'm going to cut right here. Imagine I'm taking this piece, I'm looking at it from the top. I'm now going to take this section and look at it in kind of like a more of a coronal section. So we're going to look at it more of a coronal section. So now this is the one we're going to focus on. And now you're going to be able to see the hypoblast and then epiblast. And then this little black area here is going to be the procordal plate. Here's what starts happening. At this stage, So we're getting ready, we're trying to move from week two all the way up to week three. What starts happening is there's some signaling processes that occur. And these signaling processes start causing the epiblast cells to form kind of a thick area on top of them. So you're going to get like this very thickened area of epiblast cells. And it's going to make what we call a primitive streak. So here you're going to have the... primitive streak, which is going to be this part, okay? And towards the actual cranial end, you're going to have like a little knobby, and that little knobby is going to be what's called the primitive node. So you have the primitive streak, which is kind of a thickening of the epiblast cells, and the primitive node, which is kind of like this knob-like structure towards the top of the primitive streak. Now, here's where it gets really cool. Eventually, what happens is... Some of the cells in the center of the primitive streak start dying. And some of the cells in the center of the primitive node start dying. And what's cool about this is now it makes like this little structure that we're going to have over here on the right. I just wanted to give you guys kind of like what's happening with this. So we had the primitive streak, which is a thickening of tissue. Primitive node, which is a knob-like thickening of tissue. And then what happens is some of the cells within the center of that shriek and node start actually dying and making a little cavity. Pretty cool, right? Now, why is that cavity important? Okay, let's come over here. Some of the cells near the edge of this primitive shriek, right, start secreting certain types of chemicals, right? What kind of chemicals do they start secreting? They start secreting chemicals. which are called fibroblast growth factor 8. There's other types too, maybe transforming growth factor beta, but this is the main thing that the actual cells here near the edge are going to start secreting. So they're going to start secreting these chemicals, these fibroblast growth factor 8. Now, what does this fibroblast growth factor 8 do? It's so freaking cool what it does. Let me show you. These cells, they start secreting that fibroblast growth factor 8. That fibroblast growth factor 8 is going to move these nearby, remember it starts secreting it laterally. So it's going to move from the center, moving laterally. And it's going to go to these nearby epiblast cells. When it binds onto the epiblast cells, it has a receptor. So it's a fibroblast growth factor receptor. Once it binds onto this receptor, it triggers an intracellular process, right? And this intracellular process, what it does is it activates a specific type of protein called Snail-1. Heck of a name, right? So a fibroblast growth factor binds onto the receptor, triggers an intracellular process that stimulates the formation of a protein called Snail-1. Now, what Snail-1 does is it inhibits the formation of a specific type of protein that allows for cells to be linked up with one another. Do you know what that protein is called? It's called E-cadherins. So it's called E-cadherins. Now what E-cadherins do is they allow for cells to stick with one another. So here's another epiblast cell. So two epiblast cells connected to one another through this E-cadherins. Now with these E-cadherins the cells aren't allowed for to move away from one another, right? So they're stuck to one another. They're in a localized place. If you stimulate this protein with the fibroblast growth factor, guess what it does? It inhibits the formation of these proteins. If these proteins aren't connecting the two cells with one another, what can they now do? migrate. So now this cell is free to move and this cell is free to move. Guess what they call that? Epithelial migration. Okay, so it's called epithelial migration. So now these cells that are going to be over here on the lateral side, guess what they're going to start doing? They are going to be stimulated and they're going to start migrating towards that primitive streak. Now, here's what's so darn cool. Once they move through that primitive streak, right? Remember there's a little space there? We had the primitive streak, which is the thickening. In the primitive streak, that little space that we have there now, we actually call it the primitive groove. If you really want to be particular, the primitive streak was the thickening with no space. Now that it has the space, it's called the primitive groove. What we used to have was the primitive node, which is that knob-like thickening, and it has a space now. It's technically called the primitive pit. I'll write that down. Just for the heck of it though, right? So what used to be the primitive streak is now going to be called the primitive groove, which is going to be again that space right there. And then what used to be the primitive node is now going to be called the primitive pit. And all it is is just basically saying the space within the primitive node and the space within the primitive streak. That's all it is. So we release the fibroblast growth factor 8. Those epiblast cells are now going to be Free to move where are they gonna move they're gonna move through the primitive groove once they move through the primitive groove Guess what happens now look at this. Let's make like a little space here, too So we can see how these little suckers are moving so now what they're gonna. Do is they're gonna move through this primitive Groove right when they move through the primitive groove. They're gonna move down here where the hypoblast is But guess what happens? Remember this was the hypoblast, right? These cells, the epiblast cells that are now going to be functional and able to move, they're going to move down and they're going to replace the epiblast. So again, I can't stress that enough. The epiblast cells that are moving through the primitive groove now, down through that and into where the hypoblast used to be, it's going to be replaced with these epiblast cells. So this will no longer be what we call the hypoblast. We're actually going to call it the epiblast. Endoderm. Okay, it's going to become the endoderm. Now, what happens is again the epiblast cells will continue to release more fibroblast growth factor 8 and cause more of these epiblast cells to continue to keep moving towards that primitive groove. But then remember, more growth factor is released causing more epiblast cells after the endoderm has been formed. Remember that. after the endoderm has been formed, more epiblast cells are being stimulated to