Hey guys, my name is Minas. Today I'm going to be talking about the embryological development of the tongue and I recommend that you watch a previous video that I've made on the embryological development of the pharyngeal arches and the development of the clefts and the pouches because it'll help you understand this video much more clearly. However, I made this video in a way so that if you have no understanding of embryological development of anything you should be able to understand what I'm talking about by the end of it.
Okay, as usual, we're going to begin at the beginning where we have a ball of cells called the blastula. Now that ball of cells is the result of an egg and a sperm fusing, moving down the uterine tube. It's going to go inside the uterine cavity and implant on the uterine wall like this. Initially, you were three flat layers stuck to the uterine wall.
The outer layer was the ectoderm, the middle layer was the mesoderm, and the... Inner layer was the endoderm. And this you might see in your textbooks is an oversimplification for this. We have the ectoderm, mesoderm, endoderm.
And they're all colour coded. So the ectoderm is going to be your skin and the central nervous system. And the mesoderm has three parts. It has the paraxial mesoderm, which are somites and become muscle.
the intermediate mesoderm which become kidneys and gonad and i have video on those as well and the lateral plates in green we have the endoderm which becomes your gastrointestinal tract and you can see that initially you were three layers three flat slices stuck to the uterine wall what eventually happens is that it will fold and the ectoderm will be your true covering as you are sitting right here watching the video covered in skin Okay, now let's start talking about the development of the tongue. Have a look at this week 3 embryo, where its head is over here, and its tail is over here, and we have the pharyngeal arches over here. And I'm assuming you know what the pharyngeal arch is now, because you've gone back and watched the previous videos on those. So we're going to cut it this way and look this way to get this. This is about week four to five, and the tongue appears at approximately four weeks, and it is a proliferation of the first pharyngeal arch.
Right over here, we have the first pharyngeal arch, known as the mandibular arch, and the proliferation of which starts to create the tongue. So, if we, with this being a cross section this way, have a look over here, where if we cut it this way, and look up. So, over here, imagine it's been flipped from here, and we have, where you have your first four pharyngeal outches here, you have your first four... pharyngeal arches there and we've cut it this way so looking in so your tongue begins as two lateral swellings from the pharyngeal arch one and they're color-coded so arch one is in red, two is blue, three is in orange and four is in green. So week four again we have the two lateral swellings and we have one medial swelling.
The medial swelling is called the tuberculum impar and the lateral swellings eventually grow, increase in size and overgrow the tuberculum impar and be the majority of the body of the tongue. And this is in about five-month development, whereas this is in fifth week of development. So again, we have two lateral swellings growing and a tuberculum impar, tuberculum impar, which is the medium swelling.
Eventually, the lateral swellings will outgrow the tuberculum impar and form the anterior two-thirds of the tongue. And if you've seen the... the previous video on the pharyngeal arches, you'll begin to understand what nerves supply which part of the tongue.
And we'll go over that by the end of the video. Okay, so the body of the tongue is separated from the root of the tongue by a terminal sulcus. It's a V-shaped groove called the terminal sulcus. And the root is from the the second, third and fourth pharyngeal arches.
So all of those pharyngeal arches contribute to the posterior third of the tongue, which is the root. And again, because different pharyngeal arches are contributing to different parts of the tongue, they will have their corresponding nervous supply from that specific area. Okay. We have the lateral swellings overgrowing, and then to form the anterior two-thirds of the tongue.
And then we have the mesenchyme, or the mesoderm, from the second, third, and fourth pharyngeal artery. arches proliferating and forming the root or the posterior third of the tongue. The muscles of the tongue are all formed by occipital somites and like I described in the beginning of the video, somites are from paraxial mesoderm and they help to make the muscles of the tongue.
So the connective tissue and the lymph and the blood vessels are all formed by mesoderm, bifaryngeal arch mesoderm. Okay, now, having a look at fifth to sixth week compared to the fourth and fifth week, you can see how the tongue grows or proliferates and becomes more like its final form. Over here, we have the foramen cecum in here in black.
That's between the body and the root of the tongue. And this is where the thyroid will descend from. So over here, we can...
can see the thyroid diverticulum. It's coming from the foramen cecum and eventually will migrate all the way down in front of the tracheal ring. So first two tracheal rings.
