Microvilli

Microvilli are thin, finger-like projections on the surface of cells and are particularly abundant in epithelia, where a large surface area is required for efficient functionality such as absoprtion. Microvilli increase the surface area of the cell and can be often found in large numbers. A multitude of microvilli seen lining the small intestine are described as a brush border due to their 'paint brush tip' appearance.

Each microvilli has about 20-30 parallel actin filaments that extend from the tip into the cell cortex. In all the actin filaments the plus ends point away from the cell body and are held together by actin-bundling proteins. The most important of which is villin, a protein that is specific to microvilli and cross-links actin filaments into tight bundles.

At the base of each microvillus the actin filaments are anchored into a specialized region known as the terminal web. This contains a concentrated network of spectrin molecules and a layer of intermediate filaments. It is thought that spectrin imparts rigidity to the cell cortex; because the actin filaments are anchored to the terminal web, this keeps the microvilli orientated perpendicularly to the apical surface of the cell. The actin filaments are attached to the plasma membrane by lateral bridges composed of myosin-I and serveral other molecules including the Ca²⁺.