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Where we left off after the meiosis videos is that we had two gametes. We had a sperm and an egg. Let me draw the sperm. So you had the sperm and then you had an egg. Maybe I'll do the egg in a different color. That's the egg, and we all know how this story goes. The sperm fertilizes the egg. And a whole cascade of events start occurring. The walls of the egg then become impervious to other sperm so that only one sperm can get in, but that's not the focus of this video. The focus of this video is how this fertilized egg develops once it has become a zygote. So after it's fertilized, you remember from the meiosis videos that each of these were haploid, or that they had-- oh, I added an extra i there-- that they had half the contingency of the DNA. As soon as the sperm fertilizes this egg, now, all of a sudden, you have a diploid zygote. Let me do that. So now let me pick a nice color. So now you're going to have a diploid zygote that's going to have a 2N complement of the DNA material or kind of the full complement of what a normal cell in our human body would have. So this is diploid, and it's a zygote, which is just a fancy way of saying the fertilized egg. And it's now ready to essentially turn into an organism. So immediately after fertilization, this zygote starts experiencing cleavage. It's experiencing mitosis, that's the mechanism, but it doesn't increase a lot in size. So this one right here will then turn into-- it'll just split up via mitosis into two like that. And, of course, these are each 2N, and then those are going to split into four like that. And each of these have the same exact genetic complement as that first zygote, and it keeps splitting. And this mass of cells, we can start calling it, this right here, this is referred to as the morula. And actually, it comes from the word for mulberry because it looks like a mulberry. So actually, let me just kind of simplify things a little bit because we don't have to start here. So we start with a zygote. This is a fertilized egg. It just starts duplicating via mitosis, and you end up with a ball of cells. It's often going to be a power of two, because these cells, at least in the initial stages are all duplicating all at once, and then you have this morula. Now, once the morula gets to about 16 cells or so-- and we're talking about four or five days. This isn't an exact process-- they started differentiating a little bit, where the outer cells-- and this kind of turns into a sphere. Let me make it a little bit more sphere like. So it starts differentiating between-- let me make some outer cells. This would be a cross-section of it. It's really going to look more like a sphere. That's the outer cells and then you have your inner cells on the inside. These outer cells are called the trophoblasts. Let me do it in a different color. Let me scroll over. I don't want to go there. And then the inner cells, and this is kind of the crux of what this video is all about-- let me scroll down a little bit. The inner cells-- pick a suitable color. The inner cells right there are called the embryoblast. And then what's going to happen is some fluid's going to start filling in some of this gap between the embryoblast and the trophoblast, so you're going to start having some fluid that comes in there, and so the morula will eventually look like this, where the trophoblast, or the outer membrane, is kind of this huge sphere of cells. And this is all happening as they keep replicating. Mitosis is the mechanism, so now my trophoblast is going to look like that, and then my embryoblast is going to look like this. Sometimes the embryoblast-- so this is the embryoblast. Sometimes it's also called the inner cell mass, so let me write that. And this is what's going to turn into the organism. And so, just so you know a couple of the labels that are involved here, if we're dealing with a mammalian organism, and we are mammals, we call this thing that the morula turned into is a zygote, then a morula, then the cells of the morula started to differentiate into the trophoblast, or kind of the outside cells, and then the embryoblast. And then you have this space that forms here, and this is just fluid, and it's called the blastocoel. A very non-intuitive spelling of the coel part of blastocoel. But once this is formed, this is called a blastocyst. That's the entire thing right here. Let me scroll down a little bit. This whole thing is called the blastocyst, and this is the case in humans. Now, it can be a very confusing topic, because a lot of times in a lot of books on biology, you'll say, hey, you go from the morula to the blastula or the blastosphere stage. Let me write those words down. So sometimes you'll say morula, and you go to blastula. Sometimes it's called the blastosphere. And I want to make it very clear that these are essentially the same stages in development. These are just for-- you know, in a lot of books, they'll start talking about frogs or tadpoles or things like that, and this applies to them. While we're talking about mammals, especially the ones that are closely related to us, the stage is the blastocyst stage, and the real differentiator is when people talk about just blastula and blastospheres. There isn't necessarily this differentiation between these outermost cells and these embryonic, or this embryoblast, or this inner cell mass here. But since the focus of this video is humans, and really that's where I wanted to start from, because that's what we are and that's what's interesting, we're going to focus on the blastocyst. Now, everything I've talked about in this video, it was really to get to this point, because what we have here, these little green cells that I drew right here in the blastocysts, this inner cell mass, this is what will turn into the organism. And you say, OK, Sal, if that's