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Overview of the intraepithelial glands and related structures.
Hello everyone! This is Joao from Kenhub, and welcome to another histology tutorial where this time we're going to be covering the intraepithelial glands. Essentially what we're going to be viewing here on this tutorial is to talk about the different types of intraepithelial glands and also their related structures. And before we go on and talk about the different intraepithelial glands, I just want to first list the different topics – an overview of the different topics – we're going to be covering here on this tutorial. We're going to start by defining what an intraepithelial gland is then going to talk about the different types of intraepithelial glands we find in the human body and finally, we're going to look at the ducts – the different types of ducts – that we find associated to intraepithelial glands.
And without further ado, let's start with the very first topic here on the list – definition – and by definition we mean what is exactly an intraepithelial gland. So, well, to answer this question, let's start by defining what a gland is, and here on this beautiful image you see here a gland highlighted in green, an example of a very famous gland in your body – a salivary gland – and that is the parotid gland. Now so back to defining what a gland is, a gland can be classified as an organ because it is made up of a group of specialized tissues whose function is to then secrete substances into your body for use or discharge.
Now, most glands, they are made up of either cuboidal or columnar epithelial cells and even though we're going to be defining these types of cells on other tutorials, I just want to remind you that when we talk about cuboidal or columnar epithelial cells, were talking about the shapes of the cells. So cuboidal reminds of cubes – so shapes of cubes – and columnar, the shape of a column. Now also most glands can be classified as either simple or compound which is also more related to the structure of the gland. Furthermore, we're going to also be looking at their secretory portions which can be either tubular or acinar. So we're going to be looking at all these different characteristics in this tutorial.
Okay now that we have an idea of what a gland is, it is time for us to talk about what is exactly an intraepithelial gland – so the true definition of intraepithelial gland. Intraepithelial glands as the name shows are basically glands that are found within an epithelial layer. To show you an example, I have here on this image – this is a cut of your tongue and we just magnified it a lot so you can see the different microscopic structures – to show you here these highlighted structures which are posterior lingual glands which are found within the epithelial tissue of your tongue, in particular beneath the filiform papillae – these papillae that you see here – and circumvallate papillae. In this tutorial, we will look at a few more examples of intraepithelial glands.
But before we look at the different types of intraepithelial glands, let's briefly go over the different types of secretion mechanisms displayed by these glands – in particular, exocrine glands. It should be noted that intraepithelial glands are exocrine glands that's why we're going to be focused on or focusing on this type of secretion.
Now there are three types of exocrine secretion that we're going to be looking at – apocrine, merocrine and holocrine. The first one on the list – apocrine secretion – is a mechanism of secretion that involves the discharge membrane-bound vesicles usually containing lipids. This mechanism of secretion can be seen in breasts or mammary glands seen here in this illustration on your right and also in some sweat glands. Now, the second one on the list – merocrine secretion – now this mechanism of secretion involves exocytosis of proteins or glycoproteins also from membrane-bound vesicles. This is the most common form of exocrine secretion and is the type of secretion carried out by the salivary glands. Here on this micrograph, you can then see compound tubuloalveolar mixed salivary glands.
Last on the list we have holocrine secretion which involves discharge of whole secretory cells. This type of secretion can be seen in glands such as the sebaceous glands which you now see highlighted on the micrograph on the right – this highlight right here – which secrete an oily substance called sebum.
Now that you have a brief idea of the exocrine secretory mechanisms, it is time for us to talk about the different types of intraepithelial glands. First let's look at these cells which are considered to be glands the known as the goblet cells. These intraepithelial glands are found in many organs including in the respiratory tract and also digestive system. And of course you can see here them highlighted in green – the cells right here are then the goblet cells. The main role of the goblet cells is to secrete mucous which protects the mucosa of the organs in which they are found.
Now let's focus a little bit more here on this slide of the goblet cells to show you that we have here a cross-section of goblet cells from the duodenum and as you can see they are – they appear to be a bit more lightly stained than other cells here on the micrograph – and you can also see that these cells can appear to be then round in shape or a bit more cylindrical as you can see here on this one for example. And though not visible at this magnification, it should be noted that the nucleus and other organelles of the goblet cells are located at the base of the cell while the rest of the cell is then comprised of a membrane-bound secretory granules which is the next image that we’re showing here – all these little dots highlighted in green – they are known secretory granules, and these contain then the mucus.
Now let's go back to the image here of the goblet cells. Just to say that these cells can be found in the epithelium of organs like the small intestine, large intestine, the trachea, bronchi and even in the conjunctiva of the upper eyelid.
