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Current time:0:00Total duration:12:20

Early embryogenesis - Cleavage, blastulation, gastrulation, and neurulation

Video transcript

we're going to talk about early embryogenesis say you're an egg cell you have this nice thick outer glycoprotein coat called the zona pellucida and you've got your plasma membrane just inside of that and the sperm has made its way through the zona pellucida and managed to get in through your plasma membrane and merged its genetic material with yours you're now called a zygote and you'd like to go on to form an embryo but not much is going to happen when you're stuck as a single cell and so what you've got to do is divide into multiple cells and you've got to do it fast in fact you've got to do it so fast as you don't have time to grow so you actually just split into two cells and this process of splitting without growth is called cleavage and you do this a number of times dividing from two cells to four and from four cells to eight all the while staying within the zona pellucida so you've gone from sixteen cells to thirty-two cells and at this point you look different enough that somebody decides to give you a new name so instead of being called a zygote you're called amore Allah and Morial is just a Greek word that means mulberry and you actually do look a little bit like a mulberry so here's a picture of a mulberry from my front yard to prove it this one's not quite ripe so you're finished with the cleavage stage of early embryogenesis you've gone from one cell and just divided without growing at all into 2 4 8 16 and then 32 cells and now more interesting things start to happen so you're still stuck within that zona pellucida so we'll draw it in here and I'm not going to keep track of the number of cells anymore I'm just going to draw them in but what you'll notice is I'm drawing them in quite compacted I'm drawing them all right in the middle of the structure here and I do look like they're a little bit tighter together and this is a process that's called compaction the different cells within the mauriello start to get closer and closer together and in fact the stills start to get a little bit different from each other - and you notice that these cells on the outside are a little bit different I'll draw them in here slightly different color and we can tell them apart in this process of being able to tell cells apart as they become different Ang's is called differentiation so here we have two separate populations of cells this one on the outside we'll call them trophoblast and this mass of cells on the inside we'll call embryo blasts and so you're going to continue with your process and we'll move on to the next stage and we'll draw our zona pellucida here and we'll draw all of our trophoblasts along the outside here another interesting thing is happening to those cells in the middle they're starting to clump even more in fact they clump so much that they all cluster at one end leaving a little cavity on the other end so here you can see your trophoblasts and your embryo blasts this mass of cells in the middle some people actually call that the inner cell mass and it's left you with this cavity and that cavity is called a blastocyst so we're going to give you a new name this structure that you've turned into with a blasto seal and an outer cell ring of trophoblasts is called a blastocyst in fact we name this whole process after you and after cleavage the process is called blast ulation so also about this stage in blast chelation your zona pellucida starts to disintegrate away here I'm going to draw a little bites being taken out of it here as it falls away and disintegrates and that's going to be important later on because you can't be stuck in this thing forever and the next step in blast chelation you actually just lose it completely so now we'll draw our trophoblasts completely naked without zona pellucida but more interesting things are happening to you your inner cell mass of embryo blasts is starting to look a little bit different so you still have this rim on the outside here up at the end of your blastocyst and you still have this mass here in the middle but you developed another cavity and this cavity is called the amniotic cavity and also your inner cell mass of embryo blasts has started to differentiate more and now it's got this layer on the bottom of it here and the cells in this layer are called hypo blasts while the cells in the layer just above it are called EPI blasts now at this stage we're pretty much completely free of our zona pellucida and that's going to be really important for implantation but that's a discussion for another time we're going to focus on this ball of cells here and I really want to stress that this is a ball of cells it's not flat like we've got it drawn here on the computer screen it's 3-dimensional it's spherical in nature a spherical melon so think of it like a melon here I'm going to draw a spherical melon so now I want you to picture taking a big knife or a machete or something and just slicing the top off the melon like this well here we'll erase the top part of our melon here and we're left with a flat surface so now that you've got a melon with a flat surface picture yourself taking a pancake and just putting it right on top of this flat surface like that and that's basically what we've got here with our blastocyst and this forming layer of at the blasts and hypo blasts it's really a pancake it's not two-dimensional here it's a 3-dimensional disc of tissue so we're going to draw this again but this time without the outer sphere of trophoblasts but we're going to draw our pancake of epiblast s' and hypo blasts and what you can see here is our pancake has got two layers so in fact this is a very important structure in embryology called the by laminar disc and we're going to get another look at the bilaminar disk here so I'll show you we're actually going to draw a little plane through the disc here and if we take a cut view through this plane that I've drawn it looks a little bit like this so this is still