My next video will be on the embryology of the thyroid. Okay. So moving on, we have the epiglottal swelling from the fourth pharyngeal arch in week five becoming the epiglottis in the fifth month. And this is important to know because it helps to relate... the nervous innervation of it.
The fourth pharyngeal arch gets its nervous supply from the superior laryngeal nerve. And for bonus points, you can say that the extremely posterior part of the tongue, right near where the epiglottis is, is innervated by the superior laryngeal nerve. Okay, let's talk about nervous innervation of the tongue. We know that the anterior two-thirds and the posterior two-thirds and then the extreme back portion and the epiglottis are derived from different parts of pharyngeal arches so therefore they have different innervation from the nervous system. So if we're talking about motor innovation, they are from the cranial nerve 12 hypoglossal nerve.
And that's because they're derived from occipital. somites from paraxial mesoderm and so all of the musculature of the tongue is supplied by cranial nerve 12 Let's talk about sensory innovation the Anterior two-thirds has different innovation to the posterior two-thirds, both in sensation of touch and of special sense of taste. So anterior two-thirds is by CN5, the trigeminal nerve, the mandibular branch, more specifically the lingual branch. So anterior two-thirds is trigeminal nerve V3.
V3. 3. CM5V3. Okay, the posterior third is from the glossopharyngeal nerve.
So again, first sensation of touch, anterior two-thirds, mandibular branch of the trigeminal nerve, posterior third from the glossopharyngeal nerve. Okay, now for taste. Again, the anterior two-thirds is separate from the posterior third. Anterior two-thirds is given by the facial nerve.
However, some parts of the anterior two-thirds, namely the valet papillae, which end up having taste buds, although they are part of the anterior two-thirds, aren't innervated by the facial nerve. These are innervated by the same nerve that supplies the posterior third, which is the glossopharyngeal nerve. So for taste, anterior two-thirds is facial nerve, except for the valate papillae.
which is innervated by glossopharyngeal nerve along with the rest of the posterior third of the tongue. So for bonus points in your OSCE, if you're getting examined, if you're on the ward and your physician is going to ask you what is the facial nerve, tell me everything about it, or what is the innervation of the tongue, what cranial nerve, nerves contribute to it and you're on the spot in front of 10 other people you will be able to get bonus points by saying not only that the anterior two-thirds of taste is from the facial is from the facial nerve but you'll also be able to say that the valet papillae of the anterior two-thirds of the tongue is supplied by the same nerve that supplies the posterior third of the tongue for taste which is the glossopharyngeal nerve okay So over here we have the epiglottal swelling becoming the epiglottis, innervated by the superior laryngeal nerve. And here we have the artenoid swellings, which become your artenoid cartilage. to help it if you see look over here you'll have the laryngeal orifice and the epiglottis here therefore your vocal cords should be somewhere around here and the artenoid swellings help to change voice as they become the artenoid cartilage. Okay, so again, the tongue is mostly comprised of mesoderm, and it's a combination of pharyngeal arch mesoderm from first four pharyngeal arches.
Pharyngeal arch one contributes to lateral swellings and will become the posterior two-thirds of the tongue. Pharyngeal arch two, three and four will help to form the root of the tongue and therefore... you'll be able to see to see what nerves innervate which part of the tongue if you understand the embryology okay so what can go wrong in some cases so there There is a condition called Ankyloglossia where the tongue isn't completely freed from the floor of the mouth. And so usually cell degeneration occurs which frees the tongue from the bottom or the bottom of the tongue, the mouth. However this doesn't happen in Ankyloglossia in the most common form where the frenulum will extend all the way to the tip of the tongue and therefore the tongue isn't free to move inside the mouth.
You can also have a cleft tongue where there is an incomplete fusion of both of these lateral swellings. So here we have a normal situation where both lateral swellings overgrow the median swelling and fuse together. In a cleft tongue this is incomplete. And also you can have a bifid tongue where it's a complete lack of fusion of both lateral swellings. So essentially it looks like you have two tongues or a forked tongue like a snake.
Okay, so that's the end of the video. I've broken it down very simply. I've, of course, left out a lot of other information like the molecular guidance of how all of this happens.
But if you have any questions regarding anything that I went over through this video, leave a comment or send a message on Facebook and I'll be glad to reply. Thanks for watching.