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Histological appearance of sweat glands.
Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial on the histology of sweat glands. Before we begin, let me give you a quick overview of what we'll cover in today's video. Basically, we're going to discuss the histological features of the sweat glands that we possess. First, we'll quickly review what an exocrine gland is. We'll discuss the secretory portion and excretory ducts of a general exocrine gland and then dive into the two types of sweat glands – apocrine sweat glands and eccrine sweat glands. We'll also explore a common form of benign skin tumors arising from eccrine sweat glands called cylindromas as well as a condition of excess sweating known as hyperhidrosis.
Now before we get into the specifics of sweat glands, let us refresh ourselves of the important layers of the skin that we're concerned with when it comes to these structures.
Our sweat glands live within the layers of our skin and their tubular projections form the excretory ducts that emerge at the surface. This top layer of the skin is known as the epidermis and can be seen in this image highlighted in green. If we take a closer look at this image, the epidermis can be seen as a stratified layer of cells. This layer here is tightly packed together so as to prevent the passage of any foreign matter into the body such as bacteria or viruses.
It's important to note that in areas where our skin is in constant contact with objects or surfaces like our hands or the soles of our feet, the epidermis is much thicker which is obvious to notice in this sample of skin here. This type of skin is known as acral skin and it helps to protect the hands and feet from the mechanical stresses of walking and grasping objects.
The layer just below the epidermis is called the dermis. It's made up of dense irregular connective tissue and sweat glands are found within it or at the junction between the dermis highlighted here in green and the subcutaneous fat located below the dermis. Taking a closer look at the skin, it can clearly be seen that the dermis is made up of irregularly organized collagen fibers and can be found between the epidermis and the subcutaneous fat.
Sweat glands are a type of exocrine gland. Exocrine glands are composed of a secretory portion and an excretory duct. The secretions are made in the secretory portion and are transported to their destination via the excretory duct. This is different from endocrine glands which release their contents directly into the bloodstream. The image here shows an exocrine gland highlighted in green. If we look a little closer, you can see highlighted in green various structures that look like cross-sections of little tubes. These are the excretory ducts.
Both the excretory ducts and secretory portions have characteristic epithelium associated with them. First, let's talk about the different structures that are associated with the secretory portion of the exocrine glands. The secretory portion of an exocrine gland houses the cells that make up the secretions for the gland, a part of it can be seen highlighted in green here. These cells line a sac-like space and form a small grape-like structure called an acinus. Generally as is the case with sweat glands, an acinus doesn’t occur by itself and when it's accompanied by others, we refer to them collectively as acini.
Like all structures within our body, there must be some kind of support present in order to maintain the integrity of these glands. The basal lamina of the sweat glands acts as a support for the secretory portion of our glands. Highlighted in the image in green, the basal lamina can be seen as a thin layer of connective tissue that surrounds both the secretory cells and the sac-like space in this portion. It's composed of loose connective tissue and extracellular material from the overlying secretory cells. The remaining space between the glands is filled in with loose connective tissue.
As I just mentioned, the secretory portion of a sweat gland has a large population of secretory epithelial cells. These are what lined the acinus and can be seen here highlighted in green. They produce various substances that are secreted into the gland. Secretory epithelial cells are a modified form of regular epithelial cells and we'll see later on in the tutorial that the structure of this epithelium is different between the two types of sweat glands that we'll encounter.
Another cell type that we can see within the secretory portion of a sweat gland are the myoepithelial cells. These cells are also a modified version of an epithelial cell and are capable of contraction. Highlighted in green in the image, we can see that the myoepithelial cells are found peripheral to the secretory cells – that is, they're located further away from the lumen of the acinus and closer to the basal lamina. And in this image here highlighted in green, their location is even more apparent. In the same way that we use our hands to squeeze toothpaste out of the tube, myoepithelial cells help move secretions out of the secretory portion of the gland so that they can make their way into the excretory ducts.
Highlighted in green in the image, we can see the excretory ducts in cross-section. Within the center of the excretory ducts, we find a space called the lumen. The secretions from the secretory portion of the gland will pass through the lumen of the excretory duct and with the help from myoepithelial cells, these ducts will release their secretions either onto the surface of the skin or within a hair follicle.
Along with this luminal space, another important feature of the excretory duct is the presence of stratified cuboidal epithelium. The stratified cuboidal epithelium of the excretory duct is made up of two distinct layers. The cells that make up this epithelium are referred to as duct cells and we can see them highlighted in green here. The duct cells are further organized into two cell layers. The basal cell layer of the duct is the most peripheral or outer layer of the duct that we see highlighted. The layer that is directly atop of the basal layer is the luminal cell layer. The cells here are directly in contact with the lumen, hence, the name of the layer.
Now that we've covered the two basic units of an exocrine gland, let's discuss the general shape of a sweat gland before we dive into the two main types.
