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- AuthorDan Washmuth
Dan has taught college Nutrition, Anatomy, and Medical Terminology courses. He has a B.S. in Exercise Physiology from Furman University and a M.S. in Dietetics & Nutrition from Florida International University. He is a Registered Dietitian (RD) and a Certified Exercise Physiologist (EP-C)
View bio - InstructorSarah Phenix
Explore transitional epithelium. Learn the definition of transitional epithelial tissue and understand its characteristics. See transitional epithelium functions.Updated: 11/21/2023
Table of Contents
- What is Transitional Epithelial Tissue?
- Transitional Epithelium Function
- Transitional Epithelium Characteristics
- Transitional Epithelium Location
- Importance of Stratified Transitional Epithelium
- Lesson Summary
Frequently Asked Questions
Where is transitional epithelium found?
Transitional epithelium is found lining the structures of the urinary system. These structures include the ureters (transport urine from kidneys to bladder), urinary bladder (holds urine), and urethra (transports urine form the bladder to outside the body).
What is the main function of transitional epithelium?
The main function of transitional epithelium is to allow tissue to expand and contract. For example, the transitional epithelium lines the inner walls of the urinary bladder, and this tissue allows the bladder to expand when the bladder fills with urine.
What are the features of transitional epithelium?
There are several features of transitional epithelium. This tissue has several layers of cells and the cells contain gap junctions and microvilli. Additionally, the cells are round and pear-shaped, but they become flat and thin when the tissue is stretched.
Table of Contents
- What is Transitional Epithelial Tissue?
- Transitional Epithelium Function
- Transitional Epithelium Characteristics
- Transitional Epithelium Location
- Importance of Stratified Transitional Epithelium
- Lesson Summary
Epithelial tissue is a specific type of tissue that covers and lines the internal and external organs of the body. For example, the tissue that covers the liver, forms the skin, and lines the inner walls of blood vessels is epithelial tissue. The epithelial cells that form the epithelial tissues come in several different shapes, such as flat, square, round, and columnar.
Transitional epithelial tissue is a specific type of epithelial tissue that is comprised of multiple layers of cells rather than a single layer. Epithelial tissue that is comprised of multiple layers of cells is known as stratified epithelium. Transitional epithelium cells are found lining the structures of the urinary system.
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One primary function of transitional epithelial tissue is to allow for the stretching and contracting of the organ or structure it covers or lines. For example, transitional epithelial tissue lines the inner walls of the urinary bladder, which is the organ that stores urine before it is excreted from the body during urination. The hollow portion of the urinary bladder is known as the lumen, and it needs to expand as the bladder fills with urine and contract when a person empties their bladder. Transitional epithelial tissue can expand and stretch, which allows the urinary bladder to expand as it is filled with urine. Additionally, this tissue is also able to contract, which allows the urinary bladder to contract as a person urinates to empty their bladder.
Another function of transitional epithelium is to provide a protective, impermeable layer of the urinary system. For example, the transitional epithelial tissue that lines the urinary bladder does not allow water and other substances to cross through walls of the bladder. In simpler terms, this tissue does not allow the urinary bladder and other structures of the urinary system to leak urine into the rest of the body.
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The characteristics of transitional epithelium include:
- Stratified tissue: Stratified tissue refers to tissue that is comprised of multiple layers of cells, and transitional epithelium is composed of up to six layers of cells.
- Mucus-secreting: Some of the cells of transitional epithelium can secrete mucus, and this mucus helps to protect the tissue from surrounding substances, such as acidic fluids.
- Cells are round and pear-shaped: The cells of transitional epithelium are round and pear-shaped. However, these cells become thin and flattened when the tissue is stretched.
- Gap junctions: The cells of transitional epithelium are connected by gap junctions. Gap junctions are small channels between cells that allow small molecules and ions to be transferred from one cell to another.
- Microvilli: Some of the transitional epithelial cells have microvilli, which are small, finger-like projections that extend out from the cells.
- Avascular: The cells of the transitional epithelium are avascular, which means they do not contain any blood vessels. These cells receive blood from blood vessels that supply nearby cells and tissue with blood.
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Transitional epithelium is located in several parts of the urinary system. These locations include:
- Lining of the urinary bladder
- Lining of the ureters, which are the tubes that transport urine from the kidneys to the urinary bladder
- Lining of the urethra, which is the tube that transports urine from the urinary bladder to the outside of the body
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Stratified transitional epithelium plays a very important role in the functioning and protection of the urinary system. Without the ability of this tissue to stretch and expand, the urinary bladder would not be able to hold large quantities of urine. Therefore, a person would feel the urge to urinate much more frequently.
