Transitional Epithelium - Structure, Functions, and Examples

Transitional Epithelium: Epithelium is a type of tissue that covers all internal and external surfaces of the body, lining body cavities and organs, and is the primary tissue in glands. Depending on its location in the body, epithelial tissue serves various functions such as protection, secretion, and absorption.

Transitional epithelium, a type of stratified epithelium, can change its shape when stretched (stretchable epithelium). It typically appears cuboidal when not stretched and squamous when stretched. NEET Biology Notes on Transitional Epithelium tissue structure, functions, location and more are provided in the article below.

Transitional Epithelium Definition

Transitional epithelium is a type of layered tissue composed of multiple cell layers. These cells have the ability to alter their shape based on the stretching of the organ they line. When the organ fills with fluid, the top layer of cells in this epithelium can stretch out and appear flattened.

Conversely, when fluid pressure is low, these cells can take on a cuboidal shape with rounded edges. This type of epithelium is located in the lining of the urinary bladder, ureters, and urethra, as well as in the ducts of the prostate gland.

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Transitional Epithelium Location

Transitional epithelium, also known as urothelium, is a type of layered tissue that covers much of the urinary tract. It is specifically located in:

• Urinary Bladder: I t lines the bladder walls, enabling them to stretch and expand as urine fills the bladder.
• Ureters: These narrow tubes that carry urine from the kidneys to the bladder are also coated with transitional epithelium.
• Renal Pelvis: Found in the kidney, the renal pelvis is lined with this type of epithelium.
• Urethra: Transitional epithelium extends into the proximal part of the urethra.
• Prostate Gland: The ducts within the prostate gland are similarly lined with transitional epithelium.

This type of epithelium is notable for its ability to change shape depending on the volume of fluid in the organ. When the organ is distended with fluid, the surface cells of this epithelium can stretch and appear flattened. Conversely, when fluid pressure is low, these cells can adopt a more cuboidal shape with rounded edges. This elasticity allows the organs to expand without exposing underlying tissues to harmful substances present in urine.

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Transitional Epithelium Structure

The structure of transitional epithelium is distinctive and enables it to function effectively. Here are its key characteristics:

1. Layers: Transitional epithelium consists of 3-4 layers of cells. The number of layers can vary depending on how stretched the organ is.

2. Basal Layer: The bottom layer, or basal layer, stays in contact with the basement membrane. Cells in this layer are attached to the underlying lamina propria using tonofilaments and hemi-desmosomes. These cells are less specialized and provide support to the upper layers.

4. Intermediate Layer: Cells in the intermediate layers actively divide and can replace cells lost due to wear or infection. They also contain a large Golgi network with numerous membrane-bound vesicles.
5. Superficial Layer: The top layer of cells can change shape from cuboidal to flattened when the organ is stretched. These cells have microvilli , which are cytoplasmic projections based on actin. The apical plasma membrane of these cells is covered by a network of hexameric plaques.

This unique structure allows transitional epithelium to stretch and contract, making it crucial for organs such as the urinary bladder, ureters, and renal pelvis. Additionally, it serves as an effective barrier, blocking the passage of water and most small molecules.

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Transitional Epithelium Histology

Transitional epithelium, also known as urothelium, is a special type of stratified epithelium found in the lower urinary tract. Its unique characteristic allows it to change thickness rapidly to accommodate stretching and contraction.

When the bladder or urinary tract is in a relaxed state (not stretched), the surface of the transitional epithelium is covered with a variety of cells, including large dome-shaped cells called umbrella cells. These cells are named for their role in protecting the underlying layers of epithelial cells. Active umbrella cells are found at the surface layer facing the urinary space.

During stretching (when the bladder fills), the transitional epithelium becomes thinner, and the umbrella cells stretch out and flatten.

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Transitional Epithelium Characteristics

Transitional epithelium, also known as urothelium, is a type of stratified epithelium that lines the urinary tract. It is specially adapted to endure the frequent stretching and relaxation that occurs as the urinary organs fill with urine and then empty. The following are the characteristics of Transitional Epithelium:

• Stratified: Composed of multiple layers of cells, typically 3-4 layers thick.
• Change in superficial cell shape: The cells in the outermost layer can alter their shape depending on whether the organ is stretched or relaxed. When relaxed, these cells appear cuboidal or rounded. When the organ fills with urine and stretches, these cells flatten out, resembling squamous cells.
• Allows for stretching: This ability of the superficial cells to change shape enables the transitional epithelium to expand and accommodate a significant volume of urine without tearing.
• Forms a tight barrier: Tight junctions between the superficial cells create an impermeable barrier that prevents urine from leaking into surrounding tissues.
• Provides protection: Transitional epithelium serves to shield underlying tissues from the potential harmful effects of urine, which may contain toxins and irritants.
• Regeneration: Stem cells in the basal layer of transitional epithelium continually divide to generate new cells, facilitating repair of the epithelium in case of damage.

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Transitional Epithelium Functions

Transitional epithelium, also known as urothelium, is a specialized type of tissue lining found in the urinary system. Its main roles include:

1. Stretchability and Distensibility: Transitional epithelium can stretch significantly to accommodate changes in the volume of organs like the urinary bladder and ureters as they fill with urine. This elasticity allows the urinary system to store urine effectively and then return to its original shape after emptying.
2. Protection: It acts as a barrier against the potentially harmful effects of urine, which contains waste products and can be acidic. This prevents these substances from damaging the underlying tissues.
3. Prevention of Leakage: The multiple layers of cells in transitional epithelium help prevent urine and ions from diffusing through the tissue, thereby avoiding leakage into surrounding tissues that could cause infection or injury.
4. Facilitation of Urine Flow: Transitional epithelium has a specialized surface, including umbrella cells that flatten and expand when the bladder is full. This adaptation facilitates the smooth passage of urine.
5. Barrier Function: Similar to other epithelial tissues, transitional epithelium forms tight junctions between cells that maintain a strong barrier, preventing pathogens and toxins from entering the underlying tissues.
6. Secretion and Absorption: It secretes substances to form a protective mucus layer on its surface and absorbs water and ions from urine. This helps in concentrating urine and maintaining the body's fluid balance.

Overall, transitional epithelium plays critical roles in ensuring the urinary system functions properly by efficiently storing, expelling, and processing urine, all while safeguarding the underlying tissues from urine's potentially harmful components.

Transitional Epithelium Examples

Transitional epithelium is a type of stratified tissue composed of multiple layers of cells. These cells can change their shape depending on the expansion of the organ. When the organ fills with fluid, the cells in the top layer of this epithelium can stretch and flatten. Conversely, they can also appear cuboidal and rounded when fluid pressure is low. The following are the some examples of where transitional epithelium can be found:

1. Urinary System: The most common type of transitional epithelium is found in the urothelium, which lines the urethra, ureters, and urinary bladder. This epithelium is predominantly located in the urinary system, hence it's also referred to as "urothelium". In the urinary system, the volume and concentration of urine depend on various factors.
2. Male Reproductive Tract: Transitional epithelium also lines the prostatic urethra of the male reproductive system, connecting with the urothelium of the urinary bladder.

The main functions of this tissue include serving as an effective barrier that is impermeable to water and most small molecules, and allowing organs to stretch and expand based on fluid pressure.

For example , during the expulsion of excess water from the body, large amounts of liquid pass through the ureters, urinary bladder, and urethra. The ability of the superficial layer cells of this epithelium to change shape—from a rounded cuboidal shape to a flattened squamous structure—enables these organs to stretch without exposing underlying tissues to toxic substances in the urine.

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