Video: Duodenum histology
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Okay, let’s be real here. Not every goblet deserves its own story, loved by children and adults alike. But there is one goblet which is definitely worth knowing about if you’re an anatomy student, and of course, we’re talking about goblet cells and one place we’ll find them is when studying the histology of the duodenum, which happens to be exactly what we’re looking at today.
Before we jump in, let’s have a quick overview of the topics we’ll tackle today. We’ll start with a reminder of the duodenum location, gross anatomy, and function. We’ll then move on to the histological section of a duodenal biopsy where we will identify some of the layers of the duodenum. We’ll start with the mucosa, the different cell types found in it, and its larger features such as the villi and intestinal glands. We’ll take a closer look at the muscularis mucosa, one of the layers of the mucosa, before we move on to the submucosa and the duodenal glands. Then using a diagrammatic representation, we’ll quickly discuss the muscularis externa and serosa layers. Finally, we’ll have a section on clinical notes.
To kick off this tutorial, let’s first look at the gross anatomy of the duodenum.
The duodenum is the shortest segment of the small intestine at around 25 centimeters long. Proximally, it is an extension of the pylorus of the stomach and distally, it is continuous with the jejunum. It is C-shaped and wraps around the head of the pancreas. It receives partially digested food or chyme from the stomach and plays a part in breaking it down further as well as absorbing nutrients. A distinctive feature of the small intestine known as circular folds or plicae circulares are also found in the duodenum and serve to increase the surface area for absorption. If you need more duodenum gross anatomy in your life, just head over to kenhub.com where you’ll find a video and article on just that as well as lots of anatomical illustrations.
Now allow me to introduce you to the beautiful slide we’ll be using this tutorial. I’m sure you’ll notice straight away that the stain used here is hematoxylin and eosin, more commonly known as H&E. Hematoxylin is a basic dye which stains acidic structures so things like nuclei containing lots of acidic DNA stain dark blue. Eosin is acidic so it sticks to the basic structures like the cell cytoplasm and stains them various shades of pink.
Now what we’re looking at here might be initially difficult to identify as duodenum but what you can see now is actually looking at a section from a biopsy taken from one of the circular folds we saw earlier, therefore, you won’t be able to see all the layers of the duodenum in this slide.
Now let’s dig a little deeper and take a look at the layers of the duodenum.
Let’s start by identifying all the layers of the duodenum in this image here. The first is the innermost layer known as the mucosa followed by the submucosa, the muscular coat or tunica muscularis, and the outermost layer called the serosa. So if you compare the diagram to a histological slide, you’ll be able to identify that the layers we’re seeing are the mucosa and just a little piece of the submucosa. And now we can move on to our histological slide.
First up, we’re looking at the innermost layer of the duodenum facing the lumen – the mucosa. It consists of three layers – the epithelial layer, the lamina propria, and the muscularis mucosae. The first thing we might notice with the mucosa are these long elongated structures extending from the duodenum into the luminal space. Each of these is known as an intestinal villus. These serve the same purpose as the plicae circulares – to increase the luminal surface area of the duodenum and maximize absorption. The luminal surface of the duodenum including its villi is covered in a layer of simple columnar epithelium. This means that these cells are arranged in a single layer and are taller than they are wide. The individual epithelial cells are called enterocytes. They’re easy to identify in a histological section because their dark-staining nuclei are located close to their basal border.
Another distinctive feature you’ll be able to identify at a higher magnification is what we call a striated border. It is a very thin, striated, or fuzzy-looking line on the apical surface of the epithelium. It’s also often referred to as a microvillous border due to the fact it is formed by microvilli, which are minute projections of the enterocyte cellular membrane which serve to increase the luminal surface area of the enterocytes by up to 600 times. That, of course, helps greatly with the absorption of nutrients from food, which is the main function of enterocytes. These, of course, are difficult to appreciate in light microscopy sections, but if we look at them here in this transmission electron microscopy image, we can see the morphology much clearer. Each microvillus is about 1 micrometer long and 0.1 of a micrometer in diameter.
If you’ve been paying attention so far, you might have counted three different adaptations of the duodenal luminal surface which serve to increase the available surface area for absorption. From largest to smallest, these were the circular folds, intestinal villi, and finally, the microvilli of the striated border.
Now you’ve probably noticed that among the tall and thin enterocytes are rounder, fatter cells with these empty spaces. These are known as goblet cells which we excitedly refer to at the beginning of this tutorial. In reality, goblet cells contain mucinogen, which gets washed away in routine H&E preparations so the cells appear empty. Goblet cells are actually unicellular glands producing mucus which is where this layer, the mucosa, gets its name from.