move through that primitive groove and down. Where do those cells go and how do they move is important because they're going to form the mesoderm. So let's come up here for a second. All right. So we've already had the cells. We'll draw here an orange. We've already had these cells replace the actual what? We've already had these cells move down. and replace the hypoblast and form the endoderm. And we're going to see that in another diagram in a second. Remember I told you, more growth factor is released. When more growth factor is released, more of these epiblast cells start moving through that primitive streak. But guess how they do it? This is the cool part. They move through this actual primitive groove. And when they do that, I'm going to show this in kind of like a broken arrow here. They move through and they move forward. So you had here. These cells coming in, right? And then when they come in, they move to the side and forward. Coming in, to the side, and forward. Coming in, to the side, and forward. And they're going to keep doing that. Now, as they start filling up that area, first layer we had, we replaced it, right? We had the hypoblast. What did it become now, I told you? It becomes the endoderm. I wanted to turn it into a different color. So it was this green. Now we're going to have it as a dark green. So again, epiblast cells migrated through first, they become the actual endoderm. So now this layer that used to be the hypoblast is now going to be called the endoderm. All right, that's going to be represented by this orange arrow. The cells move down through the primitive groove, move in, replace the actual hypoblast cells. Some books will say... that they replace all of it. Some will actually say that there is still some hypoblast cells that are actually remaining within that endodermal layer. Okay, then we said after the hypoblast is replaced and turned into the endoderm, then we said more epiblast cells are going to move through the actual primitive groove down, out, and forward. And they are going to make the new layer, which is going to be this red layer. That red layer that we have there is going to be the mesoderm. And then what do we have left up on the top? That didn't change right? It should still be the ectoderm. Or actually we're gonna that's what we're gonna call it now. So now instead of it being the epiblast we now call it the ectoderm. Alright, so let's recap, just because this is one of the tougher parts. Okay, first layer you have is epiblast, bottom layer, hypoblast, right? You have the primitive streak. Now remember, the primitive streak was the thickening of the epiblast tissue here, and then you're going to have a primitive node, which is the thickening of the epiblast tissue kind of in a nodular form. The tissue in the center of the primitive streak starts to disintegrate and form the primitive groove. The tissue inside of the center of the primitive node starts to disintegrate and form the primitive pit. Then the epiblast cells right around the edge of this primitive streak in the primitive node start secreting a chemical called fibroblast growth factor type 8. There's also other ones like transforming growth factor beta as well. These chemicals, what do they do? They cause the epithelial cells or these epiblast cells to no longer become adherent to one another by inhibiting the formation of E-cadherins, which are these cell-to-cell adhesion molecules. If those aren't linked up, now these epithelial cells can migrate, or the epiblast cells can migrate. Now, as they start migrating, where do they move? Remember, they move first through the actual primitive groove, downwards, and then what do they do first? They replace the hypoblast. So this was the hypoblast. What does it become next? It becomes the endoderm. And then the epiblast. is what we call the ectoderm. Now, the ectoderm is still going to release fibroblast growth factor, all right? So, more fibroblast growth factor is going to be released, causing more of these epiblasts or ectodermal cells to continue to migrate towards the actual primitive groove. But now, in this maroon or purplish color, these ones, when they move through the primitive groove, they move down, laterally, and forward, okay? As they move down laterally and forward, they make a new layer, which is called the mesoderm. Okay? So this is going to form our three germ layers. Now, this process is super critical because this, where we go from a bilaminar disc to a trilaminar disc, is a specific type of thing, and we call that gastrulation. So that's the importance that we have to understand. So this process here is called gastrulation. Now, the next thing that has to happen is, now that we have our three layers, here's the next part that gets really cool. What happens is, again, more growth factor is going to be released. This growth factor is going off the hook, all right? And it's causing more of these ectodermal cells to continue to keep migrating through. But now, instead of migrating through this actual primitive groove, they only migrate through the primitive pit. And as they start migrating, they move through the primitive pit and they move cranially towards the procordial plate. But when they move, you know what's so crazy? They form like a tube. So they're going to move through the actual primitive pit and they're going to move cranially or we can say cephalic towards the procordial plate and they're going to make this tube. Now I want us to see this tube from a different, two different ways. Once they move through They're going to make a tube. So imagine here, this is going to be the ectoderm. This red here is going to be the mesoderm. And this dark green here is going to be the endoderm. Here, you're going to have this hole inside of the ectoderm. What is that hole called? That is the primitive pit where the primitive node is. What happens is those ectodermal cells are going to migrate and move their way through the actual primitive pit. And they're going to make this nice little elongated tube that moves underneath the ectoderm, but above the endoderm. But as you can see here, where the nodal cord is, there is no mesoderm. There's three places that you have to remember that there is no mesoderm between the ectoderm and the endoderm. One is where the nodal cord is. Second is, what do we call this place right here? Where the front end, or the cranial end, where we kind of define cranial and caudal. We call that the procordial plate, right? So if I were to put right here, little black kind of lines here connecting, this right here is called our procordal plate, right? And we said right in front of that is where you're going to get your buccal pharyngeal membrane, right? Where the oral cavity forms. Right back here, what do you think is here? The cloacal plate, right? So back here, you're going to have the cloacal plate or membrane. We'll call it the plate. And that's going to form the butthole, right? Now, three places where there's no mesoderm. One is where the notochord is. Second is where the