Human Egg Implantation – Embryo Folding, Embryonic Cavities and Placenta

by John McLachlan, PhD

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    00:01 Hello, and welcome to the slideshow in which we’ll look at embryo folding and development of the embryonic cavities and the placenta. Now, I should say at the beginning that this is quite difficult to understand. It probably took me several years to get my head around it, and you may have to watch it several times before it makes complete sense for you.

    00:19 What we’re going to do is to look at, first of all, the folding of the embryo, and then how the extra-embryonic cavities and membranes develop. And then finally, we’ll look at the development and the function of the placenta. But we’ll have to begin right back at the very beginning, at the point of implantation. In this diagram, we can just see the pre-embryo beginning to implant under the uterine wall. The outer cells, the trophoblast, as we described in an earlier lecture, actually begins to form syncytiotrophoblast, which is lost at cellular boundaries, and you have nuclei in a space surrounded by cytoplasm, and this is eating its way into the uterine wall. We can see that the primitive ectoderm here marked in red, and primitive endoderm marked in yellow have already formed from the inner cell mass.

    01:11 The uterine wall has been prepared to receive this, otherwise, as we said previously, this would be quite dangerous as it implanted into the surface. Two days later, it has almost completely disappeared under the surface, and the primitive ectoderm marked in red here has formed a cavity, and that is going to be the amniotic cavity. That kind of the thing to focus on if you follow the amniotic cavity all the way through, that’s probably the most important. The other ones tend to come and go, but the amniotic cavity is going to be really important later on. So at this point, it sunk completely under the surface. Blood vessels are being engulfed by the syncytiotrophoblast and maternal blood is in direct contact with the syncytiotrophoblast. So nutrients are beginning to make their way to the developing pre-embryo. As we said, the inner cell mass has given rise to the amniotic cavity, and just where the embryo sunk under the surface, there’s a little plug or scar which marks where it is. It's quite hard to see even if you’re looking inside the uterus at this point. Little later still, 11 days after fertilization, now sunk completely under the surface, the amniotic cavity is still visible.

    02:29 The yolk sac is formed from the primitive endoderm, and we could begin to see little villi, little projections from the trophoblast marked in dark brown here, projecting into the syncytiotrophoblast.

    02:42 These villi are increasing the surface area of the developing conceptus so that nutrients can flow from the mother. So growth, from now on, can be relatively rapid.

    02:55 The primitive endoderm has formed that primitive yolk sac and the primary stem villi are forming to increase the absorptive area, so rapid growth ensuing as a result of nutrients coming from the mother.

    03:12 In this image, we can see the yolk sac separated from the rest of the embryo with the amniotic cavity shown in green up above it. That’s the amniotic cavity. As I said, that’s the key part. The body stalk is what’s going to be the umbilical cord later on.

    03:27 The embryo itself is going to develop in the bilaminar disc. That’s the part made up of ectoderm and endoderm in between the amniotic cavity and the yolk sac.

    03:41 As the cavity is expanding, this begins to push its way back into the cavity of the uterus, the uterine cavity overall. To remind you that when we talked about gastrulation, we saw the primitive streak forming in the floor of the amniotic activity, and that defines the main body axis. It also gives rise to the mesoderm of the embryo. So that two layers of the ectoderm and the endoderm between the amniotic cavity and the yolk sac becomes three layers, the mesoderm as the meat in the sandwich as it were.

    About the Lecture

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

    Author of lecture Human Egg Implantation – Embryo Folding, Embryonic Cavities and Placenta

     John McLachlan, PhD

    John McLachlan, PhD

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