Microvilli are specialized cellular extensions that project from the surface of certain epithelial cells. These tiny structures dramatically increase the cell’s surface area to enhance efficiency in absorption or secretion. Found in high concentrations where the exchange of substances is paramount, microvilli are designed for maximizing cellular function in various bodily systems.
Defining Microvilli: Structure and Cellular Location
A microvillus is a minute, finger-like protrusion of the cell’s plasma membrane, typically measuring about 0.1 micrometers in diameter and up to 2 micrometers in length. Structurally, each projection is supported by an internal core composed of tightly bundled actin filaments. These protein filaments are cross-linked by proteins like fascin and villin, providing the structural rigidity necessary for the microvillus to stand upright. This core of actin filaments is anchored to a network of proteins beneath the cell surface called the terminal web. Microvilli are most famously found lining the epithelial cells of the small intestine, but they are also present in the proximal convoluted tubules of the kidney.
The Central Function: Maximizing Surface Area for Absorption
The collective presence of thousands of microvilli on a cell’s surface creates a dense, carpet-like layer known as the “brush border.” This morphology is a direct strategy to drastically increase the surface area available for contact with the contents of the lumen. The sheer number of these projections multiplies the cell’s plasma membrane area, allowing for a much greater capacity for exchange. For instance, in the small intestine, this brush border increases the absorptive surface area of the epithelial cell by an estimated 25 to 40 times. By maximizing the interface between the cell and the digested food, the rate at which nutrients and water can be absorbed into the body is significantly enhanced, and in the kidney, this same principle is used to increase the reabsorption of water, ions, and molecules back into the bloodstream from the filtrate.
Beyond Structure: The Role of Brush Border Enzymes and Transport
The physical structure of the microvillus provides a platform for biochemical activity. Embedded within the plasma membrane of the brush border are specialized integral membrane proteins known as brush border enzymes. These enzymes are responsible for the final steps of digestion, which occur immediately before the nutrients are absorbed into the cell.
For example, disaccharidases, such as lactase, sucrase, and maltase, break down complex sugar molecules into their simpler, absorbable forms like glucose and fructose. Similarly, peptidases hydrolyze small peptides into individual amino acids or dipeptides. This terminal hydrolysis ensures that nutrients are in their simplest state for transport across the cell membrane.
Once these molecules are fully digested, the microvilli facilitate their movement into the cell through transmembrane transport proteins. These transporters are also embedded in the brush border membrane, situated in close proximity to the digestive enzymes. Specific protein channels actively move substances like glucose and amino acids across the apical membrane and into the epithelial cell, where they can then be passed into the bloodstream.
When Microvilli Fail: Understanding Malabsorption
When microvilli are damaged or improperly formed, the consequence is a failure to absorb nutrients efficiently, a condition termed malabsorption. The most well-known example of microvilli destruction occurs in Celiac Disease, an autoimmune disorder triggered by the ingestion of gluten. The immune response in genetically susceptible individuals leads to inflammation that causes the microvilli, and the larger villi, to flatten or atrophy, drastically reducing the surface area for absorption. Since the enzymes and transporters are lost along with the physical structure, essential nutrients, including fat-soluble vitamins, iron, and calcium, are not absorbed and instead pass through the digestive tract. Rare congenital defects can lead to Microvillous Inclusion Disease, where microvilli fail to form correctly from birth, resulting in severe, life-threatening malabsorption that requires specialized nutritional support.