In the image here, we can see the sweat gland highlighted in green. Sweat glands are found within the dermis and sometimes at the junction between the dermis and subcutaneous fat. From this illustration, we can see that sweat glands are both coiled seen in the secretory portion and also tubular seen mostly in the excretory duct. If we take a look at a sweat gland in an H&E stain slide, we need to imagine that where we see these cross-sections of secretory portions and excretory ducts highlighted in green that they are actually going into and out of the page. This allows us to really understand that the gland is coiling around and eventually going to form a predominantly tubular duct towards its endpoint.
Now that we have a thorough understanding of the type of gland that sweat glands are and where we can find these structures, let's now dive into the two main types of sweat glands and the features that distinguish them.
As I just said, sweat glands come in two main types – eccrine sweat glands and apocrine sweat glands. These glands are also referred to as ordinary and odoriferous sweat glands respectively. First, let us look at eccrine sweat glands in more detail.
Highlighted in green in this image is an eccrine sweat gland. Eccrine sweat glands are the most abundant sweat glands found throughout the skin of the human body. They secrete a mixture of salts, ammonia, urea, and glycoproteins in solution and release these secretions directly onto the surface of the skin. Eccrine sweat glands are found in greater numbers on the palms of the hands and the sole or plantar surface of the foot.
Let us now explore some distinct qualities of these glands.
First and foremost, the epithelium of the secretory portion of an eccrine sweat gland is composed of columnar secretory cells. If we take a look at the image here highlighted in green, it almost appears as though there are columnar cells that are all different heights. This gives the appearance of multiple layers of epithelial cells and thus we call this type of epithelium, pseudostratified columnar epithelium. This type of epithelium is characteristic of the eccrine sweat glands.
Within the epithelium of the eccrine sweat glands, there are two distinct cell types that appear slightly different in color. As the name suggests, clear cells appear clearer and secrete salts like potassium, sodium, chloride, ammonia, and urea. We can see them highlighted in green and they're somewhat polygonal in shape. To contrast clear cells, there are dark cells which we can see here highlighted in green. These cells appear darker than clear cells under the microscope and these secrete glycoproteins into our sweat.
Apocrine sweat glands are the other type of sweat gland and these sweat glands are mainly associated with hair follicles. Highlighted in green is a hair follicle. Here we can see the hair in the center surrounded by numerous layers of darker staining cells. These layers of dark staining cells are keratinocytes and are rather dense around the hair follicle. If we zoom in further to the previous image, we can see the apocrine sweat gland just beside the hair follicle. These glands are also called odoriferous glands as bacteria feed off products found within the apocrine secretions and the resulting metabolites produce a not so pleasant odor.
We find apocrine sweat glands predominantly in hairy areas like the armpit. But how we differentiate them from the eccrine sweat glands is not based solely on location but also on the type of secretory cells they contain. Unlike the pseudostratified columnar secretary cells of the eccrine sweat glands, the secretory cells of the apocrine sweat glands are cuboidal cells. Here we can see highlighted in green the stratified cuboidal cells of the apocrine sweat glands. These cells stain lighter than the keratinocytes we see adjacent to them.
Now let us briefly touch upon a clinical application involving our sweat glands.
Hyperhidrosis is characterized by excessive sweating in the areas of the body not related to the body's need for physiological heat loss during strenuous or warm activities. The image here shows an eccrine sweat gland as these sweat glands are primarily the overactive type of gland in this condition. Hyperhidrosis is due to overactivity of the sympathetic nervous input to these sweat glands resulting in a higher production and release of sweat. One form of treatment for this condition is the injection of botulinum toxin to reduce sympathetic nervous activity to the sweat glands.
The cells of eccrine sweat glands can also form a tumour known as a cylindroma. This tumour falls under the category of skin adnexal tumours as it is a tumour that arises from the accessory structures found within the skin. It can also be known as a turban tumour as multiple confluent lesions can sometimes form on the scalp in individuals with a defect in the CYLDG. Solitary cylindromas are much more common, are usually benign, and ninety percent arise in the skin of the head and neck. They are also said to have a jigsaw puzzle appearance under the microscope. Surgical excision is usually curative, however, larger tumors may require a complex reconstructive plastic surgery.
This brings us to the end of our tutorial on sweat glands. First, we reviewed both the epidermal and dermal layers of the skin and then moved into the components that make up the exocrine glands. These are the secretory portion that produces the secretions and the excretory duct that acts as a passageway for the secretions to be expelled. The glands have supporting myoepithelial cells and a basement membrane.
From there, we went into detail of the features of both eccrine and apocrine sweat glands. Eccrine sweat glands have pseudostratified columnar epithelium in their secretory portion along with both clear cells and dark cells. Apocrine sweat glands, on the other hand, are associated with hair follicles and have cuboidal cells populating their secretory portions.
I hope you enjoyed this tutorial, thanks for watching, and see you next time.