Embryo Folding – Embryo Folding, Embryonic Cavities and Placenta

by John McLachlan, PhD

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    00:01 Now, if we’re to follow the embryo itself, it begins to fold. And looking at it from the side, what happens is that as the body begins to curve into a C-shape, so the whole embryo begins to rise into the amniotic cavity and the amniotic cavity is swinging around, front and back. And also, as we’ll see from side to side, so that gradually, the embryo is going to be suspended as it were in the amniotic fluid which fills the amniotic cavity.

    00:35 So that’s the view seen from the side. And if we looked at a solid view of the embryo, you can see the same structures as it curves around to form this C-shape. At this point, you can begin to get the idea that this might be a vertebrate embryo. You might even begin to guess that it might be like a human embryo. It might be looking a bit like a human embryo from later on. Everyone see the same thing seen in transverse section, a cross-section across the long axis of the embryo. Again, we’ve marked the amniotic cavity, and you can see how it’s sweeping around from the sides as well. So the embryo rising up as the amniotic cavity filled with this fluid begins to sweep around the embryo, and pinch-off the yolk sac underneath. Little later still, that sweeping around process is continued, and here, we can see how the embryo is rising up and projecting into the amniotic cavity; and the yolk sac is contributed towards formation of the gut.

    01:42 Now, let’s look at essentially the same process again, but we’re going to step back a level. Now we’re looking at in the context of the whole of the cavity of the chorionic cavity, which is the large space that you can see here. You can see the amniotic cavity is marked again, the body of the embryo and the yolk sac. What happens is that mesoderm will cover all of those internal and external surfaces covering over the amniotic cavity, covering off the yolk sac, and lining the extra-embryonic, chorionic cavity.

    02:19 On the right-hand side of the picture, you can see our amniotic cavity again. And again, I emphasize that that’s the thing to focus on. That’s expanding as the embryo folds up into the amniotic cavity, and the yolk sac will steadily diminish in size and relative importance. You can see on the outside of the trophoblast cells, the villi have become more complicated, and they’re beginning in to develop a branched structure as they continue to increase their absorptive area. And of course, that is surrounded by syncytiotrophoblast with maternal blood running through these spaces in the syncytiotrophoblast.

    03:00 Little later still, again, focus on the amniotic cavity. And you can see as you move from three and a half weeks up to about five and a half weeks, the amniotic cavity is expanding, and gradually, the chorionic cavity is diminishing in size. So go on expanding in that way.

    03:21 The section view through the embryo, you can see here, shows the body of the embryo as it rises up into the amniotic cavity. Now, let’s move from a diagram to some actual specimens. Here’s a human embryo at roughly the same kind of stage of development.

    03:40 Here, you’re looking at the chorionic cavity. You can see the embryo inside its embryonic sac. So it’s lying inside the amniotic cavity and the little structure, just to the side of the embryo, is the diminishing yolk sac. Then surrounding all around the yolk side are the thorns of the villi, which are in contact with syncytiotrophoblast and through which maternal blood is flowing. You can already see that perhaps, they’re a little thicker towards the top of the picture than they are down at the bottom of the picture.

    04:17 What’s happening is the villi are beginning to become concentrated in one particular area.

    04:24 So looking at that in the diagrammatic view, now you can see that the amniotic cavity has almost completely excluded all of the previous chorionic cavities, and the embryo is floating freely within that. And it’s during the embryonic period that the embryo first becomes capable of moving. So it will show spontaneous movements although these are still too small and gentle for the mother to be aware of them. But you can see how the villi are concentrating at one side of the external structure.

    About the Lecture

    The lecture Embryo Folding – Embryo Folding, Embryonic Cavities and Placenta by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Embryo Folding – Embryo Folding, Embryonic Cavities and Placenta

     John McLachlan, PhD

    John McLachlan, PhD

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