Students enrolling in Embryology Courses in Bangalore often find early developmental stages like zygote formation, morula, and blastocyst development challenging to understand at first. The development of the human body begins with a single-celled zygote that begins to divide roughly 30 hours after fertilisation. It divides into two cells, then four, and so on and so on, without really gaining any extra size. So, after a few divisions, we're left with a big ball of cells that's referred to as a morula.
Now, as this is still undergoing lots of division, eventually it is going to run out of space for it to get nutrients from the surface, so what has to happen is that a cavity is formed within and that is known as the blastocyst cavity. Now if we were to take a section through the morula, you'll see that it has cells around it and inside of it and now, as it is getting bigger and bigger and the cells dividing, it is forming a blastocyst cavity so that nutrients and fluid can get to and from the dividing cells. At this stage, it is not really called a morula anymore, it's now called a blastocyst and this here is the blastocyst cavity.
Now, as division keeps happening, the blastocyst cavity gets larger and larger, and one group of cells gets kind of sequestered off to one end of the blastocyst. And the very appropriately named grouping of these cells is that you have an inner cell mass, shown right here, and an outer cell mass, which is making up the periphery. Now, the outer cell mass is going to form the placenta and other related structures.
We're going to kind of ignore that for the time being and focus on the inner cell mass, because this is the structure that's actually going to form the embryo and eventually the foetus. So cells from the inner cell mass, still surrounded by the outer cell mass, which we'll draw in right here, begin to subdivide a little bit. So, we have cells that are still in contact with the blastocyst cavity, but the cells that are kind of sequestered away from it get very tall.
And these cells, these tall cells, are known as the epiblast, right here in blue. And the cells that are in contact with the blastocyst cavity are known as the hypoblast cells. Now, the important thing about these epiblast cells, these will be what forms the actual embryo.
These hypoblast cells, despite being part of the inner cell mass, don't really contribute anything to the mature human being. Now, we move just a little further down and because all these cells are still dividing and all this diffusion is taking place, we need to have another cavity to supply those epiblast cells. So, in addition to that early blastocyst cavity in contact with the hypoblast cells, and I'm drawing in right here, and by the way, this whole area is now known as the primitive yolk sac.
It magically changes names for no good reason. We have the tall epiblast cells, and as I said before, they have their own cavity, and this cavity, shown right here, is referred to as the amnion. And the amnion, despite its location right now, will eventually surround and completely encompass the developing embryo and foetus all the way up until birth.
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Now, what we're going to do to follow up from here is take a section right through, take the lid off the amnion, so we're looking down on those hypoblast cells, and we'll follow the next step along the way, which is called gastrulation. We've gone to the point of looking at the formation of the bilaminar embryo, which is the group name for these epiblast cells in blue and the hypoblast cells in orange, and just above them we have the amniotic cavity that we sliced open. And if we were to look down, taking the lid off of that, what we'd see is kind of an elongated little disc of epiblast cells. Visit: https://www.medlineacademics.c....om/embryology-traini

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