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Structures and types of stratified epithelia.
Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial about stratified epithelium. Just to give you a bit of background information, I want to first talk about what epithelial tissue is and how it relates to other tissues of the body. Now there are four basic tissue types that make up organs and tissues found in the human body. There's epithelial tissue, connective tissue, muscle tissue and nerve tissue. In the image on the right, we can see all four tissue types. So externally, we have the epidermis of the skin highlighted in green which is our epithelial tissue. Beneath that, we have the dermis and hypodermis which essentially consists of connective tissue. Deep to that, we then have muscular tissue and nervous tissue in the form of nerve fibers.
So now that we've covered the four basic tissue types of the human body, let's focus on the one we're interested in which is epithelial tissue. Epithelial tissue is known as a covering tissue made up of uninterrupted layers of cells because it covers all external body surfaces and lines internal body cavities. It can be classified according to the number of cell layers present. For example, simple when only one layer of cells is present or stratified when more than one layer of cells is present.
In this micrograph, we see the epidermis highlighted in green which is also a type of epithelial tissue. The epidermis is the outermost layer of the skin and is made up of uninterrupted layers of cells which means it satisfies the criteria being a covering tissue because it covers the entire body. Epithelial tissue can also be further classified according to the shape of the epithelial cells it comprises. For example, whether the cells are squamous or flattened in shape, cuboidal or square in shape, or columnar or rectangular in shape. Finally, epithelial tissue can also be described according to the specializations that the cells may possess such as secretory, excretory, absorptive, keratinized, non-keratinized or ciliated.
In this tutorial, we'll focus on the different types of stratified epithelia and where they can be found in the body. As I mentioned, stratified epithelium is a type of epithelial tissue that contains multiple layers of epithelial cells. As you can see in this micrograph, the highlighted tissue contains layer upon layer of cells. The main function of stratified epithelium is to act as a protective layer. Because of the number of cell layers present in stratified epithelium and also due to the fact that it's constantly renewing itself, this type of epithelium can withstand abrasion therefore making it good for protection.
Now let's look at the types of stratified epithelium that can be found in and around the body. The first type of stratified epithelium we'll look at is stratified squamous epithelium. Stratified squamous epithelium is made up of several layers of squamous or flattened epithelial cells. In this type of epithelium, only one layer of cells has contact with the basement membrane while the rest of the cell layers adhere to one another. Although you can't really tell just by looking at this micrograph, the innermost layer of cells divide and the daughter cells move towards the surface where they mature and degenerate. In other words, the bottom layer of cells is mitotically active constantly replacing the superficial layer of cells that gets damaged and cast off at the free surface of the epithelium.
There are two types of stratified squamous epithelium – non-keratinized stratified squamous epithelium and keratinized stratified squamous epithelium. First, we'll talk about non-keratinized stratified squamous epithelium. Non-keratinized stratified squamous epithelium can be found lining the surfaces of the oral cavity, the esophagus, the vagina and cervix, the rectum as well as the anus. As you can already tell, these areas of the body are areas where a considerable amount of abrasion may occur and therefore this type of epithelial tissue – a type that can renew itself – is vital to have in such areas.
Once again as you can see in this micrograph, there are several layers of epithelial cells in this tissue stacked upon one another. If you look more closely, you can see that the cells closer to the basement membrane appear not as flattened as those closer to the surface. Keep in mind that the innermost layer of cells is mitotically active which means that they're dividing or producing daughter cells. Non-keratinized stratified squamous epithelium doesn't only function as protection in these areas of the body but also provides lubrication.
The second type of stratified epithelium we'll look at today is keratinized stratified squamous epithelium. This type of epithelium is also referred to as stratified squamous keratinizing epithelium. The best example of this type of epithelium is the epidermis of human skin, however, it also comprises some oral epithelia. Its outermost layer is made up of keratin-filled dead cells that are tightly packed together. This layer of cells functions as protection against mechanical and chemical damage as well as preventing water loss. As I just mentioned, the epidermis of the skin is a good example of this type of epithelium and is comprised of five layers. So let's look a little more closely at the five sublayers of the epidermis.
The first three layers are the stratum basale, the stratum spinosum and the stratum granulosum. The last two layers are the stratum lucidum and the stratum corneum.
So let's start internally and work our way outwards with the first layer we talked about being the stratum basale which is also known as the basal layer. Here you can see the stratum basale has been highlighted in green. It's the innermost sublayer of the epidermis and is made up of a continuous layer of cells. These cells are mitotically active which means that they divide quite frequently pushing some newly formed cells upwards. They then go on to mature while at the same time replenishing the basal layer. This is the only layer of cells that is in contact with the basement membrane. Note that the stratum basale is also known as both the basal layer and the stratum germinativum.