procordal plate is. B is where the cloacal plate or the cloacal membrane is. Now, I wanted us to get that view. So how we're looking at this, just so you guys kind of get an idea, is all I did was I'm taking this piece here, this coronal section that we're looking at here, which is in a three-dimensional structure, and I'm making a sagittal cut. So imagine I'm cutting right down the middle here of this primitive streak, kind of making a bilateral structure, okay? So I'm getting a sagittal cut here, and this is how we're looking at it. All right? So again, that's going to be the notochord. So that's an important thing to remember. The cells that are moving through the primitive pit, moving cranially towards the procordial plate, which is making a nice little tube or tubular process, is called the notochord. Why is the notochord important? Because the notochord is what helps to induce neurulation, which is another topic that we have to cover. So what is this structure here called? This is called the noto. cord. And if you guys are real good, you guys remember that what happens to the notochord after you've actually been developed. So what is it in the adult? What is it the remnant of it? All right, so imagine here we have the notochord. What happens is you have an intervertebral disc, right? And so here's the disc, right? And the disc is actually going to be made up primarily of dense fibrous kind of irregular connective tissue, okay? We call this the annulus fibrosus, right? But if I were to kind of make this into a three-dimensional structure, let's say, okay, in the center of it, in the center of this disc is a kind of a jelly-like material. And this jelly-like material is called the nucleus pulposus. So right here in the center, I'm going to have this jelly-like material, which is called the nucleus pulposus. Nucleus pulposus. That is the adult remnant of the notochord. So again, notochord forms, right? And it's going to be important because it releases certain types of growth factors that causes the thickening of the ectoderm, which forms the neuroplate, which allows for neurulation. But on top of that, as they start to develop, right, you're going to have the remnants of it, which is going to be the nucleus propulsus, which is the little jelly material in the annulus fibrosus of the intervertebral disc, which is between your vertebrae. All right, so that's the important thing. So again... This is just going to be a sagittal view. So just so you understand, this is going to be a sagittal view of that previous diagram. And then the one below it is going to be the same diagram, but just showing you now what it looks like to have the actual notochord. So imagine again, what happens is that the actual ectodermal cells move through that actual primitive pit and move and make that tube. Well, now here, look, here's your ectoderm. Here's your mesoderm and here's your endoderm. And what do you call this little tube here, which is just actually in between the mesoderm? That's going to be the notochord. And that's important to remember. Now, later on, what happens is the mesoderm, it'll actually differentiate, right? It'll become into three different components. It'll become the paraxial, which is the central part. Then the intermediate mesoderm, which is important for making like the kidneys and the gonads. And then towards the edge, you'll have the lateral platen mesoderm. And that's going to differentiate into what's called splanchnic, which is going to surround your GI organs. It's the muscle around the GI organs and some of the connective tissue. And also, you're going to have the somatic, which is going to be basically forming around a lot of the body. Okay, forming a lot of the body structures. So, that's important to remember. As we'll see in another video, the ectoderm is going to be able to make the skin and a lot of the nervous system. And the endoderm is going to be making the lining of the GI tract. It's going to be making some of the accessory organs, the glands. And we'll talk about that as well. But again, important to remember, the whole purpose of this video here is to get what happens with the outer cell mass, which is basically the cytotrophoblast and the syncytiotrophoblast. What is their significance? Implantation, right? And formation of the placenta. We'll go into more detail into that in another video. What was the other function of this video? The other purpose was for us to understand what happens with the inner cell mass, which is the embryoblast. which is going to be making what? Bilaminar disc. Bilaminar disc is important. If we blow it up, we'll see what happens. Thickening of the epiblast is going to give us primitive streak and node. That some of the cells will then break down, give us the primitive groove in the primitive pit. It'll release growth factors. The growth factors will cause the epiblast cells to move through the primitive groove, below it into the hypoblast, replacing the hypoblast and turning it into endoderm. More growth factor will cause more epiblast cells, but now what we specifically call ectodermal cells to move through the primitive groove and primitive pit and then make a new layer called the mesoderm. Then after we have the three layers are our A trilemn or disc, the process of gastrulation, we then lead to the formation of the nodal cord, which is in more ectodermal cells via growth factors, move through the primitive pit, and move forward, or we can say cranially, towards the procordial plate, making this tubal process, which is important for two reasons. One is it induces the formation of neurulation, the neural tube formation, as well as becomes the adult remnant called the nucleus pulposus. And... We can also get a better idea of this in another view just to get an idea of how this trilimiter disc is there along with the notochord. And we already talked about the function or just simple differentiation of these three layers, which is again the ectoderm, skin, nervous system, mesoderm, a lot of your connective tissues, muscles, ligaments, bones, and then on top of that the endoderm making a lot of the GI tract lining, some of the accessory organs and glands. So in this video we covered everything that we need to know particularly for the gastrulation process and the development of the nodal cord. And throughout these two videos that we've already started, we've already covered everything up until week two. So what we're going to do in the next set of videos is we're going to go into detail all the development of the embryo up until the third week and then we'll get into detail on the development of the placenta as well. If you guys like this video and if it did help, please hit that like button, comment down in the comment section. Also in our description box, you guys can go there check out our Facebook, Instagram, Patreon, GoFundMe page. any help or any assistance whatsoever. We would truly appreciate it. I hope this video made sense. I hope you guys did enjoy. We love you, Ninja Nerds. And as always, until next time.