An example to illustrate this is a water balloon. A water balloon expands as it fills up with water. Without this ability to expand, the water balloon would not be able to hold much water at all. The transitional epithelial tissue of the urinary bladder acts like a water balloon. The ability of the cells of the transitional epithelium to stretch from round cells to thin, flat cells allows the urinary bladder to expand to hold more urine. An empty urinary bladder is quite small, but a typical urinary bladder can usually expand to hold up to 16 ounces (2 cups) of urine. Additionally, the urinary bladder will shrink in size after a person empties their bladder during urination, just as a water balloon shrinks in size when water leaks out of it.
The transitional epithelium also serves to prevent urine from leaking out of the structures of the urinary system. The stratified cells of the transitional epithelium form a layer of tissue that is impermeable and prevents urine from leaking through these cells. This is vitally important since urine leaking out of the urinary system could contaminate the surrounding areas and tissues, possibly leading to infections.
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Epithelial tissue covers and lines internal and external structures and cavities of the body, and transitional epithelial tissue is a specific type of epithelial tissue. Transitional epithelium is made up of multiple layers of cells known as stratified epithelium, and it is found lining the structures of the urinary system, such as the urinary bladder (holds urine), ureters (transport urine from kidneys to bladder), and urethra (transports urine from the bladder to the outside of the body). For example, transitional epithelial cells line the lumen of the urinary bladder, which is the open space within the bladder that fills with urine.
Transitional epithelium functions to stretch to accommodate fluctuating volumes of fluid in the urinary bladder. For example, this tissue will stretch to allow the urinary bladder to expand to hold more urine. The transitional epithelium also functions to form a layer of tissue that is impermeable to fluids and other substances, which prevents urine from leaking out of the urinary system. The cells of the transitional epithelium are round and pear-shaped but become flat and thin when they are stretched. Some of these cells can also secrete mucus, which helps to protect the inner walls of the urinary system. Transitional epithelial cells are connected by gap junctions, and some transitional epithelial cells contain microvilli.
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Video Transcript
What Is Transitional Epithelium?
The term 'transitional epithelium' is actually a really descriptive one and, by taking a moment to understand the term, you will get a really good idea of the basic function of these cells. By 'transitional,' we mean that these cells can undergo a structural change in their shape and composition, while 'epithelium' refers to their location. Epithelial cells are any cells that form a tissue lining of a space, such as the internal lining of an organ, blood vessel, or the outside space of your body; your skin, for instance, is a type of epithelium. Therefore, transitional epithelium cells are a very specific type of cells that line a space and can undergo a change in their shape and structure.
What Makes It Transitional?
Well, transitional epithelial tissue, made up of a stratified, multi-layered, collection of transitional epithelial cells, is a tissue that is capable of stretching and contracting to accommodate fluctuating volumes of fluid passing through the structures they line. Now, this may not seem like a big deal but, because of these cells, your urinary bladder, urethra (the canal urine exits the body through), and ureters (the tubes that bring urine from your kidneys to your bladder for storage) are able to expand.
This means that the tissue of your bladder can stretch to accommodate more fluids, which in turn, reduces the frequency with which you need to go to the bathroom.
Why Is It Stratified?
One reason transitional epithelium is stratified is for protection. Transitional epithelium is composed of about six layers of cells that vary in shape and size. One reason that there are so many layers is that the organs these cells line (the ureters and the urinary bladder) have highly acidic environments. Therefore, stratified tissue (tissue having more than one layer of cells) always has an underlying layer of cells ready to manage the harsh environment when the outer cells lining the lumen of your bladder, the cavity of the organ that fills with fluid, slough off or get damaged. To protect themselves against the acidic environment, these cells secrete a mucous coating and, as such, are referred to as the mucosa , or mucous-producing layer, of the bladder.
Another reason why transitional epithelium is stratified is for expansion, so that the tissue can stretch. When your bladder is empty, the tissue is relaxed, resulting in the interior surface forming many finger-like folds of the muscle and mucosal layer. However, as the bladder begins to fill with urine, its walls expand, and these folds begin to smooth, just like the coils of a spring smooth as you stretch it from its relaxed state.
As the cells of the transitional epithelium tissue stretch, their shape changes from round and globular to thin and flat, and the layers of cells becomes thinner as they stretch over an increasingly wider surface area. Thanks to the elastic capacity of transitional epithelium, as well as the underlying muscular layers, your bladder can stretch from a relaxed state of about two inches long to about five inches and hold anywhere from 16-24 ounces of fluid.
Lesson Summary
Transitional epithelium is a layer of cells that forms the mucosal lining of your ureters, a portion of your urethra, and your urinary bladder. These cells are called transitional because they can undergo a change in their shape and structure. Additionally, the cells are stratified, which means that they create several layers. Stratified tissue has an underlying layer of cells ready to manage the harsh environment when the outer cells lining the lumen of the bladder, or the cavity of the organ that fills with fluid, slough off or get damaged. The stratified cells of transitional epithelium provide protection and allow the vessels to expand to accommodate fluid.
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