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Epithelial cells border the outer surfaces and inner cavities of the body. They possess five principal functions: protection, ion transport, sensory reception, secretion, and absorption.
Epithelial cells are classified according to their shapes and number of cell layers. Squamous cells are flat cells with often slightly protruding nuclei. Cuboidal cells are square in shape, while columnar cells are elongated. Pseudostratified columnar epithelia are all on the same cell layer, but some do not reach the upper surface of the layer.
If the cells reside on a single cell layer, the cells are considered to be simple. Otherwise, they are stratified. In stratified cells, the uppermost cell type is the designated classification for the epithelia. For example, if squamous cells are superficial to cuboidal cells, the epithelia are considered as stratified squamous cells.
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The apical surface of epithelia may contain a layer of glycoproteins known as glycocalyx. There are also possible additions of microvilli and cilia/flagella, which increase the membrane surface area or possess motility.
The lateral surface contains several modifications toward the apical layer, known as the junctional complex. This includes the following regions: zonula occludens (tight junction), which forms a tight seal between adjacent cells; the zonula adherens (belt desmosome); and the macula adherens (desmosome). Below the junctional complex, there are hemidesmosomes, which are structurally similar to desmosomes, and gap junctions that include collections of channels permitting the flow of ions and small molecules between connecting cells.
Below the basal surface of an epithelial group is the basement membrane. This is comprised of a basal lamina (lamina lucida and lamina densa), primarily secreted by the epithelial cell, and the lamina fibroreticularis formed by the connective tissue.
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