Video: Tongue and epiglottis

Flick it, flaunt it, roll it, or even bend it into a clover. Your tongue can be an acrobat regardless of whether your parents are capable of the same tricks. Yes, you heard me. If your parents are tongue-rollers and you aren't, no need to question your genetics. This myth has long been busted. Of course, this doesn't mean there's no genetic involvement in rolling your tongue. You need to have a tongue wide and long enough with the right muscle arrangement to do this. It's just that it doesn't come down to one single dominant gene as you may have been taught. Tell that to your biology teacher.

Yes, the tongue definitely holds its own when it comes to complexity both in function and in structure which is actually what we're going to explore today. Stay with me now as we study the histology of the tongue and the epiglottis.

Immediately, you're probably thinking but why look at these together and that's understandable. The tongue is one of the first organs of the digestive system while the epiglottis is perhaps more considered to be relevant to the respiratory system and they even have different functions. The tongue is involved in taste, moving food, and speech while the main function of the epiglottis is to prevent food from entering the airways. Despite all that, these two structures are adjacent to each other, and in some ways, structurally quite similar.

With that information in mind, let's have a quick rundown of the topics we'll tackle today. We'll start by looking at a histological slide of the dorsum of the tongue which you can see here. We'll then move to another section of the tongue which shows its ventral surface. We'll then go on to look at the histology of the epiglottis especially its main difference from the tongue – the elastic cartilage. We will, of course, finish with some clinical notes to put the histology that we're about to learn into a bit more perspective.

Let's begin by looking at our first histological section of the tongue which is our H&E section of the dorsum of the tongue.

Now I've just mentioned that the stain used for this section is hematoxylin and eosin or H&E. Just in case you're not too familiar with it or just need someone to jog your memory, hematoxylin is basic or acidophilic so it stains acidic structures such as nuclei full of DNA or deoxyribonucleic acid a dark blue color, and eosin is acidic or basophilic and stains basic structures like the cytoplasmic proteins pink.

We've zoomed in a bit to take a closer look at the tissues of the tongue now. Looking at the dorsal surface here, we can see the mucosa of the tongue which is composed of two parts – an epithelial layer and a subepithelial layer which we refer to as the lamina propria. Note that unlike most of the others of the gastrointestinal tract, there is no muscular layer in the mucosa of the tongue nor is there an underlying submucosa. Now, unfortunately, in this section, the structures known as lingual papillae which contain our taste buds have not been well preserved so we're not going to say a whole lot about the epithelial layer of the mucosa apart from the fact that it is generally nonkeratinized stratified squamous epithelium.

Squamous refers to the elongated, spindly shape of the epithelial cells and stratified means that there are multiple layers of cells stacked on top of one another. In general, most of the tongue epithelium is said to be nonkeratinized, meaning for the most, part it does not bear a harder keratin outer layer like that seen on the hard palate or gums, for example. That being said, the numerous filiform papillae of the tongue are keratinized and thus provide protection from mechanical abrasion on the tongue.

As with all stratified squamous epithelium, we have a darker staining basal layer of cells which are actually more cuboidal or round in appearance. As the epithelial cells of the basal layer mature and move more superficially, they may accumulate glycogen which gives them a somewhat clear and swollen appearance which is typical of this type of epithelium. And that is all we're going to discuss about this epithelial layer in this tutorial. If you'd like to learn more about the lingual papillae, please be sure to check out our video tutorial which will explain all about them.

Moving on to the deeper part of the mucosa now which as we said is called the lamina propria. This is a fairly thick layer of connective tissue which contains pockets of adipose tissue, nerves, blood vessels, as well as diffuse or scattered amounts of MALT – mucosa-associated lymphoid tissue.

The lamina propria is relatively uneven in thickness and sends many projections known as connective tissue papillae into the overlying epithelial layer. When these papillae are cut obliquely, they may appear as islands of connective tissue within the epithelial layer. Deep to the mucosa of the tongue and making up the rest of the tongue is a thick muscular layer meaning that there is no defined recognized submucosa in the tongue. The muscular layer contains muscle fibers coursing in various directions which you can notice by the different appearances of the fibers cut in section.

Now before we speak in more detail about this muscular layer, let's first point out that what we are looking at here is skeletal striated muscle. How do we know this? Well, if we look at a high magnification of the muscle fibers cut in cross-section, we will see nuclei mainly at the periphery of each muscle cell which is characteristic of skeletal myocytes, and in the longitudinally cut fibers, we can see classical skeletal muscle striations here.

So before we try to identify the tongue muscles in our section, let's first refresh our memory on what and where they are.

So we know that the tongue is capable of a wide range of really precise movements to accommodate speech and this is achieved by quite an unusual muscle arrangement of intrinsic and extrinsic muscles. In our histological section today, however, we will be mostly focusing on the intrinsic muscles. The image that we're looking at now shows a cross-section of the tongue with the dorsum of the tongue shown here. You'll notice there is one layer of muscle just below the dorsum and it is called the superior longitudinal muscle of the tongue. It runs from the posterior to the anterior ends of the tongue. We also have an inferior longitudinal muscle of the tongue found just deep to the ventral surface mucosa and with the same orientation of fibers.

