Microvilli are microscopic, finger-like projections found on the surface of certain cells within the human body. These slender extensions of the cell membrane typically measure about 0.1 micrometers in diameter and up to 2 micrometers in length. Their primary purpose is to significantly increase the cell’s surface area, which enhances the efficiency of various biological processes, such as absorbing substances or detecting external stimuli.
Where Microvilli Are Found
Microvilli are present in several locations throughout the human body, each tailored to a specific function. A prominent location is the small intestine, where thousands of microvilli form a dense structure known as the “brush border” on the apical surface of epithelial cells. This extensive surface area is a key adaptation for efficient nutrient absorption from digested food. Microvilli also line the proximal convoluted tubules of the kidneys, aiding in the reabsorption of water and solutes back into the bloodstream from the filtered fluid. In the inner ear, specialized microvilli, called stereocilia, are found on sensory hair cells. These structures are involved in converting mechanical vibrations from sound waves into electrical signals that the brain interprets, playing a role in both hearing and balance. Microvilli are additionally observed on the plasma surface of egg cells, assisting in the anchoring of sperm cells during fertilization. They are also present on immune cells like lymphocytes, where they help in sensing features on the surface of pathogens and other antigen-presenting cells.
Anatomy of Microvilli
Each microvillus is an extension of the cell’s plasma membrane, enclosing a small amount of cytoplasm but generally lacking cellular organelles. The structural integrity of a microvillus is provided by a dense bundle of cross-linked actin filaments at its core. These actin filaments, typically numbering 20 to 30, are tightly bundled together by specific proteins such as villin, fimbrin, and espin. The actin filaments within the microvillus are oriented with their “plus” ends positioned at the tip of the projection, where they are capped by proteins. The “minus” ends of these filaments are anchored within a specialized region at the base of the microvillus called the terminal web. This terminal web is a complex network of actin filaments, spectrin, and myosin II, which connects the microvilli to the underlying cytoskeleton of the cell. The structural core of the microvillus is also attached to the plasma membrane along its length by lateral arms composed of myosin 1a and the calcium-binding protein calmodulin. Myosin 1a acts as a bridge, binding to filamentous actin and the lipid membrane. The surface of the microvillus is further covered by a layer called the glycocalyx, a complex meshwork of glycoproteins and glycolipids that project from the plasma membrane. This outer coating is involved in binding substances for uptake and providing protection against harmful elements.
How Microvilli Perform Their Roles
The unique structure of microvilli directly enables their diverse functions. Their primary role is to dramatically increase cellular surface area for absorption. In the small intestine, thousands of microvilli form the brush border, amplifying the absorptive surface by approximately 25 times. This expanded area allows for more efficient uptake of nutrients, ingested food, and water molecules. The glycocalyx layer covering the microvilli also plays a role in digestion and absorption by containing enzymes that break down complex nutrients into simpler, more easily absorbed compounds. For instance, glycosidases, enzymes that digest carbohydrates, are present in high concentrations on the surface of intestinal microvilli. This arrangement not only increases surface area but also concentrates digestive enzymes where they are needed for the final steps of nutrient breakdown. Beyond absorption, microvilli contribute to mechanosensation, as seen in the stereocilia of the inner ear. The bending of these specialized microvilli in response to fluid motion or sound waves triggers electrical signals that are transmitted to the nervous system, allowing for hearing and balance. Microvilli also facilitate cellular adhesion, binding cells to each other or to external substances.
Microvilli and Health Conditions
Abnormalities in the structure or function of microvilli can lead to specific health conditions, primarily affecting processes like nutrient absorption. Celiac disease provides a clear example of microvilli involvement. In individuals with celiac disease, consuming gluten triggers an immune response that damages the microvilli in the small intestine, causing them to flatten or disappear. This damage to the intestinal lining significantly reduces the surface area available for nutrient absorption, leading to malabsorption of vitamins, minerals, and other nutrients. Symptoms can include severe diarrhea, bloating, and cramps, and if left untreated, it can result in deficiencies such as iron-deficiency anemia and other health complications. Another condition is Microvillus Inclusion Disease (MVID), a rare genetic disorder that typically manifests shortly after birth. MVID is characterized by severe, persistent watery diarrhea and an inability to absorb nutrients, even when no food is consumed. This occurs because the surface cells lining the intestine do not develop microvilli normally, or the microvilli are significantly blunted or absent, compromising the cells’ ability to absorb fluids and nutrients.