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.
Epithelialtissue - found as the lining and covering of organs and body cavities, the secretory parts of organs and glands, the transport membranes of capillaries and alveolar sacs, and membranes which lubricate organs.
Epithelialtissue is composed of closely packed cells, anchored by a basement membrane consisting of a basal lamina made of glycoprotein (similar to the glycocalyx on the apical surface) and a reticular lamina made of collagen.
Epithelialtissue in the skin, in the linings of organs in the GI and respiratory tracts, in the liver, in many glands, and in blood vessels can all replace and repair themselves, the limiting factor generally being the degree of damage and other nutritive and health factors.
Tissue, group of associated, similarly structured cells that perform specialized functions for the survival of the organism (see Physiology).
Animal tissues, to which this article is limited, take their first form when the blastula cells, arising from the fertilized ovum, differentiate into three germ layers: the ectoderm, mesoderm, and endoderm (see Embryology: Normal Development in Animals).
These tissues, which support and hold parts of the body together, comprise the fibrous and elastic connective tissues, the adipose (fatty) tissues, and cartilage and bone.
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