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Epithelium covers surfaces, lines body cavities or forms solid glands. It is formed by layers of tightly linked cells that are attached to a basement membrane. The basement membrane is a fibrous extracellular matrix which separates the epithelial cells from the underlying tissue. The basement membrane provides metabolic support, and regulates growth, differentiation and permeability. The basal surface of the epithelial cell is closest to the basement membrane.
Epithelium has a specialised structure for specific functions. It can act as a barrier, allow absorption (gas exchange in the lung, absorption of nutrients in the gut etc.), and secrete substances such as mucus or hormones.
Epithelium sub-types: structure matches function
Epithelium sub-types: structure matches function
1. Simple epithelium
1. Simple epithelium
Single layer of cells, all of which are in contact with the basement membrane
Single layer of cells, all of which are in contact with the basement membrane
A. Simple squamous epithelium
Structure: Flattened cells
Function
Barrier between environments
Exchange between environments
Examples: lining blood vessels, body cavities, alveoli in lungs etc.
B. Simple columnar epithelium
Structure: Tall cells, may have cilia or microvilli
Function:
Barrier between environments
Secretion of substances
Absorption
Examples: Lining gut, uterine tubes etc.
C. Simple cuboidal epithelium
Structure: Square cells
Function:
Barrier between environments
Secretion of substances
Absorption
Examples: Lining small ducts etc.
2. Stratified squamous epithelium
2. Stratified squamous epithelium
Several layer of cells, only the lowest layer of which is in contact with the basement membrane
Several layer of cells, only the lowest layer of which is in contact with the basement membrane
Structure: Flattened cells, multiple layers
Function:
Barrier between environments
Protection
Examples: Skin etc.
3. Pseudostratified columnar epithelium
3. Pseudostratified columnar epithelium
Appear to be arranged in multiple layers but all cells are in contact with the basement membrane
Appear to be arranged in multiple layers but all cells are in contact with the basement membrane
Structure: Tall cells, multiple layers but all contact BM
Function:
Barrier between environments
Secretion or absorption
Examples: Respiratory tract etc.
4. Transitional epithelium
4. Transitional epithelium
This epithelium is able to change its characteristic shape based upon its environment. Multiple layers of cells with only the lowest layer in contact with the basement membrane. The surface layer of epithelial cells can change from a domed shape to a flattened shape.
This epithelium is able to change its characteristic shape based upon its environment. Multiple layers of cells with only the lowest layer in contact with the basement membrane. The surface layer of epithelial cells can change from a domed shape to a flattened shape.
Structure: Adaptable (domed/flattened)
Function:
Barrier between environments
Allow filling
Examples: Lining bladder etc.
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