In zootomy, epithelium is a tissue composed of a layer of cells. Epithelium can be found lining internal (ex. endothelium, which lines the inside of blood vessels) or external (ex. skin) free surfaces of the body.
The outermost layer of our skin is composed of dead squamous epithelial cells, as are the mucous membranes lining the inside of mouths and body cavities. Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands.
Functions of epithelial cells include secretion, absorption and protection. Epithelial cells sit on a basal lamina (formerly called a basement membrane).
Epithelial cells are classified by the following three factors: -
Stratification (number of layers)
Squamous: Squamous cells are thin cells with an irregular shape.
Cuboidal: As the name suggests, these cells have a shape similar to a cube, meaning its width is the same size as its height. The nuclei of these cells are usually located in the center.
Columnar: The height of these cells is usually longer than its width. The nucleus is also closer to the base of the cell.
Simple: There is a single layer.
Stratified: More than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina. Stratified cells can usually withstand large amounts of stress.
Pseudostratified: This is used mainly in one type of classification (pseudostratified columnar epithelium). There is only a single layer of cells, but the position of the nuclei gives the impression that it is stratified.
Transitional: This is a specialized type of epithelium found lining organs that can stretch, such as the bladder or the ureter of mammals. Since the cells can slide over each other, the appearance of this epithelium depends on whether the organ is distended or contracted: if distended, it appears as if there are only a few layers; when contracted, it appears as if there are several layers.
Keratinization / Cornification: Cells contain keratin (a cytoskeletal protein). This process occurs mainly in skin, since it provides a tough, impermeable barrier.
Transitional: Specialized to distend (stretch) as the urinary bladder fills
These complexes are involved in cell cohesion (the first three) or cell communication (Gap Junction). These are visible at the light-microscope level as a series of dots or lines, often likened to a ladder or a zipper.
As stated above, secretion is one major function of epithelial cells. Glands are formed from the invagination / infolding of epithelial cells and subsequent growth in the underlying connective tissue. There are two major classification of glands: endocrine glands and exocrine glands.
The portions of cell membranes found on the outer surface of an epithelial tissue are specialized to act as the regulators of the tissue strictly managing the flow of materials into and out of the epithelium.
The outside surfaces of epithelial tissues often are highly specialized in other ways possessing cilia, hairs, or glands, which may arise from individual cells or groups of cells that have invaginated and which are capable of covering the outer surface of an epithelial tissue with mucous or waxy secretions.
In stratified epithelial tissues, the outermost layer of cells determines the name of the tissue: Stratified squamous, stratified cuboidal, and stratified columnar epithelia are the three types of stratified epithelial tissues.
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