Cleavage and Blastulation

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