the organism, what's all of these purple cells out here? This trophoblast out there? That is going to turn into the placenta, and I'll do a future video where in a human, it'll turn into a placenta. So let me write that down. It'll turn into the placenta. And I'll do a whole future video about I guess how babies are born, and I actually learned a ton about that this past year because a baby was born in our house. But the placenta is really kind of what the embryo develops inside of, and it's the interface, especially in humans and in mammals, between the developing fetus and its mother, so it kind of is the exchange mechanism that separates their two systems, but allows the necessary functions to go on between them. But that's not the focus of this video. The focus of this video is the fact that these cells, which at this point are-- they've differentiated themselves away from the placenta cells, but they still haven't decided what they're going to become. Maybe this cell and its descendants eventually start becoming part of the nervous system, while these cells right here might become muscle tissue, while these cells right here might become the liver. These cells right here are called embryonic stem cells, and probably the first time in this video you're hearing a term that you might recognize. So if I were to just take one of these cells, and actually, just to introduce you to another term, you know, we have this zygote. As soon as it starts dividing, each of these cells are called a blastomere. And you're probably wondering, Sal, why does this word blast keep appearing in this kind of embryology video, these development videos? And that comes from the Greek for spore: blastos. So the organism is beginning to spore out or grow. I won't go into the word origins of it, but that's where it comes from and that's why everything has this blast in it. So these are blastomeres. So when I talk what embryonic stem cells, I'm talking about the individual blastomeres inside of this embryoblast or inside of this inner cell mass. These words are actually unusually fun to say. So each of these is an embryonic stem cell. Let me write this down in a vibrant color. So each of these right here are embryonic stem cells, and I wanted to get to this. And the reason why these are interesting, and I think you already know, is that there's a huge debate around these. One, these have the potential to turn into anything, that they have this plasticity. That's another word that you might hear. Let me write that down, too: plasticity. And the word essentially comes from, you know, like a plastic can turn into anything else. When we say that something has plasticity, we're talking about its potential to turn into a lot of different things. So the theory is, and there's already some trials that seem to substantiate this, especially in some lower organisms, that, look, if you have some damage at some point in your body-- let me draw a nerve cell. Let me say I have a-- I won't go into the actual mechanics of a nerve cell, but let's say that we have some damage at some point on a nerve cell right there, and because of that, someone is paralyzed or there's some nerve dysfunction. We're dealing with multiple sclerosis or who knows what. The idea is, look, we have these cell here that could turn into anything, and we're just really understanding how it knows what to turn into. It really has to look at its environment and say, hey, what are the guys around me doing, and maybe that's what helps dictate what it does. But the idea is you take these things that could turn to anything and you put them where the damage is, you layer them where the damage is, and then they can turn into the cell that they need to turn into. So in this case, they would turn into nerve cells. They would turn to nerve cells and repair the damage and maybe cure the paralysis for that individual. So it's a huge, exciting area of research, and you could even, in theory, grow new organs. If someone needs a kidney transplant or a heart transplant, maybe in the future, we could take a colony of these embryonic stem cells. Maybe we can put them in some type of other creature, or who knows what, and we can turn it into a replacement heart or a replacement kidney. So there's a huge amount of excitement about what these can do. I mean, they could cure a lot of formerly uncurable diseases or provide hope for a lot of patients who might otherwise die. But obviously, there's a debate here. And the debate all revolves around the issue of if you were to go in here and try to extract one of these cells, you're going to kill this embryo. You're going to kill this developing embryo, and that developing embryo had the potential to become a human being. It's a potential that obviously has to be in the right environment, and it has to have a willing mother and all of the rest, but it does have the potential. And so for those, especially, I think, in the pro-life camp, who say, hey, anything that has a potential to be a human being, that is life and it should not be killed. So people on that side of the camp, they're against the destroying of this embryo. I'm not making this video to take either side to that argument, but it's a potential to turn to a human being. It's a potential, right? So obviously, there's a huge amount of debate, but now, now you know in this video what people are talking about when they say embryonic stem cells. And obviously, the next question is, hey, well, why don't they just call them stem cells as opposed to embryonic stem cells? And that's because in all of our bodies, you do have what are called somatic stem cells. Let me write that down. Somatic or adults stem cells. And we all have them. They're in our bone marrow to help produce red blood cells, other parts of our body, but the problem with somatic stem cells is they're not as plastic, which means