Biology: Epithelium Characteristics

Study Notes

Epithelium is a tissue that grows upon another tissue, composed of two words: "Epi" meaning "upon" and "Thelia" meaning "growth".

Epithelial cells are compactly packed with little intercellular matrix.
Due to limited intercellular spaces, blood vessels, lymph vessels, and capillaries cannot penetrate the tissue, and thus, blood circulation is absent.
Cells rely on the underlying connective tissue for their nutrients.

A thin, non-living, non-cellular basement membrane is present between the epithelium and connective tissue.
The basement membrane is highly permeable and secreted by both epithelium and connective tissue.
It is composed of glycoproteins, mucopolysaccharides, and protein fibers.

The tissue has a free surface that faces either a body fluid or the outside environment, providing a covering or lining for some part of the body.
All cells in the epithelium are held together with little intercellular material.
Specialized junctions provide both structural and functional links between individual cells in nearly all animal tissues.

Tight Junctions: Prevent leakage of substances across a tissue.
Adhering Junctions: Hold neighboring cells together by "cementing" them.
Gap Junctions: Allow cells to communicate with each other by connecting the cytoplasm of adjacent cells, enabling rapid transfer of ions, small molecules, and sometimes large molecules.
Interdigitation: Characterized by finger-like processes of plasma membrane that enter into the cytoplasm of adjacent cells.