Notes and transcripts - Early Human Embryogenesis Video Khan Academy PDF

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This is a little transcript extrapolated (with some images) from a video by Khan Academy regarding Human Embryogenesis - very useful for Dr Wright's lectures.
This documet is the result of vocal transcripts, lecture notes and a summary from books extrapolated....

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Early Human Embryogenesis VIDEO Khan Academy Early embryogenesis. Egg cells have a tick glycoprotein layer (outer) called zona pellucida. The plasma mebrane is inside that. The sperm managed to go into the zona pellucida and the plasma membrane and merged its genetic material with the egg one. Now this is called a zygote. However not much is going to happen if the cell is a single one so this cell has to divide. This division needs to be fast and it’s so fast that growing is not allowed. So what happens is that the cell literally split into two and the process of splitting without growing is called cleavage. This process happens for a number of times and from 2 we pass into 4 and then 8 all way staying in the zona pellucida. At 32 cells the “zygote” changes name into morula. Morula looks like a raspberry. In all of this, there was no growth. Now a more interesting thing start to happen. The material is still stucked into the zona pellucida and the cells will be quite compacted, tighter together in the centre and this process is called “compaction”. The cells starts to become different as the one from the outside are not the same from the one in the inside. This process in which can start telling them apart from each other is called differentiation. So we have two separate population of cells, the one outside are the throphoblasts and the one inside are the embryoblasts. We are going to continue with this process and what will happen is that the cells in the middle, the embryoblasts will cluster at one end leaving a cavity at the other end. That mass is called the “inner cell mass”. The cavity is called the blastocoel. The structure has now a new name which is “Blostocyst” and after cleavage the addition of everything that happened is called “Blastulation”. In here the zona pellucida starts to fade off and disintegrate. The next stage is that out thropoblast do not have a zona pellucida and the interest thing happening is that the inner cell mass starts to look a bit different; It gets holed (still at one end) and the cavity in the end is called the amniotic cavity. The inner cell mass is starting to differentiate more and at the bottom of it we have cells called hypoblasts (the layer and the cell) end the cells just above it are called epiblasts. This ”ball” is a spherical mellen (3D) and let’s just cut it and think about it such as a “pancake”. This pancake is a bilayer disk and we draw a plane and we can look that something is forming which looks like a “streakiness” that split our pancake in two. This streak is called the “Primitive streak” and it marks the beginning of the gastrulation. What this streak is it’s just a site where the epiblasts cells in the bilaminar layer start to migrate. They migrate down into the bilaminar disk and thy go down into the hypoblast layer and go over the place. As the cells migrate out from the primitive streak and the two layers are not two anymore and the cells have differentiated even further into three layer (top-middle and a lower). So instead of the bilaminar disk we have a trilaminar disk and these are called the germ layers. The top one is the ectoderm, the middle is mesoderm and the lower is the endoderm. Cells from each three layer go and do important things and form specific structure. This process of formation of the 3 layers is called gastrulation. The next step is “neurolation”. This final stage involves the middle layer that see a further differentiation and we can observe the formation of a “core” called the notochord (where the primitive streak was, underneath the mesoderm cells differentiate). Its main goal is in neurolation which induces a change in the ectoderm (thickening) which is called the neuroplate. The next thing that happens is that the neuroplate cells start to dive in into the mesoderm and start to form a ring structure which is more like a tube (it’s 3D) and as this cells dive in they become known as the neural tube. This process is literally the formation of a tube between the layers. The cells that dive in will differentiate into neurocrest cells. After this our early embryogenesis is complete....