Sandwiched between these two layers, we have the vertical muscle of the tongue with its fibers arranged somewhat vertically from the dorsum towards the ventral surface of the tongue and the transverse muscle of the tongue with its fibers coursing from the medial septum to the lateral edge of the tongue.

In our section of the dorsum of the tongue, we can identify three of the intrinsic muscles – the superior longitudinal muscle of the tongue seen here closest to the mucosa; the next intrinsic muscle which is easiest to identify is the vertical muscle of the tongue which extends from the deep parts of the tongue right up to the mucosa; and finally, interspersed among the vertical fibers are the fibers cut in cross-section which belong to the transverse muscle of the tongue.

Let's continue on to our next section of the tongue now which is our trichrome section.

We're now looking at a slide processed with an Azan trichrome stain of the ventral surface or the underside of the tongue. Let's quickly run through what you would expect to see with this stain. The nuclei are stained bright red. Connective tissue and mucin stain blue and muscle stains red. Now let's see what we have in this section beginning first with the mucosa.

Like our H&E section, we see the exact same squamous stratified arrangement of the epithelium here but you'll immediately notice that its surface is not nearly as bumpy because it lacks mucosal ridges and folds. Moving into the deeper lamina propria, this is similar to that scene on the dorsal surface; however, it is not as thick. With this particular stain, however, we can get a much better appreciation of the loose connective tissue which appears bluish-purple. Once again, we can see several connective tissue papillae extending into the overlying epithelial layer, however, it's not as pronounced as what we saw in the H&E section. Also present in this section are many mucous lingual glands, one of which you can see here. These glands mainly produce mucin which acts as a lubricant for the surface and contents of the oral cavity. We know that they are mucous glands as opposed to serous glands due to the fact that their cells have a foamy appearance which is due to the nuclei that have been displaced to the periphery.

Now let's take a look at the muscular layer or the core of the tongue in this section. Once again, we can easily identify this as skeletal striated muscle due to the fact that we can see peripherally-located nuclei of the myocytes, or muscle fibers, cut in cross-section as well as the presence of striations in the muscle fibers in longitudinal section. Once again, we can also appreciate the varying directions and cuts of the muscle fibers, all of which belong to different muscle components of the tongue.

So coming back to our Azan trichrome stain section of the tongue, we can identify three different orientations of muscle fibers in this section. The first immediately deep to the ventral mucosa is the inferior longitudinal muscle of the tongue. We know this is the inferior longitudinal muscle because it lies closest to the ventral surface of the tongue and since this muscle runs longitudinally from back to front of the tongue, its fibers appear in cross-section or kind of circular in our micrograph here.

Deep to this, we see a layer of long muscle fibers which are clearly running in a different direction, sometimes appearing to be cut in longitudinal or sometimes oblique section. This is our transverse muscle of the tongue, which as we said, runs from the median septum to the lateral border of the tongue.

As we move deeper to the core of the tongue, we can see some fibers cut in longitudinal or oblique section. These longitudinal fibers most likely belong to the vertical muscle of the tongue which extends from the dorsum to the ventral surface of the tongue.

Interspersed amongst these fibers are some horizontal fibers cut in longitudinal section which are additional fibers of the transverse muscle of the tongue which we looked at before as well as other fibers cut in cross-section which probably belong to some of the extrinsic muscles of the tongue, maybe the genioglossus or the palatoglossus muscles depending on where this section was taken from. You'll also see plenty of adipose tissue here between the muscle fibers which is common in the tongue. In fact, your tongue also increases in size when we gain weight and may contribute to snoring and disorders such as obstructive sleep apnea. Who knew?

Also in this layer of muscle, we find a branch of the lingual artery which is the main blood supply to the tongue. The lingual artery is actually a branch of the external carotid artery in the neck. This artery is convoluted, meaning, wavy to allow it to stretch with the tongue as it moves.

And that, my friends, wraps up the histology of the tongue. Let's move on to the histology of the epiglottis.

The epiglottis is a flexible cartilage that bends or folds over the inlet of the larynx when you swallow. We are once again looking at an H&E stained section. You will immediately notice that unlike the tongue, the epiglottis does not contain any muscle tissue and therefore cannot move voluntarily. If we zoom on to the surface of the epiglottis, we start seeing some similarities between it and the tongue. The first obvious feature you'll notice is the nonkeratinized stratified squamous epithelium which is present on the lingual surface and superior half of the laryngeal surface of the epiglottis.

Although you can't see them in this section, you may also find a few taste buds scattered across this layer. What we're not seeing in this section here is the laryngeal side of the epiglottis which as I mentioned has nonkeratinized stratified squamous epithelium. Its laryngeal surface is covered in ciliated pseudostratified epithelium, typical of respiratory mucosa. Pseudostratified means that the cells appear to be stacked on top of each other but are actually all connected to the basal lamina.