So we talked about fertilization, we talked about cleavage, and we talked about the formation of the blastocyst, right? And then how we got to the end of that, we're going to now go into the development throughout the embryo up until week two. We're going to mainly talk about, in this video, gastrulation. We'll have a tiny little peek into implantation, but I don't want to get into too much detail on that.

Reason why is we'll go more into that when we talk about the development of the placenta, okay? But let's see where we left off. We left off talking about how the trophoblast, right, which is the outer cell mass, used to be the outer cell mass, we said it's going to get derived into a cytotrophoblast and a syncytiotrophoblast, and we'll talk about their significance.

We also talked about how the inner cell mass differentiated, become more specialized, and it became the embryo blast. And how it starts to differentiate and become a little bit more specialized, and it forms what's called the bilaminar disc, right? It's going to become the embryo. Trophoblast is important because it becomes a part of the chorion, and it becomes a part of the placenta, which is very important for being able to provide oxygen, nutrients, get rid of waste, all right? Between the mother and the fetus.

So we finished off here with our... Kind of like the end part here where we had our trophoblast, which we said we're going to differentiate into these two, and the embryoblast. Remember, what did I say?

Embryoblast turns into a bilimiter disc. We're going to talk about this a little bit more in just a second. I want to focus on the outer layer now. That's where it can get a little bit tough.

When you focus on these things, you want to focus on one part of the time. What I want to do first is we're going to talk about that outer layer first, just a little bit. Again, we'll get into it more when we talk about the development of the placenta.

I want to talk about... what the significance of the trophoblast is. Then after that, we'll come back and we'll talk about how the embryoblast develops into the trilaminar disc via the process of gastrulation.

All right, so what I want to do is let's go back into our diagram and talk about what's happening from this point, okay? All right, so let's go ahead and take a look at our diagram again. So here we have the uterus, right? And we already talked about what had happened to this point. We already said about how the anterior pituitary releases the LH, right?

LH triggers the ovulation process where we pop out that secondary oocyte, right? And it's frozen imidaphase 2 in the ampulla. Then we said that the sperm is going to meet the actual oocyte frozen imidaphase 2 at the level of the ampulla. We're going to get fertilization and then that whole process of cleavage is occurring, right?

And we talked about how cleavage is basically going from the zygote to the 4-cell stage to the 8-cell stage to the 16-cell stage and then to the blastocyst. Well, what happens is when all this cleavage process is occurring, this actual zygote or these different cleavage processes are actually, they're moving towards the uterine cavity. So then what happens is that little blastocyst, it kind of just pops off here into the actual uterine cavity. And then what happens is it has specific types of proteins on it, certain types of selectins or integrins that allow for it to hook up with the endometrium and start implanting itself. Right?

So what we're going to do now is we're going to take a deeper look, looking at how that actual, that blastocyst is going to implant and what are the processes with that outer trophoblastic layer. Okay? So what we have over here, again, don't worry yet about the bilimiter disc. We'll talk about this a little bit later. But remember we said that there's an outer cell layer, this outer blue lining.

We call that the cytotrophoblast. Well, the trophoblast which divides into a cytotrophoblast and syncytiotrophoblast. What happens is, imagine here for a second, you have these well-defined cells, okay?

And here they are, these kind of like baby blue cells right there. We're going to have these well-defined cell margins. These are called your cytotrophoblast.

So you have the well-defined cell margins with the nucleus inside of the cell, right? What happens is, some of the other cells start proliferating. And as they start proliferating, their cell membranes actually start disintegrating. So they start proliferating outside of that zona pellucida, right? So remember how we had the zona pellucida all the way around?

It starts releasing certain types of enzymes that allows for it to kind of break outside of that. And as it does that, these cells start proliferating outside of the cell, and then their cell membranes start disintegrating. When they start disintegrating, it forms kind of a fluid, if you will, kind of a cytoplasm. which is going to be consisting of the nuclei of these cells, right? So what happens is all the cell membranes of the...

These cells start actually breaking open and releasing out the nuclei and the cytoplasm into kind of just a little area. And what happens is all of the cytoplasm starts kind of fusing with one another, forming a syncytium, right? And that's going to be our syncytiotrophoblast.

So this part here, which is going to be kind of like these finger liposomes that are kind of extending out into the uterus. So this is our uterine lining right here. This is going to be called the syncytio. trophoblast.

Okay, and again it's important to remember just so I give you guys a good idea here. Imagine what there's all the cells, right? Here's the finger process.

All of the actual nuclei are just kind of sitting out here in the cytoplasm. There's no well-defined cell margins. Okay, that's our syncytiotrophoblast. At the base, you're going to have these well-defined cells.

which have their cell membrane and the nucleus with inside of that. Eventually, what will start happening is that these cells will start actually forming kind of a defined cell, kind of like finger process or villi that extends through the center of this, up into the center of that actual villus, right? But again, for right now, I want you to remember all this cytoplasm on the side with the nuclei kind of just within that fluid is called this incisio-trophoblast. And then the cells with the well-defined margins or membranes with the nuclei inside of them are called the cytotrophoblast. So again, these are your cytotrophoblasts.

And again, the syncytiotrophoblast is going to be just the actual syncytium of all the cell cytoplasm and nuclei kind of just sitting in this kind of pool of fluid. All right? Why is that important? Well, eventually what happens is...

The syncytiotrophoblast will continue to kind of release certain types of hydrolytic enzymes that will work its way through the actual uterine lining. And guess what is out here in the uterine lining? You have these maternal blood vessels. So let's just say here's going to be like a maternal blood vessel. Here's going to be a maternal blood vessel.

And here's another maternal blood vessel. What's eventually going to happen is that this actual syncytiotrophoblast is going to continue to keep working its way out here to kind of become confluent with these maternal blood vessels. Because once this actually becomes confluent with the maternal blood vessels, now the actual embryo can receive oxygen, nutrients, hormones, everything it needs in order for it to grow. Because there's going to be a connection between the embryo and the maternal vascular system via that syncytiotrophoblast and cytotrophoblast.

So that should make sense. Once it implants via the integrins and selectins to the uterine lining, It's going to burrow itself in a little bit, and then what happens is those cytotrophoblastic cells are going to start proliferating and working its way through the zona pellucida, making these finger-like processes. The cells inside of those finger-like processes start to disintegrate and break down. Their cytoplasm spills out of them, and their nuclei spills out of them.