The next layer of the epidermis which is the layer just above the stratum basale is known as the stratum spinosum. The stratum spinosum is composed of about eight to ten layers of cells and it's also commonly referred to as the prickle cell layer. It's called the prickle layer because when you look at a histology slide, the cells in this layer have a spinous appearance. This spinous appearance occurs as a result of the cell shrinking during histological fixation making the desmosomes that connect the cells look like spines. The cells in this layer also contain and synthesize keratin. It's because of this protein keratin that gives keratinized stratified squamous epithelium its name and characteristic distinguishing it from non-keratinized epithelium.
Just above the stratum spinosum, we can see another layer of cells highlighted in green which is known as the stratum granulosum. This layer is comprised of about three to five layers of cells. The reason that it's known as the granular layer or the stratum granulosum is because the cells found in this layer possess granules that contain lipid-rich secretions that act as waterproofing sealant.
Another layer of the epidermis that is situated between the stratum granulosum and the stratum corneum is the stratum lucidum. This thin transparent layer functions to reduce friction and shear forces between the stratum granulosum and the stratum corneum. Please note that because this layer is thin, it may be hard to distinguish in some histological preparations.
The final layer of the epidermis which is located above the stratum lucidum is known as the stratum corneum. This is the outermost layer of cells and is made up of dead flattened keratin-containing cells. As you can see here, the cells are really flat and you can hardly distinguish them individually. They also lack nuclei. At the surface, dead cells flake off and these flakes are actually what make up household dust. So basically most of the dust that you find accumulating in your house or flat is just the dead cells that you've shed over time. The final layer of the epidermis comprises of up to twenty layers of cells and functions as a chemical barrier. It's also responsible for maintaining the integrity and hydration of the skin.
Okay, so let's stop for a quick breather and assess where exactly we are in this tutorial. So far, we've covered both non-keratinized stratified squamous epithelium and keratinized stratified squamous epithelium. We've also gone into a bit more detail by looking at the layers of the epidermis which is composed of keratinized stratified squamous epithelium.
Now let's go on to look at the other types of stratified epithelium. Another type of stratified epithelium is the one we can now see highlighted in green which is known as transitional epithelium. This type of epithelium can be found only in the urinary tract lining the ureters, urethra and the urinary bladder. It's unique from other types of stratified epithelium because it can accommodate stretching in the urinary tract through its ability of its epithelial cells to change their shape. When unstretched, the cells of transitional epithelium appear rounded, however, when the urinary tract is under fluid pressure, the cells flatten accommodating for the amount of stretch or distention that is necessary. When the urinary bladder is relatively empty, these epithelial cells will appear more rounded or ovoid in shape but as the bladder begins to fill up with urine, the cells begin to stretch out and flatten as it expands with fluid.
Finally, we'll look at two types of stratified epithelium that are not found widespread in the body. These are stratified cuboidal and stratified columnar epithelia. Stratified cuboidal epithelium, as the name suggests, is comprised of multiple layers of cuboidal epithelial cells like what you see here in this micrograph. This type of epithelium can be found lining the ducts of sweat glands, the ducts of salivary glands and the mammary ducts.
Now, we'll move on to talk about stratified columnar epithelium. As you probably already guessed, stratified columnar epithelium is comprised of multiple layers of columnar epithelial cells. It can be observed forming the cell layers of the ocular conjunctiva but it's also found lining structures such as the male urethra, the vas deferens, the anus, the pharynx and the uterus. In this micrograph, we can see the stratified columnar epithelium of the uterus. This type of epithelium not only acts as a protective layer but may also have secretory properties.
Although stratified epithelium acts as a protective layer against chemical and mechanical damage, it too like all other tissues of the human body can have its problems which we'll talk about briefly in our clinical notes. In our picture on the right, we can see an example of a skin condition which is called psoriasis. Psoriasis is an autoimmune disease where the squamous epithelial cells and the stratum corneum of the epidermis retain their nuclei which indicates a hyperproliferative disorder.
As we saw earlier on in this tutorial, the cells in the stratum corneum are normally dead and have no nuclei. Psoriasis results in the formation of flaky, scaly plaques on the skin but the condition can be treated with the use of phototherapy, immunosuppressants, topical agents and alternative therapies.
Before we conclude this tutorial, let us quickly summarize what we've learned today. So, we've described what stratified epithelium is seeing that it's a type of epithelial tissue that contains multiple layers of epithelial cells, acts as a protective layer and is constantly renewing itself. We've also discussed the different types of stratified epithelium. First, we looked at non-keratinized stratified squamous epithelium which can be found lining body cavities such as the anus, cervix, rectum, esophagus and oral cavity.
Next, we looked at keratinized stratified squamous epithelium which makes up the epidermis of human skin as well as some oral epithelium. Third, we had transitional epithelium which is only present in the urinary tract followed by stratified cuboidal epithelium which lines the ducts of sweat glands, salivary glands and the mammary ducts. Lastly, we looked at stratified columnar epithelium which lines the male urethra, the vas deferens, the anus, the pharynx and the uterus.
So that brings us to the end of our tutorial on stratified epithelium. I hope you enjoyed it and thank you for listening.