In this video, we're going to talk about the first type of stratified epithelial tissue in our lesson, which is stratified squamous epithelium. Recall from our previous lesson videos, the term stratified indicates this tissue has multiple layers of cells where all of the cells are not going to come into direct contact with the underlying basement membrane. The term squamous indicates that the apical surface of the tissue has cells with shapes that appear to be squished or flat. We can say that stratified squamous epithelium consists of many layers of these flat or squished cells, just like what you can see over here in our diagram. It's worth emphasizing that, concerning stratified epithelial tissues that have multiple layers of cells, the term that indicates the cell shape only applies to the cells that are closest to the apical surface of the tissue or the cells that are closest to the open space immediately adjacent to the tissue.
As you go deeper into the tissue closer to the basement membrane, the cell shape can vary from cuboidal to even columnar. It's important to note that the term that indicates the cell shape, once again, only applies to the cells closest to the apical surface of the tissue. In our little diagram, it's very clear that this tissue has multiple layers of cells. You can see these nuclei are pretty much all over the place, meaning not all of the cells come into direct contact with the underlying basement membrane. This defines this tissue as a stratified tissue with multiple layers of cells. In terms of the functions of stratified squamous epithelium, the structure of the tissue and the cells dictates the functions of the tissue. With stratified tissues that have multiple layers of cells, these tissues tend to be thicker in nature and are great for protection.
The thick nature of the tissue is going to allow it to function in protection in many cases. The case here with stratified squamous epithelium is that it has so many layers of cells, it's relatively thick, and its primary function is protection. This means that we are going to find the stratified squamous epithelium in locations of the body where protection is important. For example, in areas such as our skin, mouth, esophagus, anus, and vagina, which encounter physical stresses. In the mouth and esophagus, it encounters physical stresses from the foods we eat and the drinks we drink. In the anus, it encounters physical stresses from the passage of feces. In the vagina, it encounters physical stresses from sexual intercourse.
Some of the stratified squamous epithelium can produce a tough and waterproof fibrous protein called keratin. When this protein is present, it can increase the toughness of the tissue and help prevent water loss from the underlying tissue. The keratin itself is a waterproof protein that helps create a waterproof surface, making the tissue dry on the apical surface. Later in our course, when we talk about the skin or the integumentary system in more detail, we will revisit this protein keratin.
This leads us to two main types of stratified squamous epithelium. The first main type is keratinized stratified squamous epithelium, which, as its name implies, contains the protein keratin. The second type is non-keratinized stratified squamous epithelium, which does not contain the protein keratin. The keratinized type can be found in the outermost layers of our skin, which covers our entire body. The non-keratinized type is found in orifices near the skin, such as the mouth, esophagus, anus, and vagina. Since the protein keratin is waterproof and creates a dry surface on the apical surface of keratinized stratified squamous epithelium, our skin is usually dry. In contrast, the non-keratinized stratified squamous epithelium tends to have more moist tissue.
Notice our focus on the skin of this person here. You can see a micrograph showing the outermost layers of the skin. The outermost layers of this tissue are going to be flat in their nature. As you go lower into the tissue, deeper into the tissue, the shapes of the cells can vary, from cuboidal to even columnar. Deeper to the epithelial tissue here is the connective tissue, which you can see in this light blue color. The cells on the surface are going to encounter a lot of physical stresses that can kill the cells and rub them off. These cells need to be replaced by dividing cells that are closer to the basal surface. Cells closer to the basal surface are closer to the underlying vascular connective tissue with blood vessels that supply nutrients. As these cells get further from the basal surface, they have less access to nutrients and less capacity to divide. The outermost layers of the stratified squamous epithelial tissue are going to be keratinized in keratinized stratified squamous epithelial tissue. These cells towards the top can be dead in many cases, helping to protect the living epithelial tissue below.
We also have some helpful ID tips when it comes to stratified squamous epithelial tissue. This tissue is relatively thick, with lots of cell layers, and appears relatively thick as you can see in these images. The cells near the apical surface are going to be flat in their nature, encountering all the physical trauma and abrasions. These cells are the ones most likely to be flaking off and falling off or rubbing off, hence they need to be replaced by dividing cells closer to the basal surface. As you go deeper into the tissue closer to the basement membrane, the cell shape can vary from cuboidal to columnar. This concludes our lesson on stratified squamous epithelium, and we'll learn more about other stratified epithelial tissues and apply these concepts as we move forward. I'll see you all in our next video.