Epithelium - Structure and Functions

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Epithelium

• One of the four major tissue types of the human body.

Key principles of epithelial tissues:

• Epithelial tissues comprise sheets of cells that are connected via intercellular junctions
• Epithelial tissues are polar:
  The apical surface is "free," meaning it faces the external environment or the lumen of an internal passageway
  The basal surface attaches to the underlying basal membrane, which is a type of connective tissue
• Some general functions of epithelial tissues include:
  • Protection of underlying tissues from damage due to abrasion and/or dehydration
  • Secretion
  • Absorption

4 subcategories of epithelial tissue types

Named according to their shape at the apical surface:

• Squamous epithelium comprises flattened cells; "squamous" means scaly.
• Cuboidal epithelium comprises cube-shaped cells.
• Columnar epithelium comprises cells that are taller than they are wide; in other words, column-shaped.
• Transitional comprises irregularly shaped cells that can alter, or "transition," between flat and round.

Another dimension of epithelial nomenclature is the number of cell layers:

• Simple = single layer
• Stratified = multiple layers

Locations and Functions:

Simple Squamous:

• Thin shape facilitates rapid diffusion and filtration across surfaces
• Located in the alveolar sacs of the lungs, where gas exchange occurs between the blood and the air, and in the linings of the cardiovascular system (where it is called endothelium).

Stratified squamous epithelia:

• Keratinized = surface layers of dead, anucleic cells that contain the protein keratin.
  • This thick, toughened layer provides extra protection in areas, such as the palms of the hands, where abrasion is most common.
• Non-keratinized = No toughened layer of keratinized cells.
• Stratified squamous epithelium protects the underlying tissues from abrasion, dehydration, and penetration of foreign materials
• Located in epidermis of the skin and in the linings of the gastrointestinal tract, anus, and vagina.

Simple Cuboidal

• Comprises a single layer of block-like cells.
• Nuclei are located near the center of each cell, and line up in a row across the cells.
• Can have microvilli on the apical surface; microvilli are microscopic cytoplasmic extensions that increase the surface area for reabsorption and secretion of substances.
• Particularly important in the kidney tubules, where nutrients, ions, water, and other substances are absorbed from and secreted as part of urine formation, and, in the ducts and secretory regions of many glands.

Stratified cuboid epithelium

• Looks like stacked blocks; nuclei are centrally located.
• Stratified cuboid epithelium provide protection of underlying tissues and secretion of substances.
• It is located in the large ducts of some exocrine glands, such as the salivary glands.

Simple columnar epithelium

• Comprises mucus-secreting goblet cells and epithelial cells that are taller than they are wide.
• Nuclei of columnar epithelia are centrally located, ovoid in shape.
• Some simple columnar cells have microvilli, which increase the surface area for absorption and secretion in much of the digestive tract.
• Some columnar epithelia have cilia, which are hair-like extensions that "sweep" materials along passageways.
• Ciliated simple columnar epithelia is present in the large bronchioles, where it continuously removes foreign matter from the lower respiratory tract, and in the uterine tubes, where it moves ova away from the ovaries and towards the uterus

Stratified columnar epithelium

• Comprises more than one layer of cells; the apical layer has the truly column-like cells.
• Provides protection and secretion in the large ducts of salivary glands, in the membranous portion of the male urethra, and, in the lining of the uterus.

Pseudostratified columnar epithelium

• Appears (at first) as if it comprises multiple layers, but, upon closer inspection, does not.
• Cells fit together like puzzle pieces, and their nuclei are at various distances from the basal membrane.
• Provides absorptive and secretive surfaces.
• Is often ciliated and has mucus-secreting goblet cells; the mucus traps foreign materials, and the cilia sweep it along passageways, such as in the nasal cavities, larynx, and bronchi.
• Non-ciliated pseudostratified epithelium is rarer; it provides protection in parts of the male urogenital tract.

Transitional epithelium

• Comprises multiple layers of irregularly shaped cells, some of which are multi-nucleated.
• Is located within the ureters and urinary bladder, where its cells stretch and retract in response to changes in urine volume.

Full-Length Text

• Here we will learn about epithelium, which is one of the four major tissue types of the human body.

• To begin, start a table to denote some key principles of epithelial tissues.
  • Epithelial tissues comprise sheets of cells that are connected via intercellular junctions.

• Denote that epithelial tissues are polar:
  • The apical surface is "free," meaning it faces the external environment or the lumen of an internal passageway.
  • The basal surface attaches to the underlying basal membrane, which is a type of connective tissue.

