A microvillus, plural microvilli, is a finger-shaped outpouching of the nuclear membrane into the extracellular space. It is 4-8 micrometres long and contains actin microfilaments. The filaments are bound to the cell membrane distally and the cytoskeletal fibres supporting the plasma membrane proximally. They give the microvillus strength, although a contractile role has also been proposed.
Microvilli are most frequently found on the apical surface of cells. The number and density of microvilli bears direct relation to the absorptive role of the cell, as they increase surface area of the membrane. Hence, microvilli are particularly numerous on the luminal side of the small intestinal epithelium - the layer of microvilli is termed the brush border.
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