It is time for us to move on to the next type of intraepithelial glands. Right now we're looking at what is known to be a sebaceous gland – so this highlighted structure here in green is a sebaceous gland. This type of intraepithelial gland is a holocrine-secreting multiacinar gland. This is definitely a long, long term here to digest but just to mention that the term multiacinar just refers to the fact that the gland contains multiple secretory portions, and these multiple secretory portions are known collectively as acini which is the plural for acinus.
Now sebaceous glands, they can be found in all areas on the skin with the exception of the skin on the palm of your hand and the sole of your foot. Note here on this image that we have a cut of the skin where we highlighted the sebaceous gland and you can clearly see that these glands are usually located adjacent to this beauty here – the hair follicle.
Now let's briefly talk about the function of the sebaceous gland. Just to say that this gland secretes an oily substance called sebum and then this substance will lubricate the skin and the shaft of the hair follicle. And moving onto this image here to show you then sebaceous gland cells. Just to say that these beautiful cells are then responsible for the production of sebum that we talked about on the previous slide – so all these little cells here that you can see highlighted in green.
The next image that we're going to be seeing here on this tutorial is what we call a compound tubuloalveolar mixed salivary gland – a long name to show you all this structure here that is not highlighted in green because the entire structure is representing a compound tubuloalveolar mixed salivary gland. Now compound tubuloalveolar mixed salivary glands are basically salivary glands whose secretory portions are organized in a tubuloalveolar fashion. And what does that mean? Well, when we say these glands are compound glands, this means that the glands have branching ducts as opposed to a single duct – so remember branches of a tree instead of just having one, there are several. At the ends of these ducts, there is a convergence of what we call acinar or alveolar secretory units. As we saw at the beginning of this tutorial, these glands are merocrine secretory glands.
Compound tubuloalveolar mixed salivary glands can be found in the major salivary glands such as this one that you see here highlighted in green – the parotid gland, also the submandibular gland, and under your tongue – the sublingual gland.
It is time for us to move on and talk about the next types of glands known as serous glands. They are also a type of exocrine glands that exhibit merocrine secretion. They are classified as serous based on the type of secretory product because they produce a serous isotonic fluid. Now, the serous cells which you now see here highlighted in green – to be more specific they are known as serous acinar cells and they are then responsible for the production that the isotonic fluid that we talked about before.
Now, serous acinar cells or the actual serous glands, they can be found in the different or a major salivary gland known as the parotid gland seen here highlighted in green on this image and also in the pancreas which is this organ here highlighted in green on this image of showing the different organs, different structures belonging to the endocrine system.
We're going to move on and talk about another type of gland now seen here highlighted in green, these are known as mucous glands. This is another type of exocrine gland that exhibits merocrine secretion. These glands differ from serous glands because of the structures right here – their cells – which are known as then mucous cells. These mucous cells they produce mucin-containing mucous secretions. These secretions of mucous acinar cells, they are viscous in comparison to the watery secretions of serous cells.
Just going back here to this image if you remember that we showed you on a previous slide of goblet cells, just to show you a good example of what mucous cells are because these cells they contain secretory granules located apically in the cells which we also saw the different – we looked at a slide of the secretory granules.
Now I want to just mention here continuing on the mucous glands that mucus-secreting cells can be found in the glands such as the sublingual gland that you see here highlighted in green on this image right here as well as in the bronchial mucosa highlighted in green – you have here an image of one of your bronchus highlighted in green and the nasal mucosa.
The next type of intraepithelial glands is known as the urethral glands. They are found in the epithelium of the male and female urethras. In males, they are known as the urethral glands of Littre whereas in females, they are known as Skene's glands. Now, in both sexes, the secretions produced by these glands are mucous secretions.
The next gland we're going to be talking about that you see now on this micrograph – we've talked about it several times here on this tutorial – but the very famous parotid gland. This is one of the three major salivary glands which is responsible for production of saliva in the mouth. This gland is located bilaterally in the preauricular area as you can see here on this image and is responsible for about 30% of the saliva produced in your body. And by bilaterally, I mean that you have one on each side of your head.
Like all the salivary glands, the parotid gland is an exocrine gland. The acinar secretory portion of this gland is made up of serous cells that produce and secrete saliva. And again we have here a micrograph showing the microscopic view of the different cells if we were to cut a little bit of the parotid gland that we saw also on the previous image at a macroscopic level.
