Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, cellular adhesion, and mechanotransduction. Thousands of microvilli form a structure called the brush border that is found on the apical surface of some epithelial cells, such as the small intestines. (Microvilli should not be confused with intestinal villi, which are made of many cells. Each of these cells has many microvilli.) Microvilli are observed on the plasma surface of eggs, aiding in the anchoring of sperm cells that have penetrated the extracellular coat of egg cells. Clustering of elongated microtubules around a sperm allows for it to be drawn closer and held firmly so fusion can occur. They are large objects that increase surface area for absorption.
Microvilli are also of importance on the cell surface of white blood cells, as they aid in the migration of white blood cells Microvilli are covered in plasma membrane, which encloses cytoplasm and microfilaments. Though these are cellular extensions, there are little or no cellular organelles present in the microvilli.
Each microvillus has a dense bundle of cross-linked actin filaments, which serves as its structural core. 20 to 30 tightly bundled actin filaments are cross-linked by bundling proteins fimbrin (or plastin-1), villin and espin to form the core of the microvilli.
In the enterocyte microvillus, the structural core is attached to the plasma membrane along its length by lateral arms made of myosin 1a and Ca2+ binding protein calmodulin. Myosin 1a functions through a binding site for filamentous actin on one end and a lipid binding domain on the other. The plus ends of the actin filaments are located at the tip of the microvillus and are capped, possibly by capZ proteins, while the minus ends are anchored in the terminal web composed of a complicated set of proteins including spectrin and myosin II.
Until now, the general importance of microvilli present on the surface of almost all differentiated cells has been strongly underestimated and essential functions of these abundant surface organelles remained unrecognized. Commonly, the role of microvilli has been reduced to their putative function of cell-surface enlargement. In spite of a large body of detailed knowledge about the specific functions of microvilli in sensory receptor cells for sound, light, and odor perception, their functional importance for regulation of basic cell functions remained obscure. Here, a number of microvillar mechanisms involved in fundamental cell functions are discussed. Two structural features enable the extensive functional competence of microvilli: First, the exclusive location of almost all functional important membrane proteins on microvilli of differentiated cells and second, the function of the F-actin-based cytoskeletal core of microvilli as a structural diffusion barrier modulating the flow of low molecular substrates and ions into and out of the cell.
The specific localization on microvilli of important functional membrane proteins such as glucose transporters, ion channels, ion pumps, and ion exchangers indicate the importance and diversity of microvillar functions. In this review, the microvillar mechanisms of audioreceptor, photoreceptor, and olfactory/taste receptor cells are discussed as highly specialized adaptations of a general type of microvillar mechanisms involved in regulation of important basic cell functions such as glucose transport/energy metabolism, ion channel regulation, generation and modulation of the membrane potential, volume regulation, and Ca signaling. Even the constitutive cellular defence against cytotoxic compounds, also called "multidrug resistance (MDR)," is discussed as a microvillar mechanism. A comprehensive examination of the specific properties of "cable-like" ion conduction along the microvillar core structure of F-actin allows the proposal that microvilli are specifically designed for using ionic currents as cellular signals. In view of the multifaceted gating and signaling properties of TRP channels, the possible role of microvilli as a universal gating device for TRP channel regulation is discussed. Combined with the role of the microvillar core bundle of actin filaments as high-affinity Ca store, microvilli may turn out as highly specialized Ca signaling organelle involved in store-operated Ca entry (SOCE) and initiation of nonlinear Ca signals such as waves and oscillations.