And they form a pool of what we call a syncytium, which is going to be this syncytiotrophoblast. Then eventually what happens is... The cytotrophoblast continues to proliferate these cells and they proliferate right into the center of that actual villus. And eventually what's going to happen is this actual structure is going to become confluent with the actual maternal blood vessels. And before you know it, we're going to have a placenta, a connection between the vascular system of the mother and then the embryo.

And that's going to be the development of the placenta. We'll talk about that in more detail later. I just wanted to give you guys a little look into this.

Now. A little bit later throughout the process, we're going to kind of jump a little bit forward. As this actual embryo continues and continues to develop, right, here's the important thing.

Remember we said that the corpus luteum ejected out the secondary oocyte. And LH right now is what's driving the production of progesterone, right? And we were up to about, we said that it happened around day 14, day 15. We've already gone through about a week, all right? So we've already done about seven to eight days by now.

from the point of implantation. As we start getting closer to day 28, what starts happening? You start shedding the endometrial lining.

How do we prevent the endometrial lining from getting shed? Well, guess what happens a little bit later towards the, you know, around day 24, maybe day 21 to 24. The syncytiotrophoblast, it starts actually making a specific type of hormone. So cool.

It starts releasing what's called beta. HCG, okay, which is human chorionic gonadotropin. Now remember, I told you that progesterone levels start to drop later on in the luteal phase.

When that syncytiotrophoblast releases the beta HCG, That's what it does to the corpus luteum of the ovary. It has it continue to produce what? Progesterone. And so the progesterone is going to remain elevated. The reason why that's important is once progesterone levels start dropping, what happens to the vessels inside of the uterine lining?

They start spasming. And as they start spasming, what happens? They rupture. And as they rupture, the cells inside that endometrial lining start doing what? They start actually becoming necrotic and they start getting shed.

And if we shed the endometrial lining, what else will we shed? The embryo. And we want this to happen. We want them to be able to allow for the implantation. We don't want this to get shed out.

So, if it releases beta-HCG, it triggers the corpus luteum to continue to keep making progesterone. If progesterone levels maintain a certain level, we're not going to prevent those vessels from spasming. And so we're not going to allow for shedding.

We're going to allow for proper nutrition, glycogen. all the different types of lipids to be produced to nourish the baby. And so there's not going to be any shedding of the endometrial lining to ensure that there's proper implantation.

So that's such a cool thing. Again, we'll go into more detail about implantation and the development of the placenta later. Let's come back here and talk a little bit more about how the development of the embryo is occurring.

All right, so now we talked already about the process of the implantation. I just wanted you guys to not be like, wait, what happens to the trophoblast? Because we're primarily going to be focusing on the embryoblast and the whole... process of the bilaminar disc converting into the trilaminar disc.

So again, a little bit of information on that. We'll go into more detail on it later. But what we're going to do now is, just so we don't have to focus on that trophoblast, we're just going to take out that bilaminar disc, okay, and the cavity above it and the cavity below it, and just see what happens specifically with that, okay?

So take that out now, and here's the structure that we get. Let's make ourselves kind of understand what's going on here with this structure. So bilaminar. two discs, right? Two sheet-like discs.

The top layer is an important layer, okay? The top layer is what we call the epiblast, okay? So the top layer is the epiblast. The bottom layer is actually going to be what's called the hypoblast. And again, this is actually going to be a part of the bilimiter disc.

So this is the two components of the bilimiter disc. Now, below the bilimiter disc is going to be a little cavity. And what is that cavity called? That cavity is going to be called the yolk sac.

So it's kind of like the primitive yolk sac. So we'll put here YS. This is just going to be the primitive yolk sac. Above it is going to be a fluid-filled cavity, and this is going to be called the amniotic cavity.

So just so you guys have a little bit of an idea of what this actual microscopic structure is, is again, epiblast, hypoblast. Two layers sandwiched together. Above the epiblast is the amniotic cavity, and below the hypoblast is the primitive yolk sac. Okay?

Eventually, this will help with red blood cell production in the actual embryo. Okay? But it provides a lot of nutrients as well.

Now, what I want us to do is, and I want us to look at this in a way that helps us to kind of see what's going on, because that's kind of the biggest thing with embryology, is seeing the process, seeing the way that cells are moving, or the way things are migrating. It's the toughest part. All right, so what I want to do is I want to take a look at this actual bilimiter disc. Imagine we're looking at it from the top. Okay, so we're not going to see the hypoblast.

We're only going to see the epiblast from the top. Okay, so imagine it's kind of like this. Okay, so we're looking at it from the top. Now, what we have here is on the top you're going to have this little kind of like little fused area. It looks kind of like a thickening or kind of like a depression maybe.

You'll see it, it's called the procordial plate. And this is good because the reason why is this gives us kind of our orientation of anatomy. So here we're going to call this the procordial plate. And what the procordial plate is, is where the epiblast and the hypoblast are kind of just sandwiched together.

But the procordial plate gives us orientation of cranial and caudal. So with respect to the actual procordial plate, the top part of it, near it, is going to be the cranial. end, okay, or the head end. Away from it down towards this side is going to be the caudal or the tail portion, okay.

So here we're going to have that and then here we're going to have this. There's actually another membrane down here. If you really want to know, you'll have another membrane at the bottom here called the cloacal membrane and then you'll have a membrane right in front of the procordial plate which is called the buccopharyngeal membrane. Why that's important is buccal pharyngeal membrane, guess what that makes? The mouth.

Guess what the cloacal plate makes? The butthole. Okay? So I'll show you the two ends. Okay?

