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Histology Fact Sheet: Epithelium

 

Location of Epithelium

Epithelum is found throughout the body. Epithelium covers body surfaces, lines body cavities, and lines hollow organs.

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Functions of Epithelium

Protection, absorption, secretion, excretion, filtration, and diffusion are all functions of epithelium.

Histology of Epithelium

When looking at the histology of epithelium, it can be seen that the cells in epithelial tissue are tightly packed.  There is very little intercellular space between the cells in epithelial tissue.

Since epithelium forms surface coverings and linings, there is one free surface that is not in contact with other cells. On the other side of the free surface, there is a basement membrane.  The basement membrane is non-cellular.  It is composed of carbohydrates and proteins which are secreted by the epithelial cells and connective tissue cells.

Histology hint from Sarah Bellham: The "free surface" on epithelum can be used as an aid in idenfying epithelium on a histology slide.

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Classification of Epithelium

Epithelium is classified by the shape of the cells and the number of cell layers.

Epithelial cells may be flat, cuboidal, or columnar. If the cells are flat, the epithelium is classified as squamous. If the cells are as tall as they are wide, it is called cuboidal. If the cells are taller than they are wide, the epithelium is classified as columnar.

Epithelial cells may be arranged in a single layer or stacked upon one another in a multiple layers. Epithelium that is one cell thick is classified as simple. Epithelium that is more than one cell layer thick is classified as stratified.

histology of epithelium
llustration of the histology of epithelium

Histology of Simple Squamous Epithelium

Simple squamous epithelium is a single layer of flat cells.

Simple squamous epithelium is "simple" because it is one cell thick. "Squamous" refers to the fact that the cells are flat.

Endothelium is a type of simple squamous epithelium which lines blood vessels.  Mesothelium is a type of simple squamous epithelium which lines the body cavities.

Histology of Simple Cuboidal Epithelium

Simple cuboidal epithelium has cells which are as tall as they are deep and wide. Simple cuboidal epithelium is "simple" because it is one cell thick. "Cuboidal" refers to the shape of the cells.

The lining of most ducts is simple cuboidal epithelium. The kidney tubules are simple cuboidal epithelium.

Histology of Simple Columnar Epithelium

In simple columnar epithelium, the height of the cell is greater than the width and depth of the cell.  Simple columnar epithelium is "simple" because it is one cell thick. "Columnar" cells are taller than they are wide and have an oval nucleus.

The lining of the gastrointestinal tract is simple columnar epithelium. Goblet cells are associated with simple columnar epithelium of the gastrointestinal tract.

The mucosa of the gallbladder is made of simple columnar epithelium.

Histology hint from Sarah Bellham: the simple columnar epithelium of the gallbladder is very tall!

Histology of Stratified Squamous Epithelium

Stratified squamous epithelium is "stratified" because it is more than one cell layer thick. "Squamous" refers to the fact that the surface cells of the stratified squamous layer are flat. 

Histology hint from Sarah Bellham: In stratified squamous epithelium, the cells at the basal layer are cuboidal or even columnar. However, the epithelium is still classified as "squamous" based on the cells of the surface layer.

The epidermis of the skin is stratified squamous epithelium. The lining of the esophagus is stratified squamous epithelium. The cornea is covered by a non-keratinized stratified squamous epithelium.

Histology of Pseudostratified Columnar Epithelium

Pseudostratified columnar epithelium is a single layer of cells which looks like it is stratified. Pseudostratified squamous epithelium is "pseudostratified" because it is only one cell layer thick, yet it appears to be stratified. In reality, every cell touches the basement membrane.

Histology hint from Sarah Bellham: The prefix "pseudo" is of Greek origin and it means false or counterfeit. For example: pseudonym, pseudo-science or pseudostratified.

Pseudostratified columnar epithelium lines the trachea and respiratory tract as well as some of the male reproductive tract.

Histology of Transitional Epithelium

Transitional epithelium has domed shape cells on the apical surface.  It can be distended or stretched.

Transitional epithelium is found in the bladder and urinary tract.

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Epithelial Modifications

Keratinization

Keratinization is seen in the epidermis. Cells in the stratum corneum are essentially just bags of keratin.

Microvilli

Microvilli are finger like projections seen on the surface of some cells. Microvilli form the brush border (striated border).

Stereocilia

Stereocilia are very long microvilli. Stereocilia are seen in the epididymis and the hair cells of the ear.

Cilia

Cilia is the hair like surface modification seen on some epithelia. Cilia are made of microtubules.

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Glands

A simple gland has an unbranched duct. A compound gland has a branched duct.

If the secretory portion of a gland is shaped like a tube, it is called "tubular". If the secretory portion of a gland is shaped like a flask, it is called "alveolar" or "acinar". If the secretory portion of a gland is shape like a tube but at the end it is shaped like a flask, it is called "tubuloalveolar".

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Epitheliod Tissue

If a tissue is composed of tightly packed epithelial-like cells, but it does not have a free surface, the tissue is called epithelioid tissue. An example of epithelioid tissue is the parenchyma of the adrenal gland.

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Sources:

Histology: A Text and Atlas
Michael H. Ross/Edward J. Reith

http://www.training.seer.cancer.gov; funded by the U.S. National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program, via contract number N01-CN-67006, with Emory University, Atlanta SEER Cancer Registry, Atlanta, Georgia, U.S.A.