Primary and Secondary Palate: Development – Primary and Secondary Palate Development

by John McLachlan, PhD

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    00:01 processes to form the normal mouth. Now, what we’re going to do is to imagine that you have been slipped into the mouth, feet first, like you’ve been miniaturized beforehand, and you’re looking up at the roof of the mouth lying with your back on the tongue.

    00:14 Here we can see the same structures. So on either side, we see the maxillary processes, and in the midline, we can see the intermaxillary process. Pointing down towards this are two palatine shelves, known as the lateral palatine shelves, and they’re growing from the internal, from the medial aspect of the maxillary processes. We can see the same thing in a diagram and here, what we can see is the palatine shelves that are pointing towards us. So initially, if you were to look at the embryo face on, the two palatine shelves are coming from the maxillary processes and pointing down. In fact, the tongue will lie in between them and what will happen during the course of development is that these shelves will swing up together and meet each other in the midline. And when they do that, you can see that my fingers represent a kind of notch at the tip, and that will fit into the intermaxillary segment. So again, that process is one of the palatine shelves swinging up to meet and they will also come in contact with the intermaxillary segment at that point. Now, this is an unusual process in development, and that it takes place really quite rapidly. It is not due to differential growth or cell division and what happens is that cells along the margin will secrete hyaluronic acid, and this expands when it gets wet. So this swinging up of the palatine shelves is effectively done by a hydraulic process, and this is unique in the course of development. Obviously, it will be crucial that all of the different components come together at exactly the right time. In this image, we’re looking at a frontal section.

    02:08 So we’ve taken the front of the face, and we’re looking at the structures that are revealed. You can see the tongue, and in the top A image, you can see the lateral palatine shelves lying on either side of the tongue. But in this image, we can now also see the nasal septum projecting down from the roof of the mouth. In the diagram B, the lateral palatine shelves have swung up, so squeezing past the tongue to meet in the midline.

    02:37 But now, we can also see that they are meeting the nasal septum in the midline. Looking at a slightly larger version of these pictures, again, you can see the lateral palatine shelves having swung up to meet in the midline and also come in contact with the nasal septum.

    02:56 Now, all of these structures are initially covered with ectoderm and when ectoderm comes in contact with another layer of ectoderm, it will not fuse. Instead, what has to happen is that the underlying mesoderm has to be exposed. In order for this to happen, the ectoderm has to die. So the mesoderm produces signals which will kill the overlying ectoderm, both in the nasal septum and in the lateral palatine shelves and if everything happens in exactly the right sequence, then the ectoderm will disappear and the mesoderm will come in contact and will fuse to form the apparatus of the upper palate. As you can imagine, this is a process which will require extremely precise timing and the fact that the shelves swing up under the influence of this hydraulic effect, it means that timings are absolutely crucial. It’s also absolutely crucial that the ectoderm dies at the right time to reveal the mesoderm to permit the fusion to take place. Here’s another view. Here, we have returned to our position of lying feet first in the tongue, looking up at the roof and we can see that gradual coming together of the lateral palatine shelves, the zipping towards the back, and meeting at the front with the intermaxillary segment which represents the primary palate. The point of join is actually marked by a nerve foramen, the incisive foramen where a nerve will actually penetrate this developing palate. Here we can see the same image in larger size. So you can see the process by which the lateral palatine shelves have come together, and they are zipping up from anterior to posterior, meeting the intermaxillary segment which will define the primary palate and the nerve which exits through the palate is called the incisive foramen. Looking at this view in the adult, we can see the pits which came from the intermaxillary segment which is carrying the front four incisor teeth.

    05:05 So this is a piece which has a different developmental address, as it were, from the rest of the teeth in the upper jaw and this in itself can be significant, and that abnormalities may affect these teeth alone, whereas, the other teeth remain normal. Now, the next images contain some pictures of babies who have suffered from cleft lip and palate. So there’s this guidance that there may be a trigger warning and you may not wish to look at these pictures.

    About the Lecture

    The lecture Primary and Secondary Palate: Development – Primary and Secondary Palate Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Primary and Secondary Palate: Development – Primary and Secondary Palate Development

     John McLachlan, PhD

    John McLachlan, PhD

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