Procordial plate though, it gives you kind of an idea specifically of your positioning. So again, cranial end is where the procordial plate is, caudal end is going to be down here where the cloacal plate is, and then you'll have a membrane right in front of the actual procordial plate, which is called the buccal pharyngeal membrane. What I want us to do though is, is we're going to make a section, okay? I'm going to make a section, and we're going to look at it right here. So I'm going to cut right here.

Imagine I'm taking this piece, I'm looking at it from the top. I'm now going to take this section and look at it in kind of like a more of a coronal section. So we're going to look at it more of a coronal section. So now this is the one we're going to focus on. And now you're going to be able to see the hypoblast and then epiblast.

And then this little black area here is going to be the procordal plate. Here's what starts happening. At this stage, So we're getting ready, we're trying to move from week two all the way up to week three.

What starts happening is there's some signaling processes that occur. And these signaling processes start causing the epiblast cells to form kind of a thick area on top of them. So you're going to get like this very thickened area of epiblast cells. And it's going to make what we call a primitive streak.

So here you're going to have the... primitive streak, which is going to be this part, okay? And towards the actual cranial end, you're going to have like a little knobby, and that little knobby is going to be what's called the primitive node.

So you have the primitive streak, which is kind of a thickening of the epiblast cells, and the primitive node, which is kind of like this knob-like structure towards the top of the primitive streak. Now, here's where it gets really cool. Eventually, what happens is... Some of the cells in the center of the primitive streak start dying.

And some of the cells in the center of the primitive node start dying. And what's cool about this is now it makes like this little structure that we're going to have over here on the right. I just wanted to give you guys kind of like what's happening with this.

So we had the primitive streak, which is a thickening of tissue. Primitive node, which is a knob-like thickening of tissue. And then what happens is some of the cells within the center of that shriek and node start actually dying and making a little cavity. Pretty cool, right?

Now, why is that cavity important? Okay, let's come over here. Some of the cells near the edge of this primitive shriek, right, start secreting certain types of chemicals, right?

What kind of chemicals do they start secreting? They start secreting chemicals. which are called fibroblast growth factor 8. There's other types too, maybe transforming growth factor beta, but this is the main thing that the actual cells here near the edge are going to start secreting. So they're going to start secreting these chemicals, these fibroblast growth factor 8. Now, what does this fibroblast growth factor 8 do? It's so freaking cool what it does.

Let me show you. These cells, they start secreting that fibroblast growth factor 8. That fibroblast growth factor 8 is going to move these nearby, remember it starts secreting it laterally. So it's going to move from the center, moving laterally.

And it's going to go to these nearby epiblast cells. When it binds onto the epiblast cells, it has a receptor. So it's a fibroblast growth factor receptor.

Once it binds onto this receptor, it triggers an intracellular process, right? And this intracellular process, what it does is it activates a specific type of protein called Snail-1. Heck of a name, right?

So a fibroblast growth factor binds onto the receptor, triggers an intracellular process that stimulates the formation of a protein called Snail-1. Now, what Snail-1 does is it inhibits the formation of a specific type of protein that allows for cells to be linked up with one another. Do you know what that protein is called?

It's called E-cadherins. So it's called E-cadherins. Now what E-cadherins do is they allow for cells to stick with one another. So here's another epiblast cell. So two epiblast cells connected to one another through this E-cadherins.

Now with these E-cadherins the cells aren't allowed for to move away from one another, right? So they're stuck to one another. They're in a localized place.

If you stimulate this protein with the fibroblast growth factor, guess what it does? It inhibits the formation of these proteins. If these proteins aren't connecting the two cells with one another, what can they now do?

migrate. So now this cell is free to move and this cell is free to move. Guess what they call that? Epithelial migration.

Okay, so it's called epithelial migration. So now these cells that are going to be over here on the lateral side, guess what they're going to start doing? They are going to be stimulated and they're going to start migrating towards that primitive streak.

Now, here's what's so darn cool. Once they move through that primitive streak, right? Remember there's a little space there?

We had the primitive streak, which is the thickening. In the primitive streak, that little space that we have there now, we actually call it the primitive groove. If you really want to be particular, the primitive streak was the thickening with no space. Now that it has the space, it's called the primitive groove. What we used to have was the primitive node, which is that knob-like thickening, and it has a space now.

It's technically called the primitive pit. I'll write that down. Just for the heck of it though, right?

So what used to be the primitive streak is now going to be called the primitive groove, which is going to be again that space right there. And then what used to be the primitive node is now going to be called the primitive pit. And all it is is just basically saying the space within the primitive node and the space within the primitive streak. That's all it is.

So we release the fibroblast growth factor 8. Those epiblast cells are now going to be Free to move where are they gonna move they're gonna move through the primitive groove once they move through the primitive groove Guess what happens now look at this. Let's make like a little space here, too So we can see how these little suckers are moving so now what they're gonna. Do is they're gonna move through this primitive Groove right when they move through the primitive groove.

They're gonna move down here where the hypoblast is But guess what happens? Remember this was the hypoblast, right? These cells, the epiblast cells that are now going to be functional and able to move, they're going to move down and they're going to replace the epiblast. So again, I can't stress that enough. The epiblast cells that are moving through the primitive groove now, down through that and into where the hypoblast used to be, it's going to be replaced with these epiblast cells.

So this will no longer be what we call the hypoblast. We're actually going to call it the epiblast. Endoderm.

