[MUSIC] Hi. In this lecture, we'll talk about the embryology of the oral cavity and of tooth development. We'll learn about how the bones in the oral cavity develop and how the teeth develop inside those bones. We can see that already at 28 days or four weeks of development, the face is starting to form. I'd like to draw your attention to the green, the yellow, and the blue in this image. The green being the frontal nasal prominence where the top part of the face and nose will develop. The yellow being the maxillary prominence, where the top jaw, the upper jaw or the maxila will develop, and the blue being the mandibular prominence where the lower jaw will develop. We can see that at twenty eight days there is actually a space between the yellow, the green, and the blue. This space will need to be filled in so there is not a space within the upper jaw. At forty-eight days, approximately seven weeks, we can see that the green portion has started to grow down the nose is developing a little bit more and growing downwards to meet the maxilla, or the upper jaw. The yellow portion, the upper jaw, is growing toward the center to fuse with itself on either side. We can also see that eyes have started to develop, and, the lower portion of the image we can see that the yellow or upper jaw is fully fused with itself in the midline. The green forming the nose has come down and fused with the yellow for the upper jaw. Now that we know something about the development of the facial bones, we can understand how cleft lip and palate occurs. Cleft lip and palate is a condition that happens approximately once in every 700 live births. It's the most common congenital abnormality in the orofacial complex. We can see at the bottom of the image, a gap in the midline of the maxilla or the upper jaw. This happens when the two yellow halves that we saw, the two halves of the maxilla do not fully fuse. On the upper portions of the image, we see what happens when the yellow and the green do not fully fuse, when the maxilla does not fully fuse with the nasal bones. Now that we've seen how the bones in the upper jaw and the upper face develop let's focus on the mandible or the lower jaw in the neck region. These develope from a series of arches that we call pharyngeal arches. There are five of them, number one through six, because there were actually originally six, and the fifth disappears very early on in development. These arches will form the bones of the mandible, or lower jaw, and the cartilage in the neck. This image show us in a color coordinated fashion what these arches will form. We can see in the orange red color that the first pharyngeal arch will form the bones of the mandible or lower jaw, as well as some bones of the ear. The other pharyngeal arches are responsible for bones in the neck and cartridge of the neck forming other pharyngeal structures. We've now seen how the bony framework of the upper jaw and the lower jaw upper face, and the neck form. Now let's talk about how the teeth form within those jaws. In order to discuss tooth formation, we first have to start with a roadmap. So this is what a mature tooth looks like in histological section. The enamel is the topmost structure. It is the hardest structure in the human body, and it takes the brunt of all the biting forces. Underneath that is the dentin, which forms a cushioning for the enamel and extends all the way down to the bottom of the tooth in the root. Covering the rest of the root is a structure called cementum, and in the middle is a soft tissue structure called pulp. This houses the nerve and blood supply of the tooth. This is what a mature tooth looks like and let's see how that develops. Looking at an image that we're now familiar with I want to highlight the white portions of this image. This is where tooth development occurs. It occurs in the maxillary prominences, mandibular prominences, and the middle portions of the nasal prominences. Tooth development happens in three stages that we call the bud stage, the cap stage, the bell stage, and after these stages, hard structure forms, to form the crown of the tooth. These stages form a spectrum, and are only distinguishable histologically, based on how they look. This is a histological image, which shows us the initial forms of tooth formation, or also called the bud stage. In the middle portions of the images is the tongue. Surrounding that is oral mucosa. We're now highlighting the tooth bud, which is actually epithelial cells or the top cells that have been signaled to grow downwards into the underlying ectomesenchyme that we're highlighting now. This image shows us the early cap stage of tooth development. We can see that the tooth bud has taken on more form. The epithelial cells are denser and thicker, and the underlying ectomensenchymal cells are also starting to aggregate. The epithelial cells and the underlying cells are constantly signaling to each other to create tooth formation. This histological slide shows us the late cap stage of tooth development and highlights an important structure called the enamel knot. The enamel knot is a dense aggregate of epithelial cells that will orchestrate and shape the form of the crown. The next stage of development is the bell stage of tooth development. We can see that the tooth bud has now taken on great shape. We are highlighting an area that will be the eventual shape of the actual tooth. Now that the cells have all lined up, the epithelial cells and the underlying ectomesenchymal cells, they are ready to start making hard tooth structure. Another interesting aspect of this image is that on the right hand side you can see a developing tooth bud. This will be a permanent tooth that replaces the primary tooth which is currently in the bell stage of tooth development. This image is also showing us the bell stage of tooth development but for a molar. Note the difference in the shape, that's because molars have a different cusp anatomy and these cells are lining up to create that different shape. This image, is showing us the late bell stage where we can now see the beginning of formation of hard tooth structure. All the way at the top, and we are now highlighting the outline of what was the bell stage, we can see the beginning of formation of hard tooth structure, the enamel and the underlying dentin. These start to form at the cusp tip all the way at the top of the tooth and they form sideways and downwards. This image shows us a later stage of crown formation. We are now highlighting where the hard matrix was first created, and you can see that from there, it has spread downwards and sideways. And in this slide, actually, most of the hard structure of the crown, or the top half of the tooth, is formed. Now that the hard structure in the top half of the tooth, the crown is formed, we have to form the structure in the root. Once the enamel is formed, it does not continue downwards into the root. The dentin however does continue downwards and forms the hard structure of the root. This is a great histological section which shows us different stages of tooth development. We're now highlighting the primary molars that are currently in the mouth, and the first permanent molar that has erupted behind it. Underneath the primary molars we can see the permanent premolars developing. The first permanent premolar is in the crown stage of development, hard tooth structures forming. Second permanent premolar is in the late bell stage of development, and only just now we're beginning to see hard tooth structure. This last image is a panoramic radiograph of a seven year old child and pulls together some of the concepts of tooth development and eruption that we've been talking about. Firstly, I'd like to highlight the second molars developing. They have essentially finished the crown stage of tooth development and you can see that most of the tooth structure of the crown is formed but the roots are not yet formed. The roots will take several more years to form, and as they form it will push the crown into the mouth. Another interesting aspect of this image is just how complicated it is. If you were to look into the mouth of this child you would see approximately 24 teeth, but really you can see that many more are developing in the bone and at different stages of development. One last thing I want to highlight is that we can see that when a tooth erupts into the mouth, it’s actually not fully formed yet. When a tooth erupts, the root is approximately two-thirds to three quarters developed, which means that even after a child gets a tooth erupted, it is still developing.
