A layer of cells lining the internal surface of a hollow organ, and covering the external surface of the body; the internal lining may also be known as endothelium. Its function varies in different regions of the body (eg protection, secretion, absorption). Several types have been identified according to the shape and disposition of the individual cells (columnar, cuboidal, squamous), their arrangement (stratified, pseudostratified), or the presence of hairlike processes on their free surface (ciliated). Transitional epithelium is a type characteristically found lining hollow organs subject to large mechanical changes arising from contraction and distension (such as the bladder). In closed body cavities (eg pericardial, peritoneal, pleural) the epithelium provides a smooth, moist surface which facilitates movement.
In dermatology epithelium is a tissue composed of a layer of cells. The outermost layer of our skin is composed of dead stratified 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, protection, transcellular transport, sensation detection, and selective permeability.
Classification
Epithelial cells are classified by the following three factors:
Shape Stratification (number of layers) SpecializationsShape
Squamous: Squamous cells are flat cells with an irregular flattened shape. Places where squamous cells can be found include the alveoli of the lungs, the filtration tubules of the kidneys, and the major cavities of the body. Cuboidal: As the name suggests, these cells have a shape similar to a cube, meaning its width is the same size as its height. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. Unicellular glands called goblet cells are scattered throughout the simple columnar epithelial cells and secrete mucus. The free surface of the columnar cell has tiny hairlike projections called microvilli. 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;Stratification
Simple: There is a single layer of cells. Stratified: More than one layer of cells. Stratified cells can usually withstand large amounts of stress. There is only a single layer of cells, but the position of the nuclei gives the impression that it is stratified.Specializations
Keratinized cells contain keratin (a cytoskeletal protein). Ciliated cells have apical plasma membrane extensions composed of microtubules capable of beating rhythmically to move mucus or other substances through a duct. also found in masticatory oral mucosa (attached gingiva, dorsum of tongue, hard palate, etc.) Non-Keratinised stratified squamous: Found in human oesophagus (Oral Mucosa) specifically non-masticatory "movable" mucosa, and vagina Cuboidal Simple cuboidal: Found in thyroid follicles Stratified cuboidal: Exclusively found in sweat gland ducts Columnar Ciliated simple columnar: Found in intestine and kidney (specifically, proximal convoluted tubule) Stratified columnar: Ducts of submandibular glands Transitional: Specialized to stretch as the urinary bladder fillsJunctional complexes
A cell junction is a structure within a tissue of a multicellular organism. Cell junctions are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between a cell and the extracellular matrix, or they built up the paracellular barrier of epithelia and control the paracellular transport.
Secretory epithelia
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.
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