Derivatives – 3 Germ Layers

by John McLachlan, PhD

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    00:01 Now, I have represented it by a schematic diagram with ectoderm, mesoderm, and endoderm as it represented in these simple ways, and we’ll use that consistently. But if we were to relate that to our image on an embryo, here we can see the amniotic cavity shown in yellow, and then the ectoderm shown in blue, the mesoderm shown in red, and the endoderm shown in yellow and the endoderm, of course, is lining the yolk sac. There’s also a chord like structure which runs down the length of the embryo, and that’s called the notochord, and that’s present in all vertebrates, although it will largely disappear later on. So here’s the notochord seen in between the two leaflets of mesoderm on either side and that notochord, we’ll find, has an important signalling purpose. In other lectures, we’ve looked at the formation of the neural tube from the ectoderm, and we’ll touch on that again briefly here. But I’ve represented that as an ectodermal derivative. It’s lying underneath the ectoderm and just above the notochord. And that neural tube will later form the spinal cord in the adult. Here’s an image of how it’s formed. Now, the tissue in either side, the ectodermal tissue on either side will roll up and meet to fuse and form the spinal cord. The tissue at the very margins gives rise to another tissue called neural crest, which is important in itself and will give rise to a number of interesting and different derivatives.

    01:39 Now, the mesoderm will also begin to segment into different areas. So moving out from the center on either side, there’ll be little cubes of tissue of mesodermal tissue called somites. Then there’ll be a patch of the intermediate mesoderm, and then lateral mesoderm out to the sides from that. So the mesoderm begins to specialize out from the center out to the sides into different kinds of mesoderm, and this will give rise to different derivatives later on in the course of development. In this image, we can see a picture of an actual embryo seen in a scanning electron micrograph. Off at the top, we can see the brain, running down the center is the spinal cord, and in either side are the segmented structures which represent the somites. The sac lying underneath the embryo is the yolk sac. So, that gives you an idea of what this looks like in the three-dimensional form of a real embryo.

    02:37 Back to our diagrams again, in this one I’ve added some arrows to show some of the signallings that takes place. For instance, the ectoderm is signalling to the upper part of the neural tube, and the endoderm is signalling to the lower part of the neural tube. The endodermal signalling will help turn the lower part of the neural tube into motor nerves, and the ectodermal signalling will help turn the upper part of neural tube into sensory nerves, and this will migrate out from the neural tube through the rest of the body. However, the neural tube itself also signals. It will signal to the somites, and the somites will begin to differentiate as we will see into different regions which give rise to different parts of the body. Similarly, the endoderm is signalling to the lateral mesoderm, and that interaction will help form blood cells. The lateral mesoderm, in turn, is signalling to the ectoderm.

    03:36 And as we’ll see, that will help provide all of the different forms of ectodermal covering that we see on the surface of the body. So structures like hair and teeth, etc, are all things derived from the ectoderm under the influence of signals from the mesoderm.

    03:53 In this picture, we can see neural crest cells at the very tip, at the very ethicist of the parts of the neural tube that are growing together. When the neural tube fuses, then the neural crest cells are isolated and left separate. There, they give rise to a variety of different kinds of cells. They will migrate individually throughout the body, and give rise to pigment cells. So the cells that indicate skin color derive largely from the neural crest cells. But they also give rise to nerve cells, autonomic nervous cells and sensory nervous system cells. In addition, and very crucially to nerve function, they will give rise to the supporting cells of the nervous system. For instance, the Schwann cells which wrap around the nerve fibers and cells. However, perhaps unexpectedly, they’ll also give rise to cells which are like mesoderm even though neural crest is originally derived from ectoderm. In fact, they can also give rise to endocrine secreting cells as well.

    04:53 So a very wide variety of cells all coming from the neural crest are rather unexpected in terms of the range of differentiation states that you can get from cells of a single origin of this kind.

    About the Lecture

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

    Author of lecture Derivatives – 3 Germ Layers

     John McLachlan, PhD

    John McLachlan, PhD

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