Transitional epithelium definition

Transitional epithelium: The morphology of the cells of a transitional epithelium—a type of stratified epithelium made up of many layers—varies depending on the function of the organ. Except for the apical layer, that seems to flatten when stretched, the other layers do not appear to be affected by stretching. The epithelium has a variety of appearances. When relaxed, it appears spherical or cubical. This epithelium is also known as “urothelium” since it almost exclusively covers the urinary system.

Structure of the transitional epithelium

• Transitional epithelium is an epithelial tissue that, when at rest, resembles stratified cuboidal epithelium.
• The pear-or round-shaped cells of the transitional epithelium flatten down as the tissue is stretched, generating the look of stratified squamous epithelium.
• According to the extent of extension, the cells in the apical layer flatten, whereas cells in the base layer have a cuboidal or columnar appearance.
• Three groups of cells make up the layers of the epithelium.
• The lowest layer is the basal layer, which is directly connected to the basement membrane. This membrane is responsible for feeding the continuing cell regeneration of the higher layers.
• The cytoplasm of basal layer cells is abundant with tonofibril-forming proteins, which converge with hemidesmosomes to establish a strong bond with the basement membrane.
• Since the basal layer cells need more energy for epithelial renewal, they have a high concentration of mitochondria.
• When existing cells are injured or damaged, the highly proliferative, rapidly dividing cells that make up the middle layer—also known as the intermediate layer—can regenerate quickly.
• Numerous cells in this layer include the Golgi apparatus, which helps transport proteins like keratin to the surface layers.
• High keratinization levels in the superficial layer’s cells serve as a protection from water and salts.
• The apical layer, also known as the superficial layer, borders the lumen and protects the underneath layer of cells from potentially hazardous pathogens and waste products.
• Certain superficial skin cells feature mucus-coated microvilli and are covered in mucus.
• Desmosomes and gap junctions are used to link the cells in the epithelium. These structural components enable the extension of the epithelium, but they also make the cells more delicate.
• The transitional epithelium lacks blood vessels and is avascular like all other epithelial tissues. The blood vessels of the surrounding connective tissues provide oxygen, nutrition, and excrement for the cells in this epithelium.
• However, each cell has its own nerve supply.

Functions of the transitional epithelium

The transitional epithelium serves two basic purposes, which are determined by the composition and structure of the cell:

1. Permeability barrier

• The tissue offers high impermeability to water and other molecules because of the significant keratin deposits present in the cells.
• Even when the cells are fully stretched, desiccation is prevented by the tissue cells’ great resistance to osmotic pressure.
• Additionally, the bloodstream is shielded from chemicals and toxins returning.
• The urinary system is a well-known example of this function, where the urothelium cells do not become dehydrated even in the presence of hypertonic urine in the lumen.

2. Volume control

• This epithelium’s capacity to stretch when fluid pressure rises is its second crucial role in allowing the organs to alter their shape and grow in size.
• When the urinary bladder and ureters fill with more fluid, the cells in the outermost layer of the excretory system expand and become flattened.
• Stretching the organs increases their volume while protecting the underneath tissue from the poisons in the urine.

Location and examples

• The urothelium is the most well-known type of transitional epithelium.
• The transitional epithelium, also known as urothelium, lines the urethra, ureters, and urine bladder.
• Similar to this, the transitional epithelium that lines the prostatic urethra in the male reproductive system is likewise continuous with the urothelium of the urine bladder.

References and Sources

• Mescher AL (2016). Basic Histology. Fourteenth Edition. McGraw-Hill Education.
• Tortora GJ and Derrickson B (2017). Principles of Physiology and Anatomy. Fifteenth Edition. John Wiley & Sons, Inc.
• Waugh A and Grant A. (2004) Anatomy and Physiology. Ninth Edition. Churchill Livingstone.
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