The mucinogen vesicles accumulate close to the apical surface of the cells which expands to the top portion distorting the shape of the neighboring enterocytes. The cells actually have a really thin basal extension which reaches down to the basement membrane forming the stem of the goblet. The stems are highly basophilic because they contain the cell nuclei and numerous ribosomes so they stain a dark blue in H&E sections.
Now let’s briefly turn our attention to the biggest structures observed in this section of the duodenum. You’ll notice these long elongated structures extending from the duodenum into the luminal space. Each of these is known as an intestinal villus. These serve the same purpose as the microvilli and plicae circulares to increase the luminal surface area of the duodenum and maximize absorption. If you look at the lumen of the duodenum on a microscopic scale, it will appear velvety due to the countless villi found on its surface.
Histologically, the villi are covered in the simple columnar epithelium with the striated border which we’ve already discussed earlier in the tutorial. In its core, we find extensions of lamina propria mucosa made up of loose connective tissue, lymphocytes, and tiny blood vessels. In the center of the villus, you’ll find what is known as a lacteal. It is a blind-ended terminal branch of a lymphatic vessel which drains the lymph into progressively large lymphatic vessels. Lacteals are surrounded by smooth muscle fibers extending from the muscularis mucosa. It is thought that these muscle fibers create contractions in the villi which help propel the lymph from the lacteals into larger lymphatic vessels.
So the next layer of the mucosa is, of course, its lamina propria. We’ve already seen its projections into the villi but it actually extends all the way down from the simple columnar epithelium to the muscularis mucosa layer. You’ll immediately notice that it has a very different appearance to the loose connective tissue forming the core of the intestinal villi. The connective tissue is still there, of course, but it also features these structures known as the intestinal crypts or intestinal glands. They are tubular structures which extend from the base of the villi to the lamina propria. What you’ll normally see in histological section are some glands which are cut longitudinally and some which are cut transversely and therefore appear as circular structures. They are also lined with simple columnar epithelium, similar to what you see on the intestinal villi.
At the base of these glands, however, we find a different type of cell which we haven’t discussed yet. We’re talking about Paneth cells. These cells play a part in maintaining the bacterial flora in the small intestine. They’re really easy to identify in a routine H&E section because their basal ends contain a nucleus, and therefore, are highly basophilic, meaning they stain dark blue. At their apical ends, we find large accumulations of zymogen granules, which are highly eosinophilic, meaning they stain bright red or pink. These vesicles store secretory enzymes which break down the cell walls of certain bacteria.
There’s another type of cells scattered throughout the mucosa of the duodenum known as the APUD – the APUD or enteroendocrine cell. This is actually an umbrella term for eight different cell types which secrete peptide hormones and regulate the functions of the gastrointestinal tract. These act as endocrine glands by secreting hormones straight into the bloodstream. The most active endocrine hormones secreted in the duodenum are cholecystokinin, secretin, gastric inhibitory peptide, and motilin. It also secretes somatostatin and histamine, which are paracrine hormones not released into the bloodstream but rather having a local effect. The cytoplasm of these cells are rich in vesicles which become lost in routine H&E preparations. These cells can be difficult to spot but their clear cytoplasm can be used as an indicator.
Now that we have explored all we can see in the epithelial layer and lamina propria, now would be a good time to look at the final layer of the duodenal mucosa. We’re talking about a thin layer of smooth muscle known as the muscularis mucosae. It’s actually composed of a few layers of muscle orientated in different directions which helps expel contents of the glands located in the submucosa, which is this layer here. It is made up of dense irregular connective tissue.
Besides the blood vessels and the lymphatics usually found within connective tissue, we have something pretty unique in the submucosa. Housed within the connective tissue is what we call the duodenal submucosal glands or Brunner’s glands which expel an alkaline secretion known as mucin into the intestinal glands, which protects the duodenal wall from the acidic chyme which enters from the stomach. The duodenal glands are branched tubular structures that in an H&E section, they’ll usually appear as light and somewhat foamy circular structures as many of them will be cut transversely. Now these glands are very important not just functionally but also as a histological marker. That’s because all three parts of the small intestine have the same general composition and can look very similar in histological section. Duodenal glands are usually localized to the proximal part of the duodenum and therefore make it very easy to identify.
Just a quick note here – although the duodenal glands are normally contained in the submucosa, sometimes you can find localized displacement of the glands into the lamina propria of the mucosa. We actually have a beautiful example of that on our slide.
Now since we are working with a section of duodenal biopsy in this tutorial, the submucosa is as deep as we’re going to go in our images. Nevertheless, let’s take a moment to discuss the two outermost layers which are missing from our histological slide. Let’s first talk about the muscular coat of the duodenum, also known as the tunica muscularis. This is located deep to the submucosal layer and consists of two smooth muscle layers. The outer layer is known as the longitudinal layer due to the fact that its muscle fibers run in parallel with the direction of the duodenum. The inner layer, on the other hand, is known as the circular layer as its fibers course parallel to the circumference of the duodenal wall.