Okay, it's going to become the endoderm. Now, what happens is again the epiblast cells will continue to release more fibroblast growth factor 8 and cause more of these epiblast cells to continue to keep moving towards that primitive groove. But then remember, more growth factor is released causing more epiblast cells after the endoderm has been formed. Remember that. after the endoderm has been formed, more epiblast cells are being stimulated to move through that primitive groove and down.

Where do those cells go and how do they move is important because they're going to form the mesoderm. So let's come up here for a second. All right. So we've already had the cells.

We'll draw here an orange. We've already had these cells replace the actual what? We've already had these cells move down.

and replace the hypoblast and form the endoderm. And we're going to see that in another diagram in a second. Remember I told you, more growth factor is released. When more growth factor is released, more of these epiblast cells start moving through that primitive streak. But guess how they do it?

This is the cool part. They move through this actual primitive groove. And when they do that, I'm going to show this in kind of like a broken arrow here.

They move through and they move forward. So you had here. These cells coming in, right? And then when they come in, they move to the side and forward. Coming in, to the side, and forward.

Coming in, to the side, and forward. And they're going to keep doing that. Now, as they start filling up that area, first layer we had, we replaced it, right? We had the hypoblast. What did it become now, I told you?

It becomes the endoderm. I wanted to turn it into a different color. So it was this green.

Now we're going to have it as a dark green. So again, epiblast cells migrated through first, they become the actual endoderm. So now this layer that used to be the hypoblast is now going to be called the endoderm. All right, that's going to be represented by this orange arrow. The cells move down through the primitive groove, move in, replace the actual hypoblast cells.

Some books will say... that they replace all of it. Some will actually say that there is still some hypoblast cells that are actually remaining within that endodermal layer. Okay, then we said after the hypoblast is replaced and turned into the endoderm, then we said more epiblast cells are going to move through the actual primitive groove down, out, and forward. And they are going to make the new layer, which is going to be this red layer.

That red layer that we have there is going to be the mesoderm. And then what do we have left up on the top? That didn't change right? It should still be the ectoderm.

Or actually we're gonna that's what we're gonna call it now. So now instead of it being the epiblast we now call it the ectoderm. Alright, so let's recap, just because this is one of the tougher parts. Okay, first layer you have is epiblast, bottom layer, hypoblast, right? You have the primitive streak.

Now remember, the primitive streak was the thickening of the epiblast tissue here, and then you're going to have a primitive node, which is the thickening of the epiblast tissue kind of in a nodular form. The tissue in the center of the primitive streak starts to disintegrate and form the primitive groove. The tissue inside of the center of the primitive node starts to disintegrate and form the primitive pit.

Then the epiblast cells right around the edge of this primitive streak in the primitive node start secreting a chemical called fibroblast growth factor type 8. There's also other ones like transforming growth factor beta as well. These chemicals, what do they do? They cause the epithelial cells or these epiblast cells to no longer become adherent to one another by inhibiting the formation of E-cadherins, which are these cell-to-cell adhesion molecules.

If those aren't linked up, now these epithelial cells can migrate, or the epiblast cells can migrate. Now, as they start migrating, where do they move? Remember, they move first through the actual primitive groove, downwards, and then what do they do first?

They replace the hypoblast. So this was the hypoblast. What does it become next? It becomes the endoderm. And then the epiblast.

is what we call the ectoderm. Now, the ectoderm is still going to release fibroblast growth factor, all right? So, more fibroblast growth factor is going to be released, causing more of these epiblasts or ectodermal cells to continue to migrate towards the actual primitive groove.

But now, in this maroon or purplish color, these ones, when they move through the primitive groove, they move down, laterally, and forward, okay? As they move down laterally and forward, they make a new layer, which is called the mesoderm. Okay? So this is going to form our three germ layers. Now, this process is super critical because this, where we go from a bilaminar disc to a trilaminar disc, is a specific type of thing, and we call that gastrulation.

So that's the importance that we have to understand. So this process here is called gastrulation. Now, the next thing that has to happen is, now that we have our three layers, here's the next part that gets really cool. What happens is, again, more growth factor is going to be released.

This growth factor is going off the hook, all right? And it's causing more of these ectodermal cells to continue to keep migrating through. But now, instead of migrating through this actual primitive groove, they only migrate through the primitive pit.

And as they start migrating, they move through the primitive pit and they move cranially towards the procordial plate. But when they move, you know what's so crazy? They form like a tube. So they're going to move through the actual primitive pit and they're going to move cranially or we can say cephalic towards the procordial plate and they're going to make this tube.

Now I want us to see this tube from a different, two different ways. Once they move through They're going to make a tube. So imagine here, this is going to be the ectoderm.

This red here is going to be the mesoderm. And this dark green here is going to be the endoderm. Here, you're going to have this hole inside of the ectoderm.

What is that hole called? That is the primitive pit where the primitive node is. What happens is those ectodermal cells are going to migrate and move their way through the actual primitive pit.

And they're going to make this nice little elongated tube that moves underneath the ectoderm, but above the endoderm. But as you can see here, where the nodal cord is, there is no mesoderm. There's three places that you have to remember that there is no mesoderm between the ectoderm and the endoderm.

One is where the nodal cord is. Second is, what do we call this place right here? Where the front end, or the cranial end, where we kind of define cranial and caudal. We call that the procordial plate, right?

So if I were to put right here, little black kind of lines here connecting, this right here is called our procordal plate, right? And we said right in front of that is where you're going to get your buccal pharyngeal membrane, right? Where the oral cavity forms. Right back here, what do you think is here?

The cloacal plate, right? So back here, you're going to have the cloacal plate or membrane. We'll call it the plate.

And that's going to form the butthole, right? Now, three places where there's no mesoderm. One is where the notochord is.

