Video: Trachea histology

We all know the sound of that nasty, hacking cough associated with respiratory tract infections. You've probably felt it, too, at some point. You're also probably more than familiar with the experience of that gunky, green, sticky phlegm spluttering up into your mouth with each cough. But where does all this phlegm come from and how on earth does it get carried up into our mouths?

Well, in this tutorial, we're going to find out all about this and lots more as we study the histology of the trachea.

First, we're going to take a brief look at the gross anatomy of the trachea and discuss the tracheal lumen. Then we're going to look at the tissues lining the walls of the trachea, and these include the mucosa, the respiratory epithelium, the basal lamina, the lamina propria, and the submucosa. Then we'll look at the structures making up the trachea itself. These include the tracheal cartilage, the trachealis muscle, and the tunica externa.

The trachea, sometimes called the windpipe, is actually a series of C-shaped cartilage rings which are connected by a thin layer of smooth muscle called the trachealis muscle and a connective tissue membrane. It runs from your larynx, or voice box, to a structure called the carina. The carina is the point at which the trachea splits or bifurcates into two smaller branches that enter your lungs. These are the primary bronchi. The middle of your trachea is, of course, hollow so that air can pass through it. This hollow area is called a lumen.

So we're now looking at a section of the trachea under a microscope. This green part is actually an empty space representing the tracheal lumen where air passes in and out of the lungs. The lumen of the trachea is lined by a moist mucosa and submucosa, and we're going to spend the next part of this tutorial looking at the microscopic anatomy of these structures.

This is the mucosa. It is a moist layer of epithelial tissue lying directly adjacent to the tracheal lumen and the underlying connective tissue known as lamina propria. The cells making up the epithelium of the mucosa are specifically characterized as ciliated pseudostratified columnar epithelial cells - and I know that's a really long name, but it does tell us everything that we need to know.

Epithelium means that these cells line an organ. Columnar means that the cells are rectangular or column-like in shape. Pseudostratified means that although the cells appear to be stratified, they're really not. Each cell has a surface on the basal lamina - a structure we'll talk about in just a minute - but not every cell reaches the free surface. Ciliated means that the cells have small cytoplasmic projections on one surface. You can find this kind of epithelium in several places in the body, but in the trachea, it gets a special name - respiratory epithelium. Let's take a look at the small projections found on these cells.

If you look closely at respiratory epithelium, you'll notice that there is a small line along the surface of the cells that faces the lumen. These are the cilia. And if we look at the cells really closely using scanning electron microscopy, we can see the projections like little fingers extending up from the cells.

The cilia are motile - meaning that they can move and beat in a coordinated pattern to propel particles that have become trapped in the mucous membrane of the trachea upwards to be expelled. So when you hear someone with a raspy cough, often when they're sick, this is the result of particles being pushed towards the pharynx so they can be removed from the trachea when the person coughs.

Interspersed within the respiratory epithelium is a special type of cell called a goblet cell, also known as a mucous cell. It's called a goblet cell because of its shape. It looks like a kind of water goblet stuck in between the epithelial cells.

The goblet cells secrete mucus into the tracheal lumen to protect the mucosa from injury and trap dust particles that may have entered the tracheal lumen. They're easiest to see when they have accumulated mucinogen granules in their cytoplasm like these have. Goblet cells increase greatly in number during chronic inflammation of the airways.

Now that we've seen the cells next to the tracheal lumen, let's take a look at the cells farthest from the lumen, and these are called basal cells. The word basal refers to the cells being at the bottom or the base of the epithelium. Basal cells - the progenitor cells of the respiratory epithelium - are responsible for replacing cells that have been sloughed off in the tracheal lumen.

Similar to the cilia along the tracheal lumen, the basal lamina looks like a thin line along the respiratory epithelium except now that line is at the deepest part of the epithelium instead of the surface - and we can see it here in green. The basal cells overlie the lamina propria and some mucosa. Let's take a look at those now.

The lamina propria seen here in green is a sheet of loose connective tissue directly deep to the basal lamina. It's rich in lymphatic tissue and blood vessels and contains the ducts from small submucosal glands that lubricate the respiratory mucosa.

Deep to the lamina propria, we next encounter a large sheet of connective tissue called the submucosa. The word submucosa means that this tissue is beneath the mucosa. Although the submucosa surrounding most organs in your body consists of dense connective tissue, the connective tissue of the trachea is rather loose as we can observe by these blank spaces.

Earlier, we mentioned glands found in the submucosa. There aren't any actually visible on our histological slide because in the trachea, these glands are not very abundant, but this diagram will allow you to appreciate their location in the submucosa. The glands found in the trachea are exocrine glands, meaning that they release their secretions through ducts most easily seen in the lamina propria. We can actually see a couple of these ducts cut transversely right below the epithelium in this image here.