How these fibers will look in histological section of course depends on the orientation of the section so it’s best to remember that the inner layer is always the circular layer and the outer layer is longitudinal. These layers produce two different movements in the small intestine. The circular layer produces a movement called segmentation which refers to locally moving chyme around to mix it with digestive juices and bring it in contact with the mucosa. The other type of movement is called peristalsis and is produced by the coordinated action of both the circular and longitudinal layers which serves to move along the contents of the intestine.
Between these two muscle layers, we find the myenteric plexus of Auerbach. These postganglionic parasympathetic neurons are part of the enteric nervous system responsible for producing peristaltic movements. So, it’s no wonder, the plexus is located between the two layers of the muscularis externa innervating the smooth muscle within them.
The outermost and the last layer we’re discussing in this tutorial is the serosa. It is made up of a layer of simple squamous epithelium called mesothelium and a small amount of loose connective tissue. Where the mesothelium is absent, the outermost layer is called adventitia. Serosa is associated with intraperitoneal structures such as the liver, stomach, and the first part of the duodenum whereas adventitia is associated with retroperitoneal structures such as the pancreas, the ascending and descending parts of the large intestine, and the rest of the duodenum.
That wraps up all the histological features of the duodenum we wanted to teach you today. But the show is not over because it’s time to put the histology into perspective and look at its clinical significance.
Today we’re going to be looking at duodenal ulcers in a histological context. Duodenal ulcers are a type of peptic ulcer which refers to the breakdown of the mucosal lining of the proximal part of the duodenum. Let’s look at its symptoms first. Really, it’s what you would expect with most digestive disorders – abdominal pain, nausea, vomiting (sometimes with blood), bloating, weight loss – you get the gist. The most common cause of duodenal ulcers is the bacterium Helicobacter pylori which causes the mucosa of the duodenum to become inflamed. Chronic use of certain medications particularly nonsteroidal anti-inflammatory drugs such as aspirin and ibuprofen as well as smoking and alcohol are also known to cause duodenal ulcers to occur.
So what does an ulcer look like in a histological section? The Helicobacter pylori bacteria attack the epithelium of the mucosa on the luminal surface of the duodenum. They secrete inflammatory proteins which attach to the epithelium and cause inflammation. Eventually, this causes the epithelium to break down which leaves the lamina propria exposed to the stomach acid. If you remember, we find blood vessels and nerves in the lamina propria so it’s no wonder that this continuous aggravation can cause pain and bleeding. Treatments include removing agents which may have caused the ulcer like medications, smoking tobacco, or drinking alcohol. If the ulcer is caused by bacterium, the first step in managing it is administration of antibiotics.
That wraps up everything I wanted to talk to you about today. But before we finish up, let’s recap what we learned.
First, we looked at the mucosa, the layer closest to the luminal surface of the duodenum. Here we identified its simple columnar epithelium made up of enterocytes with a striated border formed by microvilli on the cell’s apical surface. Also scattered in the epithelial layer, we saw mucus-producing goblet cells. We also identify these finger-like projections of mucosa known as villi which help to increase the luminal surface area of the duodenum. In their connective tissue core continuous with the lamina propria of the mucosa, we found a small blind end of lymphatic vessel known as the lacteal surrounded by smooth muscle fibers.
We went on to talk a little bit more about the lamina propria with its most distinguishing feature, the intestinal glands. Overall, they have the same covering as the rest of the mucosa but at their base, we find Paneth cells which play a role in maintaining the bacterial flora of the small intestine. Also at the base of the intestinal glands, we saw APUD cells – APUD cells – responsible for peptide hormone section into the bloodstream.
We moved on to identify the muscularis mucosae, a thin layer of smooth muscle considered part of the mucosa and separating it from the submucosa, before moving on to the dense irregular connective tissue layer, the submucosa. We moved on to talk about the duodenal submucosal glands or Brunner’s glands, normally found in the submucosa but in our slide, focally displaced into the lamina propria of the mucosa. We learned that these glands produce alkaline mucin which is transported into the lumen of the duodenum by intestinal glands to neutralize the acidic chyme from the stomach.
We took a quick peek at a diagrammatic representation of the tissue layers of the duodenum to identify the muscular coat or the tunica muscularis, a layer made up of a circular and longitudinal layer of smooth muscle found below the submucosa. Finally, we looked at the outermost covering of the duodenum called the serosa made up of loose connective tissue. And finally, we looked at the formation of duodenal ulcers in our clinical note section.
That brings us to the end of this tutorial. Hope you enjoyed it and happy studying!