Gastrulation – the Most Important Thing – Week 2 of Embryogenesis

by John McLachlan, PhD

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    00:01 Hello, and welcome to our lecture on gastrulation. Now, you know, the most important thing that ever happens to you is not birth, it is not marriage, it is not even death, it's gastrulation.

    00:13 And yet, it might be something that you’re not really familiar with. So obviously, it’s going to be very interesting to explore just what it actually means. So, what we’re going to do is to look at the formation of something called the primitive streak, and that gives rise to the main body axis. At the same time, we’ll go from having two body layers to three body layers. Just after this process, we also have neurulation when the neural tube begins to form and obviously, this is a key time in the formation of neural tube defects.

    00:45 Finally, we’ll look briefly at twinning and the different varieties of twinning that you can get. So, First of all, gastrulation. If we look at the embryos at the moment, we are studying the second week after fertilization and we have the formation of the body cavities.

    01:04 So the upper cavity in green is the amniotic cavity, and the lower cavity in yellow is the yolk sac. If you could imagine being a tiny diver, a boat, a millimeter long swimming in the green amniotic cavity and looking down at the floor, what you would see round about the second week is formation of a little streak growing in from the margin, growing in from the edge of the amniotic cavity and it’s growing in the ectoderm in the upper layer.

    01:38 That little streak is called the primitive streak, and that is the formation of the main body axis. So round about 16 days after fertilization, we’d expect to see that in the floor of our amniotic cavity. It doesn’t look very impressive, but this is actually the moment of individuation. Up till now, you could have been twins or triplets or quads, but after this, then you’re an individual for the first time and therefore, it’s the really significant moment in the process of human development.

    02:14 Also, of course, it determines left and right, and head and tail. In this whole diagram, the head is towards the center of this area in the floor of the amniotic cavity, and the tail is towards the edge. So, all of these important axes are laid down at this particular time. It’s a shallow depression. So if we’re to section through it, it would appear like a little trough in the floor of the amniotic cavity, and it actually grows towards the midline. It grows towards the center of the amniotic cavity. Shortly, we’ll look at a section through the part marked with the red line in the diagram. At that point, we could imagine cutting through it and we were to look at it end on. So, let’s just review with labels what we’ve been looking at. You can see the amniotic cavity marked in green. The yolk sac is marked in yellow. And in between the two are the primitive ectoderm and the primitive endoderm, together called the bilaminar disk. The yolk sac is extending beneath the bilaminar disk and the amniotic cavity is extending above the bilaminar disk.

    03:27 The body stalk is what will be the umbilical cord in time that’s connecting it to the rest of the developing embryonic cavities. We’ll look at that again in more detail in the lecture on the formation of the placenta. Then we can see the primitive streak, and at the very tip of it, a structure called the primitive node. It’s just the kind of starting point of the primitive streak. And we’d labeled a cranial towards the head and caudal towards the tail. These are the anatomical terms for those directions in the body. Now, I said that we are cutting a section through the red line two slides back.

    04:05 This is what the section looks like. So, up above, we have the ectoderm, epiblast that is marked here, and down below, we have the endoderm and what happens is that the cells are actually tracking towards the primitive streak. They are moving towards the primitive streak from either side, when they reach the midline, when they reach the streak itself, a little depression that you can see, and then what happens is that they dive down and then migrate out to the sides. They are actually detached from the upper epiblast, the ectodermal layer and then they migrate out again laterally, and that gives rise to a third layer.

    04:44 That third layer, helpfully, is called mesoderm, the in-between derm layer. So we then have three layers, ectoderm on top, mesoderm in the middle, and endoderm underneath.

    04:57 The cells are spreading out from the ectoderm to give rise to the mesoderm. In the later lecture, we will look at the different kinds of tissue that come from these three different body layers. So, that’s our third layer, the mesoderm between the primitive ectoderm and the primitive endoderm.

    About the Lecture

    The lecture Gastrulation – the Most Important Thing – Week 2 of Embryogenesis by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Gastrulation – the Most Important Thing – Week 2 of Embryogenesis

     John McLachlan, PhD

    John McLachlan, PhD

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