Microvilli are microscopic, finger-like extensions of the cell membrane, protruding from the surface of various cell types. These tiny structures increase the cell’s available surface area without dramatically increasing its volume. This change in cellular architecture significantly enhances the rate at which substances can be exchanged, optimizing the efficiency of numerous cellular processes that rely on interaction with the outside environment.
The Physical Structure of Microvilli
The shape and rigidity of a microvillus are maintained by a dense, internal scaffolding made up of the protein actin. Each projection contains a tightly packed bundle of 20 to 30 parallel actin filaments, which forms the structural core. Cross-linking proteins, such as villin and fimbrin, bind these filaments together, creating a stable, rod-like structure that resists bending.
The actin core is anchored directly to the cell membrane along its entire length by lateral arms composed of proteins like myosin-1a and calmodulin. The base of the microvillus extends into the cell’s cytoplasm where the actin filaments are embedded in a dense horizontal network called the terminal web. This web is a meshwork of actin, spectrin, and myosin II filaments that stabilizes the entire microvillar array and connects it to the rest of the cell’s internal support system.
Maximizing Efficiency Through Surface Area
The primary biological purpose of microvilli is to dramatically increase the surface area-to-volume ratio of the cell. Diffusion and transport across a membrane are proportional to the available surface area. By creating thousands of these slender projections, a single cell can amplify its working surface area by 25 to 40 times.
This massive increase allows for a much greater number of transport proteins and channels to be embedded on the cell’s exterior. These specialized proteins actively move nutrients, ions, and water across the barrier. In the intestine, for example, the microvillar membrane is packed with digestive enzymes that perform the final step of breaking down complex molecules right at the point of absorption.
A larger surface area ensures that the rate of nutrient uptake or waste reabsorption can keep pace with the body’s metabolic demands. Without this amplification, the cell’s volume would quickly outgrow its surface area, leading to a bottleneck that would severely limit the efficiency of life-sustaining processes.
Key Locations in the Body
Microvilli are most prominently organized on the apical surface of epithelial cells that form the lining of internal organs. A high concentration of microvilli in these locations creates the brush border, a structure visible under a microscope that resembles a dense bristle brush. The most well-known location for this brush border is the small intestine.
Here, the microvilli on the enterocytes are responsible for nearly all final nutrient absorption, including the uptake of monosaccharides, amino acids, and fatty acids. Their presence allows the body to efficiently harvest calories and building blocks from digested food.
The cells lining the proximal convoluted tubules in the kidney also rely heavily on an extensive brush border. The kidney filters the blood and reabsorbs water and beneficial solutes before they are lost in urine. The microvilli provide the necessary surface for the high-volume reabsorption of molecules like glucose, sodium, and water back into the bloodstream. Microvilli also serve specialized functions in sensory systems, acting as mechanoreceptors in the inner ear to detect sound or movement.
When Microvilli Function Fails
Impairment of microvillar structure or function can lead to severe health consequences, primarily affecting the body’s ability to absorb nutrients. A common example is Celiac disease, an autoimmune condition where gluten ingestion triggers an immune response that damages the intestinal lining. This damage causes significant shortening and blunting of the microvilli and the larger intestinal villi.
The resulting loss of functional brush border surface area leads to malabsorption, causing symptoms like diarrhea, weight loss, and malnutrition. A more severe and rare structural failure is Microvillus Inclusion Disease (MVID), a genetic disorder that typically manifests shortly after birth. MVID is often caused by mutations involved in trafficking proteins needed to build the microvilli.
In infants with MVID, the microvilli are either severely malformed or absent. The affected cells contain characteristic internal pockets of misplaced microvillar membranes called inclusion bodies. This complete failure of the absorptive surface results in intractable, life-threatening diarrhea and a total inability to absorb nutrients, requiring lifelong intravenous nutrition.