Branchial Arches Development – Pharyngeal Arches Development

by John McLachlan, PhD

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    00:00 remained in an abnormally low position. Now, let’s look at the branchial arches.

    00:03 What I’ve done is I’ve drawn a red line which shows the plane of section of the diagram on the right-hand side. You can see that I’ve cut through the frontonasal prominence.

    00:13 So there’s a little piece that’s separated where the opening of the mouth will be.

    00:17 And it happens that we’ve sectioned that through the optic vesicles. Then we can see the four arches, and these are represented by grooves on the outside of the head and neck, and on the inside, the corresponding grooves are known as pouches. Each groove and pouch is numbered from the arch in front. So, groove one and pouch one come immediately after the first arch, and so on through the rest of the arches. So, numbering is in this particular way, and we’ll use this consistently all the way through. Now, we don’t need to bother out the detail in this image. All I’m going to do is to make the general point here, that each arch contains a supply of blood vessels and nerves, there is muscle and there is cartilage and this reflects as evolutionary origin where this is involved in being gill arches.

    01:10 In human embryos, the gill has never become open. There’s always a connection between the layers so that it never opens to the outside, unlike fish of course. But because we are no longer using them as gills, it means that they are then freed in evolutionary history to take on other functions. You could describe it as a kind of general toolkit of cartilage muscle, blood vessel, and nerve which can give rise to a whole range of structures in the developing head and neck, and in the face. And this is a summary of some of the derivatives of those branchial arches. Here we can see that I’ve numbered what I’ve called somitomeres.

    01:50 Now, these are analogs to the somites, the segmental blocks of structure, which runs down in either side of the neural tube. And we’re looking in the very forepart of the neural tube which will mostly give rise to the brain. Again, back in the evolutionary history, these were segmented and we’re looking at the relics, the descendants of these segmented structures later on. What we can see is that there are muscles which are derived from these somitomeres. So, for instance, the superior, inferior, and medial rectus muscles derived from the first and second somitomeres, and so on down the list.

    02:27 Key ones we will emphasize are those from the fourth somitomere, which will provide the large muscles of mastication, and those from the sixth somitomere, which will provide the muscles of facial expression and those were associated with arch one and arch two, respectively. The other muscles are relatively small, although important in their function of course. At this point, we can also note that the cartilage elements arising from arches one and two represent mainly the malleus and incus of the inner ear bones, of the middle ear bones, and the core of the mandible. Arch two will give rise to the stapes, the styloid process and the upper part of the hyoid bone. If we are to continue down the sequence, we can see the first of the somites which follow caudally from the somitomeres, and you can see the first somites contributing to the laryngeal muscles. This is part of the fourth arch, and it will also give rise to one of the cranial nerves associated with them.

    About the Lecture

    The lecture Branchial Arches Development – Pharyngeal Arches Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Branchial Arches Development – Pharyngeal Arches Development

     John McLachlan, PhD

    John McLachlan, PhD

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