Chapter 2 - Epithelium
Epithelium forms continuous sheets of cells that line internal surfaces and cover the external surface of the body. It is a selective barrier that protects tissues and is often involved in absorption or secretion. A basement membrane separates an epithelium from the underlying connective tissue.
Epithelia are classified based on three criteria:
- Number of cell layers ( simple or compound )
- Shape of surface cells ( squamous , cuboidal or columnar )
- Specializations ( cilia , keratin or goblet cells )
Epithelial cells are polarized:
-
Apical surface
- faces the lumen or the external environment
- Microvilli, cilia, stereocilia
-
Lateral surface
- faces the sides of adjacent cells
- Tight junctions (zonula occludens), adherens junction (zonula adherens), desmosomes (macula adherens), gap junctions
-
Basal surface
- attaches to the basement membrane
- Basement membrane, hemidesmosomes
An epithelium does not contain blood vessels and receives nourishment via diffusion from the underlying connective tissue.
Glands are formed by the down growth of an epithelium into the underlying connective tissue (discussed in Chapter 12 - Exocrine Glands ).
It is not necessary to learn the names of specific tissues for this chapter, but rather learn to recognize variations in epithelia.
Simple Squamous Epithelium
Simple squamous epithelium consists of a single layer of flattened cells. The thinness of these cells facilitates the transfer of materials ( e.g. , gases, fluids or nutrients) across the epithelium.
Simple Cuboidal Epithelium
Simple cuboidal epithelium consists of a single layer of cuboidal cells. This epithelium is often associated with absorption, secretion, or excretion of waste matter.
Simple Columnar Epithelium
Simple columnar epithelium consists of a single layer of cells that are taller than they are wide. This epithelium is often associated with absorption or secretion.
Pseudostratified Columnar Epithelium
Pseudostratified epithelium appears to be stratified because the nuclei of the epithelial cells are at different levels. However, every cell is in contact with the basement membrane, but not all cells reach the lumen.
Stratified Squamous Epithelium
Stratified squamous epithelium has multiple layers of cells becoming flattened as they move from the basal layer to the apical layers. It provides protection from abrasion and is keratinized on the external surface of the body.
Stratified Cuboidal Epithelium
Stratified cuboidal epithelium is a relatively rare type of epithelium consisting of multiple layers of cuboidal cells. It's found in only a few locations in the body, such as the larger ducts of glands, parts of the male urethra, and some areas of the conjunctiva of the eye.
Stratified Columnar Epithelium
Stratified columnar epithelium is a relatively rare type of epithelium consisting of multiple layers of cells, where the outermost layer is columnar in shape. The other layers are typically cuboidal to polyhedral. It's found in only a few locations in the body, such as larger ducts, small areas of the conjunctiva (eye), parts of the pharynx, larynx, and male urethra.
Transitional Epithelium
Transitional epithelium is a specialized type of stratified epithelium that has the unique ability to stretch and change shape. The specialized surface cells are much larger, dome-shaped cells ( umbrella cells ) that can flatten out when stretched. It is found exclusively in the urinary tract.
This epithelium is sometimes called "urothelium."