Second is where the procordal plate is. B is where the cloacal plate or the cloacal membrane is. Now, I wanted us to get that view. So how we're looking at this, just so you guys kind of get an idea, is all I did was I'm taking this piece here, this coronal section that we're looking at here, which is in a three-dimensional structure, and I'm making a sagittal cut. So imagine I'm cutting right down the middle here of this primitive streak, kind of making a bilateral structure, okay?

So I'm getting a sagittal cut here, and this is how we're looking at it. All right? So again, that's going to be the notochord.

So that's an important thing to remember. The cells that are moving through the primitive pit, moving cranially towards the procordial plate, which is making a nice little tube or tubular process, is called the notochord. Why is the notochord important? Because the notochord is what helps to induce neurulation, which is another topic that we have to cover.

So what is this structure here called? This is called the noto. cord.

And if you guys are real good, you guys remember that what happens to the notochord after you've actually been developed. So what is it in the adult? What is it the remnant of it?

All right, so imagine here we have the notochord. What happens is you have an intervertebral disc, right? And so here's the disc, right?

And the disc is actually going to be made up primarily of dense fibrous kind of irregular connective tissue, okay? We call this the annulus fibrosus, right? But if I were to kind of make this into a three-dimensional structure, let's say, okay, in the center of it, in the center of this disc is a kind of a jelly-like material.

And this jelly-like material is called the nucleus pulposus. So right here in the center, I'm going to have this jelly-like material, which is called the nucleus pulposus. Nucleus pulposus.

That is the adult remnant of the notochord. So again, notochord forms, right? And it's going to be important because it releases certain types of growth factors that causes the thickening of the ectoderm, which forms the neuroplate, which allows for neurulation. But on top of that, as they start to develop, right, you're going to have the remnants of it, which is going to be the nucleus propulsus, which is the little jelly material in the annulus fibrosus of the intervertebral disc, which is between your vertebrae. All right, so that's the important thing.

So again... This is just going to be a sagittal view. So just so you understand, this is going to be a sagittal view of that previous diagram. And then the one below it is going to be the same diagram, but just showing you now what it looks like to have the actual notochord. So imagine again, what happens is that the actual ectodermal cells move through that actual primitive pit and move and make that tube.

Well, now here, look, here's your ectoderm. Here's your mesoderm and here's your endoderm. And what do you call this little tube here, which is just actually in between the mesoderm?

That's going to be the notochord. And that's important to remember. Now, later on, what happens is the mesoderm, it'll actually differentiate, right? It'll become into three different components. It'll become the paraxial, which is the central part.

Then the intermediate mesoderm, which is important for making like the kidneys and the gonads. And then towards the edge, you'll have the lateral platen mesoderm. And that's going to differentiate into what's called splanchnic, which is going to surround your GI organs.

It's the muscle around the GI organs and some of the connective tissue. And also, you're going to have the somatic, which is going to be basically forming around a lot of the body. Okay, forming a lot of the body structures. So, that's important to remember.

As we'll see in another video, the ectoderm is going to be able to make the skin and a lot of the nervous system. And the endoderm is going to be making the lining of the GI tract. It's going to be making some of the accessory organs, the glands. And we'll talk about that as well.

But again, important to remember, the whole purpose of this video here is to get what happens with the outer cell mass, which is basically the cytotrophoblast and the syncytiotrophoblast. What is their significance? Implantation, right?

And formation of the placenta. We'll go into more detail into that in another video. What was the other function of this video? The other purpose was for us to understand what happens with the inner cell mass, which is the embryoblast. which is going to be making what?

Bilaminar disc. Bilaminar disc is important. If we blow it up, we'll see what happens.

Thickening of the epiblast is going to give us primitive streak and node. That some of the cells will then break down, give us the primitive groove in the primitive pit. It'll release growth factors. The growth factors will cause the epiblast cells to move through the primitive groove, below it into the hypoblast, replacing the hypoblast and turning it into endoderm.

More growth factor will cause more epiblast cells, but now what we specifically call ectodermal cells to move through the primitive groove and primitive pit and then make a new layer called the mesoderm. Then after we have the three layers are our A trilemn or disc, the process of gastrulation, we then lead to the formation of the nodal cord, which is in more ectodermal cells via growth factors, move through the primitive pit, and move forward, or we can say cranially, towards the procordial plate, making this tubal process, which is important for two reasons. One is it induces the formation of neurulation, the neural tube formation, as well as becomes the adult remnant called the nucleus pulposus.

And... We can also get a better idea of this in another view just to get an idea of how this trilimiter disc is there along with the notochord. And we already talked about the function or just simple differentiation of these three layers, which is again the ectoderm, skin, nervous system, mesoderm, a lot of your connective tissues, muscles, ligaments, bones, and then on top of that the endoderm making a lot of the GI tract lining, some of the accessory organs and glands.

So in this video we covered everything that we need to know particularly for the gastrulation process and the development of the nodal cord. And throughout these two videos that we've already started, we've already covered everything up until week two. So what we're going to do in the next set of videos is we're going to go into detail all the development of the embryo up until the third week and then we'll get into detail on the development of the placenta as well. If you guys like this video and if it did help, please hit that like button, comment down in the comment section. Also in our description box, you guys can go there check out our Facebook, Instagram, Patreon, GoFundMe page.

any help or any assistance whatsoever. We would truly appreciate it. I hope this video made sense.

I hope you guys did enjoy. We love you, Ninja Nerds. And as always, until next time.

Transcript for:Embryo Development Overview: Weeks 1-2

Transcript for:
Embryo Development Overview: Weeks 1-2