• Some general functions of epithelial tissues include:
  • Protection of underlying tissues from damage due to abrasion and/or dehydration;
  • Secretion;
  • Absorption;

• Denote that in this tutorial we will learn 4 subcategories of epithelial tissue types, which are named according to their shape at the apical surface:
  • Squamous epithelium comprises flattened cells; "squamous" means scaly.
  • Cuboidal epithelium comprises cube-shaped cells;
  • Columnar epithelium comprises cells that are taller than they are wide; in other words, column-shaped; And,
  • Transitional, which comprises irregularly shaped cells that can alter, or "transition," between flat and round.

• Another dimension of epithelial nomenclature is the number of cell layers: if the epithelial tissue comprises just a single layer, it is "simple";

• If it comprises multiple layers, it is "stratified."

Be aware that there are multiple ways to categorize epithelial tissues, and the classification system one uses often depends on the field; in this tutorial, we'll use a classification system used by histologists.

Now, we'll draw the tissues types and learn some of their key functions and locations.

Let's begin with simple squamous epithelium.

• First, draw a basement membrane and a single row of flattened cells; notice that their nuclei create bulges; under a microscope, they look like fried eggs.

• Write that their thin shape facilitates rapid diffusion and filtration across surfaces;

• Thus, simple squamous epithelium is located in the alveolar sacs of the lungs, where gas exchange occurs between the blood and the air, and, in the linings of the cardiovascular system (where it is called endothelium).

• To show stratified squamous epithelium, draw a basement membrane and multiple layers of flattened cells.

• Then, indicate that there are two types of stratified squamous epithelia:
  • Keratinized, which has surface layers of dead, anucleic cells that contain the protein keratin; this thick, toughened layer provides extra protection in areas, such as the palms of the hands, where abrasion is most common.
  • Non-keratinized, which does not have the toughened layer of keratinized cells.

• Write that stratified squamous epithelium protects the underlying tissues from abrasion, dehydration, and penetration of foreign materials;

• Hence, the epidermis of the skin comprises stratified squamous epithelium, as do the linings of the gastrointestinal tract, anus, and vagina.

• Next, we'll show simple and stratified cuboidal epithelium.

• Show that simple cuboidal epithelium comprises a single layer of block-like cells; notice that their nuclei are located near the center of each cell, and line up in a row across the cells.

• Then, show that, in some locations, simple cuboidal epithelium has microvilli on the apical surface;

• Write that microvilli are microscopic cytoplasmic extensions that increase the surface area for reabsorption and secretion of substances.

• This is particularly important in the kidney tubules, where nutrients, ions, water, and other substances are absorbed from and secreted as part of urine formation, and, in the ducts and secretory regions of many glands.

• Then, that show stratified cuboid epithelium looks like stacked blocks; again, notice that their nuclei are centrally located.
  • Stratified cuboid epithelium provide protection of underlying tissues and secretion of substances;
  • It is located in the large ducts of some exocrine glands, such as the salivary glands.

Next, we'll draw simple, stratified, and pseudostratified columnar epithelia.

• First, draw a section of simple columnar epithelium, which comprises mucus-secreting goblet cells and epithelial cells that are taller than they are wide.

• Notice that the nuclei of columnar epithelia are centrally located, as we saw in the cuboid epithelia, but that they are move ovoid in shape.

• Indicate that some simple columnar cells have microvilli, which increase the surface area for absorption and secretion in much of the digestive tract.

• Show some columnar epithelia have cilia, which are hair-like extensions that "sweep" materials along passageways; thus, ciliated simple columnar epithelia is present in the large bronchioles, where it continuously removes foreign matter from the lower respiratory tract, and, In the uterine tubes, where it moves ova away from the ovaries and towards the uterus.

• Now, show that stratified columnar epithelium comprises more than one layer of cells; notice that it's the apical layer that has the truly column-like cells.
  • Stratified columnar epithelium provides protection and secretion in the large ducts of salivary glands, in the membranous portion of the male urethra, and, in the lining of the uterus.

Next, let's take a look at pseudostratified columnar epithelium; as its name implies, it appears (at first) as if it comprises multiple layers, but, upon closer inspection, does not.

• To show this, draw a basement membrane and a few cells that are wider at the basal surface than they are at the apical surface;

• Then, between these cells, draw new cells that are narrower at their basal surface;

• Notice that the cells fit together like puzzle pieces, and their nuclei are at various distances from the basal membrane;

• Write that pseudostratified epithelium provides absorptive and secretive surfaces;
  • For example, show that pseudostratified epithelium is often ciliated and has mucus-secreting goblet cells.
  • The mucus traps foreign materials, and the cilia sweep it along passageways, such as in the nasal cavities, larynx, and bronchi.
  • Non-ciliated pseudostratified epithelium is rarer; it provides protection in parts of the male urogenital tract.

Finally, let's show transitional epithelium.

• Draw a basement membrane and multiple layers of irregularly shaped cells, some of which are multi-nucleated.

• Transitional epithelium is located within the ureters and urinary bladder, where its cells stretch and retract in response to changes in urine volume.