Microvilli are extensions of cells. They are found, for example, in the intestine, uterus, and taste buds. They improve the absorption of substances by increasing the surface area of the cells.
What are the microvilli?
Microvilli are filamentous projections at the tips of cells. Microvilli are particularly common in epithelial cells. These are the cells of a pressurized or glandular tissue, such as those found in the intestine. The purpose of microvilli is often to improve the absorption of substances from the cell’s environment. Resorption can refer to the uptake of substances from the digestive tract as well as endogenous substances. Cells equipped with microvilli usually occur in groups; they often form a so-called brush border. In addition to microvilli, other types of protrusions exist with which microvilli should not be confused. Cilia, unlike microvilli, are not protrusions from the membrane but from the plasma and consist of microtubules. Stereocilia, on the other hand, are composed of actin filaments like microvilli but grow from the plasma like cilia.
Anatomy and structure
The microvilli average 0.8-0.1 µm in diameter. Their length is approximately 2- 4 µm. The protrusion is located on the apical side of the cell, i.e., at the tip. This side faces the basement membrane, which is a specific section of the cell membrane. Under the light microscope, this area can be distinguished from the rest of the membrane. According to their respective functions, the basement membrane is oriented toward other tissues, while the microvilli form the free surface of the cell or project into a lumen. Externally, the microvilli are surrounded by a layer of various sugars and proteins known as the glycocalyx. The microvilli each have a central bundle of fibers inside them. This is composed of actin filaments. This is a protein that is also found in muscles and the cytoskeleton. The actin filaments stabilize the microvilli and contribute to their elongated to oval shape. Between the individual actin filaments are other proteins that hold the bundles together: Fimbrin and Fascin. On the sides of the microvilli, myosin I filaments attach the actin bundles to the surface of the cell. Spectrin anchors the fibers to the cytoskeleton. Myosin and spectrin are also protein structures.
Function and roles
Microvilli increase the surface area of cells, thereby enhancing the exchange of substances between the cell and the environment. In addition, diffusion resistance at the microvilli is particularly low, which further enhances the transport of substances across the cell membrane. Within the microvilli, the cell passes on the absorbed substances with the help of actin filaments. They not only serve as a rail for transport, but also contract rhythmically. Through the pumping movements, they accelerate the forwarding of the substances into the interior of the cell. The glycocalyx, which forms a layer on the microvilli, determines the antigenic properties of the cell. Antigens represent structures on the surface. They make it possible for the immune system to identify objects and recognize potentially harmful foreign bodies. In addition, the glycocalyx allows recognition of the cell. Cell adhesion – i.e. the attachment of tissue cells – also depends on the glycocalyx on the microvilli. In the intestine, epithelial cells, which have microvilli, sit on the intestinal villi. The intestinal villi are protrusions in the intestinal mucosa. In the same way that microvilli are extensions of cells, intestinal villi are extensions of the lamina propria (inherent skin) of the intestine. A thin layer of smooth muscle surrounds the lamina propria. In the duodenum, it is also home to glands that secrete digestive juices. Intestinal villi and microvilli significantly increase the surface area of the intestine. In an adult human, it averages 180 m². The increased surface area allows the organism to absorb nutrients more efficiently and in this way make optimal use of the food consumed.
Diseases
The microvilli represent the target for rotavirus. The double-stranded RNA virus spreads through feces and causes diarrhea, which is often mucopurulent and yellow-brown to colorless. Other symptoms of infection include vomiting and fever.Rotavirus infects the microvilli, which are located in the intestinal mucosa. It chooses only the tips of the microvilli for infection and no other cell types. Once the cell is infected, the virus takes over the metabolism by causing the cell to execute its genetic material. In this way, the virus triggers vacuolization: Bubbles form in the cell body, surrounded by their own membrane. During vacuolization, several vacuoles are always formed, which have no function for the cell itself. Furthermore, the rotavirus manipulates the structure of the cell’s outer membrane, which thereby loses its integrity. As a result, the cell loses its protective skin and disintegrates. This process is called cytolysis in biology. It leads to the death of the cell. The epithelium, whose cells with their microvilli play a central role in resorption, can no longer perform its task adequately. This results in the severe diarrhea characteristic of Rota infection. The immune system eventually forms antibodies against the virus, while the organism replaces the dead cells and forms new microvilli.