We’re going to move on and talk about now these glands here which are known as mixed seromucous glands to be more precise, and as the name indicates, they produce a mixture of both serous and mucous secretions. These glands contain mucous cells that have crescent-shaped serous cells partially surrounding them. Now these cells that will be now highlighted here on the screen, you can clearly see that they have almost like a shape of a moon. And for that reason, we're going to call them then von Ebner's demilunes or simply as serous demilunes. Remember that these are crescent-shaped serous cells. Mixed seromucous glands can be found in this gland highlighted in green which is the submandibular gland. The submandibular gland is one of the three major salivary glands responsible for producing saliva in the body.
We're going to be talking about these glands that are now seen here highlighted in this image of the skin, these are very well-known especially when you exercise – the sweat glands. They are intraepithelial glands which are then found distributed throughout most of the skin. There are two types of sweat glands found in the human body – the so-called merocrine sweat glands which are found in most areas of the skin then we have the apocrine sweat glands found in many areas of the body – this time the axillary region, the areolar and nipple regions as well as the perineal region and external genitalia.
Now let's have a quick look here at the sweat gland but this time on a micrograph – so we're looking at a microscopic level – to show you that they are simple tubular glands. But to show you a bit more clearly the shape of a sweat gland, we're going to go back to this image here where you can see the simple tubular shape of the sweat gland. Essentially, it's a tube that is coiled on a portion and then continues on. So you find this area here which is essentially a secretory portion called secretory tubules or tubulus that is coiled and attached to the secretory-excretory duct which you see here. Now this excretory duct is then lined with stratified cuboidal or columnar epithelia. And if we go back here to the micrograph, you can see that some of the cells have the shape – have cuboidal or columnar shapes.
Now so far let's just look at where – what we've covered so far and what we're going to cover next. So, so far we've looked at a few examples of intraepithelial glands. We have seen that there are three types of salivary glands and we have also looked at how glands can be defined according to the types of secretion and secretory products. We have even looked at the different types of cells that produce the secretions. Now we're going to be looking at the types of ducts that make up the excretory portions of these glands. Note that these ducts are basically tubes lined with epithelium that transport the secretory products of a gland to the surface.
So now let's look at the different types of ducts. We're going to start with these that you see here on this micrograph which are known as intercalated ducts. These ducts are the terminal ducts of an exocrine gland and can be found in glands such as the submandibular gland and the parotid gland as well as the pancreas. They are narrow and lined with simple or low columnar epithelium and they connect the secretory unit with the striated ducts and they are then surrounded by myoepithelial cells, which you can now see here on this micrograph highlighted in green.
The next types of ducts that we're going to be seeing here now highlighted in green, they are known as striated ducts which are a continuation of the intercalated ducts that we talked before – so they're, this continuation of the intercalated ducts of the excretory portion of the glands. And they connect the intercalated ducts to the interlobular ducts. And because the basal membrane of these ducts is then folded, this allows for active transport of water and ions to take place within these ducts.
Finally, we're going to be looking at these ducts that you see now highlighted in green known as the interlobular duct. Right now we're looking at one here on the screen highlighted in green. They are connected to the striated ducts and are also sometimes referred to excretory ducts. These ducts can run within the connective tissue septa that divides these glands into lobules and they are lined with tall columnar epithelium. You can see here that the cells are a bit more elongated than some of the other cells that we saw that have a bit more of a cube shape but these are a bit more elongated and have the shape of a column – that's why we say that these interlobular ducts are lined with tall columnar epithelium.
As I mentioned, this is the last part – the last duct – and we're going to briefly talk about a few clinical notes connected to this topic – the intraepithelial glands. Earlier, we gave an example of sebaceous glands as a type of intraepithelial glands found in your skin and we saw that they're main function was to secrete sebum which lubricates the skin and the hair shaft. While sebaceous glands have also been strongly linked to the development of acne vulgaris which usually occurs around adolescence as a result of an increase in the production of sebum among other factors. Most people suffer from acne vulgaris or pimples at some point in their lives. And even though every well-known, I have here an image for you to see what acne vulgaris or pimples look like.
Before we complete this tutorial, I want to do a quick summary of what we talked about so let's quickly recap what we've learned. We defined intraepithelial glands as glands that are found within an epithelial layer. Then we'll look at different examples of intraepithelial glands like goblet cells which you see now on the image highlighted in green, sebaceous glands, compound tubuloalveolar mixed salivary glands; other types of intraepithelial glands included serous glands, mucous glands, urethral glands, the famous parotid gland; also mixed seromucous glands and finally, the also famous sweat glands. After looking at the different types of glands, we then explored the types of cells that make up the secretory units of these glands and the types of ducts that transport the secretory products of these glands as well as associated structures.
Hopefully now you have a good understanding of not only what glands are but what the intraepithelial glands are and their mechanism of secretion.
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Now, good luck everyone, and I will see you next time.