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Video transcript

- [Voiceover] So it's been about a week since you've been fertilized. You used to be an egg until a sperm found you, you went through a zygote stage and you cleaved, dividing into multiple cells, and you've finally become a blastocyst. You've been bouncing around in the fallopian tube for a little while, but you finally made it into the endometrial cavity within the uterus, and you start bumping against this nice endometrial lining, and that looks like a great place to get nutrients. In fact, you'd like to start the process of implantation. But at this point, you're still stuck inside this shell, this thick layer of glycoproteins that are surrounding you called the zona pellucida. Well a good thing, about this time your zona pellucida is starting to finally disintegrate away, so I'm going to erase bits of it here, you can see it's starting to fall apart, and your outer cells are becoming exposed, you're hatching out of your zona pellucida. But the endometrium isn't sitting still either, in fact, it's anticipating your arrival, and the endometrial lining is proliferating. In fact, it's developing these valleys here, and you come to rest in one of these valleys, and that's called a crypt. And here, your zona pellucida is still disintegrating away, and your outer layers of cells can come directly in contact, and this contact between the two cell layers is called apposition. Now at this point, you're not really firmly embedded at the endometrium, you're just kind of resting on top of it. You could be easily dislodged. What you'd like to do is get really stuck in there, so that you can start the process of nutrient transfer. And this outer cell layer that we've mentioned before, these are called trophoblasts. They start to multiply, in fact, they don't only multiply, but they start invading in, and you can see them here, invading into the endometrial tissue. Now this gets you good and stuck, and this is called adhesion. But the uterine endometrial cells aren't sitting still either. They continue to divide, they get larger and larger, and pretty soon, you're entirely embedded within the endometrium, but that's not the only thing that your endometrium is doing. Your endometrium has these blood vessels within it, and really what they are is actually just collections of blood that's slowly moving around. They are fed by blood vessels from the uterine arteries, but as they get bigger, they become somewhat irregular, and then they they start to coalesce, and they form these large pools of blood. And at the same time, your trophoblasts keep dividing, but something interesting is happening to your trophoblasts, you can see I'm drawing them here in pink. They're starting to get bigger. They're actually starting to fuse, and you get these large cell conglomerations, and these are actually multi-nucleated cells that are growing out into the endometrium. And because they look so different, we've got to give them a different name, so we call them syncytiotrophoblasts. Now I know that's a mouthful, but remember we started with trophoblasts, so we've got that as part of the name, and syncytio- is just a root that means 'combined' or 'fused', and a syncytium is just a fusion of a bunch of cells, and so here you can have a syncytium of trophoblasts, and so we call it a syncytiotrophoblast. But that leaves us with the problem of what we call our original cells, and we've got to give them a new name too, so these ones that haven't formed a syncytium we call cytotrophoblasts. And same thing, they come from trophoblasts, and we add the prefix cyto- just to remind us that they've maintained their unicellularity. Now, our syncytiotrophoblasts continue to grow, and they continue to grow, and they form these finger-like projections that go out into the endometrium, and these finger-like projections are called villi. But the uterine blood vessels, which are now really just pools of blood, continue to grow and fuse as well. So over time, these structures continue to grow. You get more cytotrophoblasts that line the edges of these villi as they creep out into the endometrium. And within these villis, you start to develop little fetal blood vessels. And as you can see, the fetal blood vessels are in really close contact to the uterine blood vessels. Now they're not actually mixing together, because there's this membrane of trophoblasts in between them, but they're close enough that nutrients from the uterine blood can diffuse into the fetal blood, and waste products from the fetal blood can diffuse out into the uterine blood. And over time, this structure continues to grow with the developing embryo, and as it gets larger, more and more nutrients can be transferred, and waste can be transferred, and the structure gets bigger and bigger, and eventually lines almost the entire inside of the uterine cavity, and this structure is known as the placenta.