Microvilli are microscopic, finger-like projections found on the surface of certain epithelial cells throughout the body. These cellular extensions dramatically increase the surface area available for cells to interact with their surrounding environment, enhancing the cell’s ability to absorb substances.
Primary Locations of Microvilli in the Body
Microvilli are most famously found lining the inner surface of the small intestine, where they are densely packed to form a structure commonly called the “brush border.” This appearance resembles the bristles of a paintbrush when viewed under a light microscope. Each epithelial cell in this region can possess as many as 1,000 microvilli, creating a vast surface for processing nutrients.
They are also prominent on the epithelial cells of the proximal convoluted tubules in the kidneys. In this location, the microvilli assist in the reabsorption of water, ions, and other molecules that the body needs to retain from the fluid passing through the renal tubules. Microvilli can also be found on other cell types, including some white blood cells and on the surface of egg cells, where they aid in anchoring.
Cytoskeletal Components and Structure
The structural material of microvilli is primarily the protein actin, which forms a dense core of parallel filaments. Each microvillus is essentially a protrusion of the cell’s plasma membrane, enclosing a bundle of about 20 to 30 cross-linked actin filaments (F-actin). Specific bundling proteins, such as villin and fimbrin, cross-link the actin filaments together to maintain the structural integrity of the core. Along the sides of the core, lateral arms made of proteins like myosin 1a and calmodulin tether the actin bundle directly to the surrounding plasma membrane.
The entire structure is anchored at its base within the cell by a dense network of proteins called the terminal web. The terminal web is a meshwork composed of spectrin and myosin II, which connects the microvillus cores to the underlying cytoplasm and stabilizes the apical surface of the cell. Covering the exterior of the plasma membrane is a carbohydrate-rich layer known as the glycocalyx, which consists of glycoproteins that play a role in binding substances for uptake and housing digestive enzymes.
Maximizing Surface Area for Biological Uptake
In the small intestine, the structural arrangement of microvilli is crucial for maximizing the rate of nutrient and water absorption. The sheer number of microvilli, with up to a thousand on a single cell, collectively expands the absorptive area by an estimated 25 to 40 times.
This immense increase in surface area provides abundant space to embed transport proteins and channels directly into the microvillar membrane. These specialized proteins are responsible for actively moving digested nutrients, such as amino acids and glucose, from the intestinal contents into the cell. The concentration of transport machinery on this amplified surface area ensures that absorption is highly efficient.