Gastrulation – 3 Germ Layers

by John McLachlan, PhD

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    00:01 Hello, and welcome to this lecture on the derivatives of the three germ layers.

    00:06 What we’ll be looking at is the formation of each of the following: the ectoderm, the mesoderm, and the endoderm. We will look briefly at tissue signalling between the different layers that gives rise to different structures of the body, and some interactions between tissues themselves. Finally, we’ll note that there are some implications for cancer. So let’s go back to the beginning, to the stage of gastrulation when the main body axis is formed. It has been described that it’s the most important thing that ever happens to you. In our diagram, in the upper image, we have the amniotic cavity marked in green, and beneath that, the yolk sac marked in yellow. Between the two of them is a bilaminar disc made up of ectoderm on the top, and endoderm in the bottom. If you were inside the amniotic cavity looking down, in the floor, you would see the primitive streak start to form.

    01:01 So, you can see that marked A and B in the diagram, roundabout day 16 after fertilization.

    01:07 This is a key moment. It’s the moment of individuation. From now on, you could no longer be twins in any form. It also determines the main body axis. So it establishes left and right, and the head is towards the center of the disc, and the tail is towards the margin of the disc. But also, crucially, for our purposes today, it leads to the formation of a third germ layer in between the ectoderm and endoderm and that third layer is the mesoderm.

    01:37 What will happen is that the ectoderm on the top surface will begin to migrate towards the midline. So moving towards the middle, and if we saw that in the section where we have the ectoderm or epiblast on the top and the endoderm in the bottom, as the cells move towards the midline. As they reach that midline, they form bottle cells, they change their shape, and then they detach and fall off and then they will migrate laterally out to the sides.

    02:08 So, forming that third mesodermal layer in between the ectoderm and endoderm, a bit like the filling in a sandwich. So, spreading out from either side to give rise to the mesoderm.

    02:22 That primitive mesoderm will give rise to a variety of other tissues subsequently.

    About the Lecture

    The lecture Gastrulation – 3 Germ Layers by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Gastrulation – 3 Germ Layers

     John McLachlan, PhD

    John McLachlan, PhD

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