Ectoderm and Mesoderm – 3 Germ Layers

by John McLachlan, PhD

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    00:00 So, looking at the ectoderm and the epidermis, we already indicated that the neural tube will give rise to the brain and the nervous system. But in combination with the mesodermal signals, it will also give rise to a variety of derivatives that are visible on the surface. For instance, tooth development is initiated by a signal from the mesoderm in a particular place, and that induces the overlying ectoderm to begin to form teeth. Similarly, hair is induced by the underlying mesoderm but begins as an ectodermal growth. Incidentally, it’s the mesoderm that determines which forms.

    00:42 So a piece of the ectoderm from almost anywhere in the embryo could be made to form teeth or hair depending on the signal from the underlying mesoderm.

    00:52 Sweat glands and salivary glands are also developed in the same way, and mammary glands which are also secretary in function are also initiated by the signals from the underlying mesoderm talking to the ectoderm lying above it, and giving rise to the development of the mammary glands. Of course, this form identically in early stages in both males and females, as female sexual signals which will subsequently give rise to the full development of the mammary gland in the female, but not so in the male. The final and perhaps the strangest of tissues that develop from the ectoderm is the transparent cornea. Your eye is actually covered by a layer of ectoderm but it’s transparent at that point so you can see light signals passing through the cornea to reach the retina and again, that transparency could be in just in any ectoderm with the appropriate signals from the underlying mesoderm. There are also two special senses and structures which arise from the ectoderm, and those are the placodes that will form the eye itself. They will thicken and form an indentation to finally form the lens of the eye, and the ear placode, another thickening which will sink under the surface and form the inner ear apparatus, the semi-circular canals and the cochlea. Now, let’s look at the mesoderm in our same schematic diagram. I’ve arranged it in the same way except up above where I’ve placed the heart mesoderm, because if we are looking down on the embryo from above, heart mesoderm actually starts out in front of the head. It starts out above where the head is going to be, and it’s the folding of the embryo which brings the heart mesoderm underneath the head. And we will look at folding of the embryo in another lecture.

    02:46 We’ve already mentioned the somites, but so much happens with the somites that we will have to give some special attention to those. At this point, we’ll merely comment that the vertebrae and some striated muscle cells migrate out from the somites. The intermediate mesoderm will give rise to kidney, and the lateral mesoderm will give rise to blood, to blood vessel linings, the endothelial cells, to the cartilage which will eventually be replaced by bone and by the smooth muscle in the body. Also contributes to the mesoderm of underlying skin itself forming the dermal layer. Now, I mentioned that the somites are particularly complex and interesting. Here we can see a suggestion of that complexity.

    03:33 So looking down from the top, the dermatome is going to contribute to the dermis of the skin. The myotome will give rise to all of the striated skeletal muscle cells of the body. So cells will migrate out into the body and give rise to all of the striated skeletal muscle. And then the sclerotome parts will begin to grow around the neural tube to form the vertebrae. This is a rather complex process, and in fact, four somites will contribute to each single vertebra. But this is a general principle in development where if you wish to make a complicated bone, you often do it by fusing together a number of different elements.

    04:17 It seems hard to instruct a bone to form a very complex shape like the vertebrae.

    04:22 So instead, what the body does it to build it up from a number of different components.

    About the Lecture

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

    Author of lecture Ectoderm and Mesoderm – 3 Germ Layers

     John McLachlan, PhD

    John McLachlan, PhD

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