that they can't form any type of cell in the human body. There's an area of research where people are actually maybe trying to make them more plastic, and if they are able to take these somatic stem cells and make them more plastic, it might maybe kill the need to have these embryonic stem cells, although maybe if they do this too good, maybe these will have the potential to turn into human beings as well, so that could become a debatable issue. But right now, this isn't an area of debate because, left to their own devices, a somatic stem cell or an adult stem cell won't turn into a human being, while an embryonic one, if it is implanted in a willing mother, then, of course, it will turn into a human being. And I want to make one side note here, because I don't want to take any sides on the debate of-- well, I mean, facts are facts. This does have the potential to turn into a human being, but it also has the potential to save millions of lives. Both of those statements are facts, and then you can decide on your own which side of that argument you'd like to or what side of that balance you would like to kind of put your own opinion. But there's one thing I want to talk about that in the public debate is never brought up. So you have this notion of when you-- to get an embryonic stem cell line, and when I say a stem cell line, I mean you take a couple of stem cells, or let's say you take one stem cell, and then you put it in a Petri dish, and then you allow it to just duplicate. So this one turns into two, those two turn to four. Then someone could take one of these and then put it in their own Petri dish. These are a stem cell line. They all came from one unique embryonic stem cell or what initially was a blastomere. So that's what they call a stem cell line. So the debate obviously is when you start an embryonic stem cell line, you are destroying an embryo. But I want to make the point here that embryos are being destroyed in other processes, and namely, in-vitro fertilization. And maybe this'll be my next video: fertilization. And this is just the notion that they take a set of eggs out of a mother. It's usually a couple that's having trouble having a child, and they take a bunch of eggs out of the mother. So let's say they take maybe 10 to 30 eggs out of the mother. They actually perform a surgery, take them out of the ovaries of the mother, and then they fertilize them with semen, either it might come from the father or a sperm donor, so then all of these becomes zygotes once they're fertilized with semen. So these all become zygotes, and then they allow them to develop, and they usually allow them to develop to the blastocyst stage. So eventually all of these turn into blastocysts. They have a blastocoel in the center, which is this area of fluid. They have, of course, the embryo, the inner cell mass in them, and what they do is they look at the ones that they deem are healthier or maybe the ones that are at least just not unhealthy, and they'll take a couple of these and they'll implant these into the mother, so all of this is occurring in a Petri dish. So maybe these four look good, so they're going to take these four, and they're going to implant these into a mother, and if all goes well, maybe one of these will turn into-- will give the couple a child. So this one will develop and maybe the other ones won't. But if you've seen John & Kate Plus 8, you know that many times they implant a lot of them in there, just to increase the probability that you get at least one child. But every now and then, they implant seven or eight, and then you end up with eight kids. And that's why in-vitro fertilization often results in kind of these multiple births, or reality television shows on cable. But what do they do with all of these other perfectly-- well, I won't say perfectly viable, but these are embryos. They may or may not be perfectly viable, but you have these embryos that have the potential, just like this one right here. These all have the potential to turn into a human being. But most fertility clinics, roughly half of them, they either throw these away, they destroy them, they allow them to die. A lot of these are frozen, but just the process of freezing them kills them and then bonding them kills them again, so most of these, the process of in-vitro fertilization, for every one child that has the potential to develop into a full-fledged human being, you're actually destroying tens of very viable embryos. So at least my take on it is if you're against-- and I generally don't want to take a side on this, but if you are against research that involves embryonic stem cells because of the destruction of embryos, on that same, I guess, philosophical ground, you should also be against in-vitro fertilization because both of these involve the destruction of zygotes. I think-- well, I won't talk more about this, because I really don't want to take sides, but I want to show that there is kind of an equivalence here that's completely lost in this debate on whether embryonic stem cells should be used because they have a destruction of embryos, because you're destroying just as many embryos in this-- well, I won't say just as many, but you are destroying embryos. There's hundreds of thousands of embryos that get destroyed and get frozen and obviously destroyed in that process as well through this in-vitro fertilization process. So anyway, now hopefully you have the tools to kind of engage in the debate around stem cells, and you see that it all comes from what we learned about meiosis. They produce these gametes. The male gamete fertilizes a female gamete. The zygote happens or gets created and starts splitting up the morula, and then it keeps splitting and it differentiates into the blastocyst, and then this is where the stem cells are. So you already know enough science to engage in kind of a very heated debate.