Microvilli are microscopic, finger-like protrusions found on the surface of certain cells. They are specialized extensions of the cell membrane, designed to significantly expand the cell’s surface area without substantially increasing its volume. These cellular extensions primarily contribute to processes like absorption, secretion, cellular adhesion, and mechanotransduction.
Primary Locations and Their Roles
Microvilli are prominently located in the small intestine, where they form a dense layer known as the “brush border” on the surface of absorptive cells. This brush border dramatically increases the surface area available for nutrient absorption, enhancing it by approximately 25 times. Beyond increasing surface area, the microvillar membrane in the small intestine contains enzymes that break down complex nutrients into simpler compounds, facilitating their uptake into the bloodstream. For example, glycosidases on microvilli help digest carbohydrates.
Another significant location for microvilli is within the kidney tubules, particularly the proximal convoluted tubules. Here, they also form a brush border, which is important for the reabsorption of water and solutes back into the bloodstream. Research indicates that these microvilli can function as mechanosensors, detecting fluid flow within the tubules and influencing sodium reabsorption. This mechanosensory function helps regulate the balance of substances in the body.
Microvilli are also present in other areas, contributing to specialized sensory functions. In taste buds, microvilli on gustatory receptor cells interact with dissolved food molecules (tastants) to detect taste. These interactions trigger signals sent to the brain, allowing for taste perception. Specialized structures called stereocilia, found in the inner ear, are involved in hearing and balance by converting mechanical stimuli into electrical signals.
The Structure and Function of Microvilli
Microvilli exhibit a characteristic finger-like or bristle-like shape, projecting from the cell’s apical surface. Each microvillus is encased by the cell’s plasma membrane and contains an internal core composed of tightly bundled actin filaments. Typically, 20 to 30 actin filaments are cross-linked by proteins such as fimbrin and villin, providing structural support. This actin core is anchored to the plasma membrane by lateral arms, which include proteins like myosin 1a and calmodulin.
The dense packing and elongated shape of microvilli increase the cell’s surface area-to-volume ratio. This expanded surface allows for more efficient interaction with external substances, whether for absorption, secretion, or sensation. Enzymes and transporters within the microvillar membrane facilitate processes like nutrient breakdown and uptake.
Distinguishing Microvilli from Similar Structures
Microvilli are distinct from other cellular protrusions like villi and cilia, despite some superficial resemblances. Villi are larger, macroscopic finger-like folds of tissue that line organs such as the small intestine. These villi are comprised of many cells, and it is the individual cells lining these villi that possess microvilli on their surfaces. Therefore, villi represent a larger-scale folding of the organ lining, while microvilli are microscopic extensions of individual cell membranes within those villi.
Cilia are longer, hair-like projections that differ from microvilli in both structure and primary function. While microvilli are non-motile and primarily involved in absorption and increasing surface area, cilia are typically motile and function in movement or sweeping substances across a surface. The internal structure of cilia is based on microtubules, arranged in a characteristic (9+2) pattern, whereas microvilli are supported by actin microfilaments. Stereocilia, found in the inner ear, are considered a specialized form of microvilli, sharing their actin filament core but being generally longer and thicker than typical microvilli.