Mikrovillit

Mikrovillit, or microvilli, are microscopic, finger-like projections on the apical surface of many epithelial cells. They are especially prominent on enterocytes lining the small intestine and on cells of the kidney proximal tubules, where they greatly expand the apical membrane area to enhance absorption and secretion. In the intestine, microvilli form the brush border, which bears a dense array of digestive enzymes and transporters essential for final steps of nutrient digestion and uptake.

Structure and composition: Each microvillus contains a core bundle of actin filaments, typically 0.5–2.0 micrometers tall. The actin filaments are cross-linked by proteins such as fimbrin, fascin, and espin and capped by villin. The actin core is anchored to the apical plasma membrane and to a basal network known as the terminal web, which includes spectrin, myosin II, and other cytoskeletal elements. Unlike motile cilia, microvilli are generally non-mmotile; they remain stationary while the cell absorbs and transports substances.

Function: The primary role of microvilli is to increase the surface area of the cell membrane, dramatically enhancing absorptive capacity. The dense membrane over the microvilli hosts brush border enzymes (for example, disaccharidases) and various transporters that aid in nutrient digestion and absorption. They also contribute to barrier function and influence membrane dynamics at the cell surface.

Clinical relevance: Disruption or loss of microvilli can impair absorption and is seen in certain enteropathies. Microvillus inclusion disease is a rare congenital disorder affecting microvillar formation due to MYO5B mutations. Conditions such as celiac disease can damage the brush border, reducing absorptive efficiency.