our by laminar disk but now we're looking at a slice through the bilaminar disk as opposed to here we're looking at the entire bilaminar disk from the outside so now we're looking at our pancake of our bilaminar disk here and we notice we start to see something forming on the edge of it here and that something looks like a little bit of streakiness that kind of splits our pancake right into you can think of it as pouring a little streak of syrup along the top of the pancake and dividing it into two halves so here I'm going to draw it on the surface of the pancake and then over here on our cut view you can see that this little streak happens right about here in the middle as it reaches the plane of our image and this streak of syrup on our pancake is got a name it's actually called the primitive streak and the formation of the primitive streak marks the beginning of the next stage of early embryogenesis and that's called gastrulation now what that primitive streak actually is is just the site where the cells in this epitome inner disk start to migrate here I'll draw the paths of the migrating cells they keep themselves up right at the primitive streak and then they start burrowing their way down into this bilaminar disk they go out into the hypo blast layer and I just kind of go all over the place here but it all happens from that primitive streak so as the cells migrate out from the primitive streak we can see that our structure has changed a little bit now instead of having two layers one layer of epi blasts and one layer of hypo blasts all of those migrating cells have now differentiated even further and we're left with a layer of cells on the top a middle layer of cells as they go out into the body of the bilaminar disk and we still have a lower layer of cells down here and so now you can see instead of the bilaminar disk that we have we actually have a tri laminar disk so we have three layers one two and three and these are actually called our germ layers so we have a single layer on top and instead of epiblast this layer is now known as ectoderm and our middle layer is mesoderm and our lower layer is endoderm and cells from these three layers go off to do very important things and each layer forms on specific structures so that process of the formation of the three layers is called gastrulation once our three layers are formed we can move on to the next step in embryogenesis and the next step is called neural ation so we'll draw our three germ layers in they have a layer of ectoderm on the top in the middle we have a layer of mesoderm and then on the bottom we have our endoderm so this final stage in early embryogenesis is called neural ation and as you might expect with neuro in the name we're going to see some neural elements formed here and now we have our tri laminar disk with our three germ layers and in the middle of the mesoderm the central layer here we start to have further differentiation of cells and right in the middle we get this cord that starts forming and I'm going to draw it is a little purple dot here but what that really is is underneath where the primitive streak was mesoderm cells are differentiating into a chord structure and that's called a notochord now in humans the nodal cord doesn't go on to do a whole lot it does form part of the intervertebral discs and very rarely it will cause a tumor called a cord oma but in general its main goal and its main purpose is here in neural ation now this differentiated bit of music Durham actually induces a change in the ectoderm above it so here we get a change within the ectoderm I'll draw that in is kind of a thickening of the ectoderm right here and that thickening in the ectoderm has a name since it's kind of plate like when we look at it in section here this is called the neural plate now we have our nodal cord formed and we have our neural plate form the next thing that happens so we'll redraw our three layers here or ectoderm mesoderm and our endoderm and here at the bottom we have our nodal cord forming within our mesoderm and right above it those neural plate cells actually start to dive into the mesoderm and as those neural plate cells dive in they start to form a ring structure and actually since this is a three-dimensional thing this ring is more like a tube so you can picture this tube going off into the rest of this pancake that we've drawn here and as this neural plate zips up and dives down into the mesoderm it becomes known as the neural tube now this isn't a perfect process and as the neural tube is zipping up from one end of our pancake to the other little cells are breaking off from the ectoderm and going out into the mesoderm now these cells actually have a very important role as well and will go off into differentiate into their own special tissues and these are called neural crest cells so once we have the formation of our neural tube and our neural crest cells are diving off into the tissues and differentiating and we have a pretty good idea of what the ectoderm mesoderm and endoderm are our early embryogenesis is complete so just to recap we started off as a zygote a single cell that was fertilized by a sperm cleavage happened we didn't grow but we split into a whole bunch of cells and developed into a more Yola the more EULA cells started to differentiate and we developed trophoblasts on the outside and embryo blasts in the middle we formed a little cavity and became known as a blastocyst after the blastocyst cavity formed or the blast Osio a second cabin II called the amniotic cavity formed and a pancake of cells across the sphere became apparent those cells continued to differentiate and we formed a primitive streak the epiblast cells dove into the primitive streak and started to differentiate into our three layers that became our ectoderm mesoderm and endoderm and in the final stage of neural ation we had development of a notochord which induced a neural plate to form and the neural plate dove into the miso Derman formed the neural tube neural crest cells associated with the neural plate also went off into the tissues to further differentiate