Face Development – Pharyngeal Arches Development

by John McLachlan, PhD

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    00:01 Hello, and welcome to this lecture on the development of the pharyngeal arches and their associated clefts and pouches, and also the thyroid gland. What we’re going to do is to look at the mechanics of face development, and then the derivatives of the branchial arches, and we’ll see that there are muscle cartilage and nerve derivatives in the head and neck coming from each of the arches and then we’ll look at the derivatives of the pharyngeal pouches and arches in the head and neck. This is quite a complex process, and it takes places against the background of growth, which makes it hard to understand in still images. Previously, with funding from the Wellcome Trust and working with Helen Storey, I made a video which illustrates some of those processes. When the video begins, we’re looking into the face of a very early human embryo, perhaps, round about three weeks after fertilization. As we go through the video, we’ll see various changes taking place.

    00:59 Now, I’m not going to talk you through in detail. Really what I want you to get is the gestalt feeling from the whole process of growth, of movement, and of fusion. And we’ll look at the individual structures by name later on. So, let’s have a look at the video. Now we can see the formation of the nose in the center, wide mouth.

    01:27 The eyes are beginning to come round from the side. The ears are visible low down on the neck.

    01:34 And you can see those gradual fusions taking place between the components and it will eventually become recognizable as a human baby. Of course, growth and development continue for many years after birth. So we can move through adult stages where there are consistent changes in the shape of the face, all the way through to old age.

    02:09 So that’s to give you an idea of the nature of what’s happening and you can always go back and look at that video again. So let’s look at some of the details here. Here, we begin with a very early human embryo, perhaps three weeks after fertilization. In the diagram, we have numbered the branchial arches. So, there are three visible on the surface and one deep under the surface and these correspond to the gill arches in evolutionary developmental stages. The image on the left shows scanning electron micrograph of a real human embryo, and you can see that these arches are really quite visible and well presented. So three visible in the surface, a fourth one lies deep and it’s less easy to spot from the surface. But you can tell from sections that this is present. The first arch actually folds in two. So the maxillary process is the upper part of the first arch, and the mandibular arch is the lower part of it.

    03:07 Sometimes it can give the impression of the big and extra arch there, but this is misleading this is just the first arch folded over. The frontonasal prominence is the large part projecting towards the heart. So this lies quite far forward. Bear this in mind when we’re looking at some of the images because you have to imagine that frontonasal prominence coming towards you. In this image, for instance, that frontonasal prominence is projecting towards you out of the plane of the screen. Marked in green, we have some thickenings or placodes and this indicates where the nostrils are going to form on that frontonasal prominence.

    03:49 Gradually, this becomes deeper and the walls surrounding them become higher. We can imagine there are an aspect towards the midline, the medial aspect, and an aspect towards the sides, the lateral aspect. You can see these thickenings indicating where the nostrils will form, and deepening as structures continue. But you can also see the maxillary and mandibular processes. You’ll recall that these are derived from the first arch, and these are growing towards the midline. From being paired one in either side, now they’re beginning to grow towards each other, and we can begin to see them out forming during this process.

    04:32 The walls that surround the developing nostrils are not complete. They open towards the mouth, and this will be significant when we come to look at abnormalities in this region during the course of development. You’ll note that the mouth is still quite wide at this point compared to the status in the adult. So the maxillary process is still separated by grooves which separate it from the nasal walls and also from the mandible. Now, the ear is beginning to become evident, but it has a series of swellings, and these are relatively far down on the neck of the embryo at this point. So they’re going to have to shift quite a lot in position. But we can just begin to see the eyes coming around as the head enlarges.

    05:18 So they shift their relative position from the sides of the head towards the front of the head. And of course, as we saw in the video, the whole process takes place against the background of growth, which is not well reflected in still images. So the ears begin to become a little more recognizable and they’re still low down on the neck. And again, that may be significant for some abnormalities as we can examine in another lecture.

    05:47 The eyes are continuing to move towards the front and eyelids are beginning to develop during this process. Now, if we look at the nasal part of the frontonasal prominences, as they meet in the midline, so they begin to fuse, and they will also fuse with the maxillary process.

    06:06 And the maxillary process is beginning to fuse with the mandibular process.

    06:10 So the mouth from being very wide begins to move towards these normal proportions in the development of the head and neck. It’s instructive to look at the same process, the same sequence, but this time seen from the side. In particular, what we can see is that the eyes are shifting towards the front, and the ears are moving up from being relatively low down on the neck.

    06:35 So if something was to happen to disturb this process, then we might find that the ears remained in an abnormally low position. Now, let’s look at the branchial arches.

    About the Lecture

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

    Author of lecture Face Development – Pharyngeal Arches Development

     John McLachlan, PhD

    John McLachlan, PhD

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