Just deep to the epithelium covering the epiglottis, we can see the lamina propria. As with other tissues, the lamina propria is a loose connective tissue structure that contains arteries, nerves, and lymphatics that support the overlying epithelium of the epiglottis. As we mentioned before, the epiglottis is a flexible structure. This is all down to the elastic cartilage making up the core of the structure. It contains special collagen fibers that stretch and recoil to permit its bending motion. In fact, if we zoom any closer, you just might be able to see these pink-stained fibers forming a network around chondrocytes or cartilage-producing cells. The elastic cartilage is surrounded by perichondrium, an envelope of connective tissue around the cartilage.

Both the tongue and epiglottis feature two types of small exocrine glands that keep the tissues moist and lubricated. Exocrine glands are made up of a duct that opens onto the surface of the epithelium and a secretory portion that forms the products to be expelled. The glands of the tongue and epiglottis have a bulbous secretory portion that is referred to as an acinus. Let's take a look at those now.

We have zoomed in on the epiglottis slide to be able to better see and appreciate the serous acini. Serous glands release a watery, poorly glycosylated protein secretion. Unsurprisingly, the cells of the serous acini are simply referred to as serous acinar cells. These cells present nuclei which are oval-shaped and tend to appear acidophilic or pink as we discussed previously if their secretory granules are preserved. Around the borders of these acini, you may also be able to see long slender nuclei which belong to contractile myoepithelial cells.

The second type of exocrine gland we will see are formed by what are known as mucous acini. These glands release a slimy, viscous secretion containing substantially glycosylated proteins. Like we saw with the serous acini, the cells here are simply referred to as mucous acinar cells. Compared to bright pink serous acini, the cytoplasm of a mucous acinus appears pale and foamy as the mucinogen granules it contains are often lost during tissue preparation. Once again, the long slender nuclei of myoepithelial cells may be visible around the perimeter of the acinus.

Seromucous acini, also known as mixed acini, produce both serous fluid and mucus and contain both mucous and serous cells. On first glance, you might confuse one with the mucous acinus due to the fact that the acinus appears to be mostly composed of foamy-looking mucous acinar cells. However, if you look a little closer, you will see that the acinus is kept by one or more serous acinar cells which form a crescent-shaped structure known as a serous demilune.

All acini regardless of their type empty into excretory ducts which convey the serous and mucus secretions to the surface of the epiglottis. They may be lined with cuboidal columnar or stratified columnar epithelium as you can see in our example here.

And that wraps up the histology of the tongue and the epiglottis. But we're not done yet. Let's put what we learned into some clinical context.

In our clinical section, we're looking at a really interesting condition. It is called geographic tongue due to the resemblance of affected tongues to a map. It is also clinically known as benign migratory glossitis or erythema migrans, which gives you some additional information about the condition. Benign lets you know that it is not cancerous and migratory refers to the fact that the map-like spots on the tongue can over time change their pattern. The condition presents as a pattern of red, irregularly-shaped spots on the dorsum of the tongue with occasional discomfort or burning sensation. Histologically, the red spots appear due to the loss of the epithelium on the dorsum of the tongue which then leaves the lamina propria and vasculature in it more exposed, making the spots appear reddish in color.

It's sometimes suggested that this condition can occur due to stress but really there is no agreement over it, and officially, there's no cause for geographic tongue. Similarly, there's also no treatment and the symptoms tend to disappear within a few days. So the worst part about this condition? It can sometimes be hard to reassure patients that there is nothing seriously wrong with them due to the strange symptoms of this condition.

That brings us to the end of this tutorial, but before we finish up completely, let's summarize what we've learned today.

We started by looking at two different slides of the tongue representing its dorsal and ventral surfaces. Both of these surfaces are covered by a protective layer of nonkeratinized stratified squamous epithelium. On both slides, we saw the vascularized connective tissue called the lamina propria located deep to the epithelial layer. Then we identified some of the intrinsic muscles of the tongue – the superior longitudinal muscle right below the dorsal mucosa, the vertical muscle, the transverse muscle, and the inferior longitudinal muscle of the tongue right immediately deep to the ventral mucosa.

Then we moved on to the epiglottis. On its lingual surface, the tissue structure was very similar to that of the tongue. The nonkeratinized stratified squamous epithelium forms the outermost layer with the lamina propria just deep to it. The main difference was the elastic cartilage that makes the epiglottis so flexible.

Finally, we looked at the glandular acini of the epiglottis – the serous acini that look pink in routine stains and mucous acini that appear empty when the mucinogen granules are lost in preparation. And finally, a seromucous acinus which was composed of mucous acinar cells capped with a small number of serous acinar cells forming a serous demilune. And finally, we looked at the stratified columnar epithelium of the excretory ducts which received the product of this acini.

We finished up with a mysterious but thankfully not very serious condition called a geographic tongue.

And that brings us to the end of this tutorial. Hope you enjoyed it. Happy studying and see you next time!