Microvillus
Microvillus
The microvilli (singular: microvillus) are structures that increase the surface area of cells by approximately 600 fold (human), thus facilitating absorption and secretion.
Locations
There are several thousand microvilli present on the apical surface of a single cell in human small intestinal cells (intestinal villus). Microvilli also occur in sensory cells of the inner ear (as stereocilia), in the cells of taste buds, and in olfactory receptor cells. They 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. Microvilli are also of importance on the cell surface of white blood cells, as they aid in the migration of white blood cells.
Structure
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 and villin to form the core of the microvilli. The actin filaments render the microvilli capable of contracting motion, though the motion is limited, similar to flexing of fingers.
The structural core is attached to the plasma membrane along its length by lateral arms made of myosin Ia and Ca2+ binding protein calmodulin. Myosin Ia 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 collected in the tip of the microvillus, while the minus ends bind to a ‘terminal web’ composed of thin filaments, linked together by a complicated set of proteins including spectrin and myosin II.
Relationship to cell
As mentioned, microvilli are formed as cell extensions from the plasma membrane surface. Actin filaments, present in the cytosol, are most abundant near the cell surface. These filaments are thought to determine the shape and movement of the plasma membrane. The nucleation of actin fibers occurs as a response to stimuli in the universe, allowing the cells surface to alter its shape and stiffness. This could account for the uniformity of the microvilli, which are observed to be of equal length and diameter. This nucleation process occurs from the minus end, allowing rapid growth from the plus end.
It is catalyzed by a complex made of two conserved actin related proteins (ARPs). Interestingly, though the length and composition of microvilli is consistent within a certain group of homogenous cells, it can differ slightly in a different part of the same organism. For example, the microvilli in the small and large intestines in mice are slightly different in length and amount of surface coat covering.
Information Courtesy of Wikipedia
