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FOCUS: Epithelial Tissue THE FACTS

Overall Structure of the Tissue Epithelial cells are usually close together have little or no “matrix” (intercellular material) often attached to each other with desmosomes and/or tight junctions. always have a free surface Bottom attached to basement membrane

What is a free surface? A surface, which is exposed to the outside, or opens into a space internally. This means that all substances which are absorbed / secreted by the body must pass through some type of epithelium.

Two categories of epithelial tissue External v Internal External epithelium Comes in contact with the external environment Functions: protects the body from bacteria, water loss and mechanical damage (abrasions). May also contain nerve endings that function as sensory receptors (ex: skin). Upper jaw, lower jaw and head of all crocodilians, extending over most surfaces of neck, body, tail and inside legs of subfamily Crocodylinae and Gavialinae.

Internal epithelium It is modified to function in absorption and secretion of nutrients, water and other substances. It may contain nerve endings that act as chemical receptors (ex: tastebuds, nostril linings).

Other characteristics Most epithelium lacks blood vessels, therefore nourishment must diffuse from capillaries located in the connective tissue below the epithelium. Epithelium reproduces rapidly. This allows for the replacement of damaged cells.

Epithelial tissue may contain one or more layers of cells. The free surface may have microvilli or cilia. (see H.O. on Classification of Epi Tissue)

Microvilli Finger-like projections that increase the surface area and allow for greater absorption. Note: especially important in the lining of the digestive tract

The microvilli are coated on the surface with a combination of glycoproteins and glycolipids called glycocalyx. The glycocalyx has several functions. One of them is to make the microvilli slippery. This allows organs to move over each other without damage and substances to move along without causing damage to the surface.

Cilia Free surface may also have cilia. These structures are able to move back and forth. Functions: helps move substances along from cell to cell. Help remove dust / dirt / debris out of the body (ex: lungs, trachea, windpipe & also eggs through oviducts, sperm through seminal tubules).

A magnified image of a trachea covered with allergens A magnified image of a trachea covered with allergens. Stringy parts are cilia and spheres are pollen. (Reproduced by permission of Custom Medical Stock Photo)

Basement membrane The basement membrane is the inner surface (opposite the free surface). The basement membrane is a combination of the following: 1). glycoprotein from the epithelial cells 2). a meshwork of protein and protein fibers (collagen and reticular) produced by the connective tissue below the epithelium Connective tissue

Epithelial layer + Connective layer Joined by basement membrane Epithelial layer + Connective layer Epithelial Basement membrane Connective