Microvilli

Microvilli are tiny, finger-like extensions of the apical plasma membrane found on certain epithelial cells. They markedly increase the surface area available for absorption and secretion, with the brush border of intestinal enterocytes and the proximal tubule cells of the kidney being the most well-known examples.

Structure and components: Each microvillus contains a core bundle of actin filaments aligned along its length, typically consisting of dozens of filaments and measuring roughly 0.5 to 1.5 micrometers in length. The actin bundle is cross-linked by proteins such as villin, fimbrin (plastin), and fascin, and is anchored to a supportive network known as the terminal web beneath the apical membrane. The apical membrane hosts digestive enzymes, including lactase, sucrase–isomaltase, and various peptidases. Motor and linker proteins, including myosin I, help attach the membrane to the cytoskeleton and contribute to maintenance of the structure.

Function and distribution: In the small intestine, microvilli line the surface of enterocytes to maximize nutrient uptake from the intestinal lumen. In the kidney, microvilli on proximal tubule cells increase surface area for reabsorption of solutes and water. Microvilli are distinct from stereocilia, which are longer and found in sensory cells; both share an actin-based core but differ in organization and function.

Clinical relevance: Defects in microvillar structure or trafficking can impair absorption and cause disease. For example, microvillus inclusion disease involves mutations affecting apical membrane trafficking, leading to abnormal microvillar formation and severe malabsorption. Changes in microvillar density or integrity can also accompany various gastrointestinal and renal disorders.