The cells of these ducts are distinguishable from the surrounding tissue by their cuboidal cells and organized pattern. Most exocrine glands are classified as either serous glands or mucous glands. The glands in the trachea, however, secrete a substance that is a mixture of the two types, so instead they are referred to as seromucous glands.

The tissues of the trachea receive their blood supply by tiny little arteries called arterioles, and we can identify them by their relatively round shape and the littered spots inside. So these spots are actually red blood cells carrying oxygen molecules. Blood leaves the trachea through small veins and these veins also contain red blood cells, but as they have thin walls, they tend to collapse and do not look as round as the arterioles.

So we mentioned before that the trachea is made up of a series of cartilage C-rings held in series by connective tissue and smooth muscle. Let's take a closer look at their structure now.

Like most cartilage in the body, the hyaline cartilage making up the C-ring of the trachea is surrounded by perichondrium. The perichondrium seen here is a connective tissue layer containing blood vessels, nerves, lymphatics, and immature cartilage cells. Cartilage tissue lacks its own blood supply and depends on the perichondrium to deliver nutrients and oxygens to the cartilage mix.

As we just said, the cartilage of the trachea is hyaline cartilage, and this type of cartilage consists of sparse cells known as chondrocytes surrounded by a large amount of extracellular matrix. This matrix contains a lot of water that gives it a smooth, glossy appearance under a microscope. Because of these matrix characteristics, hyaline cartilage provides a flexible, yet firm support. The C-rings are capable of some bending, but remain open or patent to permit air flow.

A band of smooth muscle attached to the ends of the cartilage rings seen here highlighted in green completes the circle posteriorly to create the lumen of the trachea. This involuntary muscle is called the trachealis muscle.

Besides closing the rings of the cartilage, the trachealis performs two very important functions for the trachea. First, it relaxes when you swallow something so that the bolus can pass behind the trachea without displacing it as we can see on the cross-section, and second, the trachealis contracts when you cough to help propel air and particles superiorly.

Finally, let's look at the adventitial layer. The adventitial layer is a layer of connective tissue on the outer surface of the tracheal cartilage. It lies just on top of the perichondrium, just here. The adventitial layer is the layer of the trachea that opposes other tissues in the neck and thorax.

So now we'd finished with looking at the histology of the trachea. Let's take a couple of minutes to have a look at some clinical correlations.

So, of course, as you know, cold and flu season is absolutely no fun at all, especially not for the trachealis muscle, the cilia, and the mucous glands of the respiratory epithelium that end up working overtime when you get sick. Of course, viral infections cause your glands to secrete the mucus into the tracheal lumen and this can be very uncomfortable, and your doctor may prescribe expectorants that loosen the mucus so that your cilia can push it out of your trachea more easily. As you cough, the trachealis muscle contracts to help push air and particles out.

So now we've reached the end of this tutorial, let's summarize what we learnt today.

So, in this tutorial, we looked at the microscopic structure of the trachea - the cartilaginous tube that transmits air between your pharynx and the lungs. First, we looked at the tracheal lumen - the open space that air passes through - then we looked at the mucosa that lines the inner surface of the trachea. Within the mucosa, we saw several layers of cells - the respiratory epithelium - directly opposing the trachea lumen.

We saw a small cilia which extend from these cells to move foreign particles towards the pharynx. We saw special goblet-shaped cells in the epithelium releasing mucus to keep the mucosa moist. We saw the deepest cells of the respiratory epithelium which are called basal cells, and these cells overlay the basal lamina, which is a small layer of tissue separating the mucosa from the lamina propria.

The lamina propria itself is a layer of connective tissue rich in lymphatics and blood vessels, and directly beneath the lamina propria, the submucosa is a thicker piece of connective tissue containing copious blood vessels and small glands. The seromucous glands of the submucosa secrete their products via ducts classifying them as exocrine glands and the blood supply of the submucosa comes from a series of small arterioles.

After we made our way through the epithelium and connective tissue layers lining the tracheal lumen, we then looked at the cartilage rings that create the shape of the trachea and these rings are made up of firm, yet flexible hyaline cartilage. Like most cartilage in the body, the rings of the trachea are covered in a supportive layer of connective tissue called the perichondrium.

The cartilage rings are C-shaped. The lumen of the trachea is closed by a thin layer of smooth muscle called the trachealis, and finally, the outer surface of the cartilage rings is covered by an adventitial layer which opposes the surrounding tissues of the neck and thorax. To close, we looked at the microscopic anatomy of the trachea and how it facilitates coughing.

And now we've reached the end of the tutorial. Thanks very much for watching and happy studying.