So it may be a little bit of a surprise to you that the second type of stratified epithelial tissue in our lesson is transitional epithelium because it is an exception to our structural naming system from our previous lesson video. Transitional epithelium is the only stratified epithelial tissue that does not have the term "stratified" in its name. But again, it is a stratified tissue, which means that it's made of multiple layers of cells where all of the cells do not come into direct contact with the underlying basement membrane. We're covering transitional epithelium at this point in our lesson because structurally and functionally it is similar to stratified squamous epithelium, which we covered in our last lesson video. We'll be able to point out some of those similarities in this video shortly.

Transitional epithelium is called transitional because its cells have the ability to transition in shape depending on the conditions. More specifically, transitional epithelial tissue cells have the ability to transition from a cuboidal or a cube shape, as you can see here in this region of our diagram up above, to a squamous or a squished flat shape, as you can see down below in this part of our diagram. This transition from a cuboidal shape to a squamous shape occurs when the tissue is stretched. A notable characteristic of transitional epithelium is its elasticity, or its ability to be stretched but then return back to its original shape after being stretched. The structure of the tissue and the cells that make up the tissue determine its function.

Transitional epithelium is a type of stratified tissue with many layers of cells. Like stratified squamous tissue from our last lesson video, because it's made of many layers of cells, this tissue is relatively thick. As a relatively thick tissue, its primary function is going to be protection. Transitional epithelium provides protection by blocking the diffusion of harmful molecules.

Transitional epithelium is found only in the urinary system. We can find transitional epithelium lining the bladder, which stores urine until it needs to be excreted. The bladder needs to have the ability to stretch when it is full and then return back to its original shape when it is not full. We can also find transitional epithelium in the ureters, which are tubes that carry urine from the kidneys to the bladder, and in parts of the urethra that are closer to the bladder, which is the tube that allows for the excretion of urine from the body.

Sometimes, transitional epithelium is referred to as urothelium. It provides protection by blocking the diffusion of harmful molecules in the urine so that those molecules cannot diffuse back into the internal regions of our body and cause us harm. In our image, we've divided it into two sections separated by a dotted line. There's a top half showing an empty bladder and a bottom half showing a full bladder. In the empty form of the bladder, the transitional epithelium is in a relaxed state, and the cells are in a cuboidal shape, as seen in the diagram. In the micrograph shown, the inside of the bladder is depicted, revealing the multiple layers of cells characteristic of this stratified epithelial tissue.

When the bladder is filled with urine, the transitional epithelial tissue cells transition from a cuboidal shape to a squamous shape. This happens because the pressure of the urine stretches the tissue, allowing the cells to widen and flatten. Tight junctions and desmosomes ensure that the cells remain connected. On the apical surface of the empty bladder version of the transitional epithelia, the cells appear pillow-shaped, indicating their larger size and curved shape. This analogy helps illustrate the stretching dynamics, as shown by arrows in the diagram.

Transitional epithelia have more layers of cells than stratified cuboidal epithelia, which we will cover in our next video, but fewer layers than stratified squamous epithelia. Surface cells on the apical surface of transitional epithelia in its relaxed state are pillow-shaped. This lesson concludes our discussion on transitional epithelium, and moving forward we will apply these concepts in practice problems and learn about other stratified epithelial tissues in our next video. I'll see you all then.