EPITHELIAL TISSUE: Classification & Function

~Epithelial tissue is characterized according to the shape of its cells and number of layers.

~Squamous cells are thin, flat cells that look like fish scales and are found in the tissue that forms the surface of the skin, the lining of the hollow organs of the body, and the lining of the respiratory and digestive tracts. if the cells are in a single layer, the tissue is simple squamous epithelium. If the cells are in multiple layers, the tissue is stratified squamous epithelium.

~Cuboidal epithellal cells are cube-shaped. In single layers, a tissue like this is known as simple cuboidal epithelium. In multiple layers, stratified cuboidal epithelium.

~Cells shaped like rectangular columns are columnar cells, In a single-cell layer it is simple columnar epithelium, and in multiple layers, stratified columnar epithelium.

~Aside from normal intracellular tasks, epithelial cells are capable of specialized activities that contribute to the function of a particular organ of which they are a part. In many cases, the function of the epithelial cell reflects the shape of the cell. Some of the functions carried out by epithelial tissue include:

• Protection against abrasion, ultraviolet light, etc.

• Filtration or diffusion.

• Absorption of material into a cell.

• Secretion of material to the outside of the cell.

• Sweeping material away by ciliary action.

• Flexibility, ability to withstand variable pressures and volumes.

• Reception of sensory stimuli, such as touch and pressure.

~Consider the function of protection. What surfaces of the body require protection against abrasion,

"weathering," wear-and-tear forces, and the like? Certainly the skin, for starters. And what is the epithelium lining the skin? Stratified squamous epithelium. Other surfaces requiring the services of this kind of epithelium include the mouth, anal canal, and vagina.

~Simple squamous epithelium, on the other hand, cannot handle any trauma at all, and lines surfaces through which substances of molecular dimensions shuttle, as in filtration. It is found in such places as blood vessels and lymph vessels (endothelium), the filtering capsules and certain tubules of the kidney, and lining of closed cavities (mesothelium). The thinner the lining, the more improbable it is that protection of a wall or organ is ane of its functions.

~Cuboidal and columnar epithelial tissue is often single-layered and involved in the secretion and absorption of substances, as in the lining of the gastrointestinal tract, glands, and other places.

~The respiratory structures the trachea and related tubes, nasal cavity and such-require a multifunctional epithelium such as pseudostratified columnar epithelium, a single layer of cells that secretes mucous to trap foreign particles and has motile cilia (tiny hair-like structures) on its free surface to sweep the particles away to an orifice.

~Certain parts of the kidney, the ureters (kidney-to-bladder tubes) and the urinary bladder, are subject to stretching due to variations in the volume of urine. The epithelium best adapted for this kind of stress is transitional epithelium.

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