Microvilli

Microvilli are finger-shaped protrusions on plasma membrane, present on the surface of a wide range of cell types, although they are most abundant and elaborated on simple epithelial cells, such as the mucosa of the intestine and the epithelium of proximal tubule of kidney. Microvilli, unlike other plasma membrane extensions, are not linked to the extracellular matrix and instead cover free cell surfaces that are often exposed to a lumen where exchange takes place. Because they increase the absorptive surface of the gut, these protrusions were given the name microvilli by analogy with the intestinal villi.

Microvilli

• Microvilli are supported by the actin cytoskeleton. 

• They can be found on almost all cell types.

• In the microvilli diagram, it can be seen that the dynamic lymphocyte microvilli, having highly curved membranes, appears to play a key role in signal transduction leading to immunological responses, according to a rising body of research. 

• Microvilli are one of the most common types of membrane protrusions found on lymphocytes in the immune system.

• Microvilli, can be seen on the surface of a range of cell types, including intestinal epithelial cells, dendritic cells and neurons. They are maintained by F-actin filaments, which are structured in parallel bundles of 10–30 filaments and resemble the actin network that makes up filopodia. 

• Filopodia, on the other hand, frequently emerge from the lamellipodial and lamellar actin networks,  but not from the microvilli actin network. Microvilli were seen on the surface of both T cells and B cells in lymphocytes.

Structure of microvilli

• Plasma membrane encloses cytoplasm and microfilaments and is found on the surface of microvilli. Despite the fact that the microvilli are cellular extensions, they lack cellular organelles.

• The structural core of each microvillus is made up of a dense bundle of cross-linked actin filaments.

•  The centre of the microvilli is formed by the cross-linking of 20 to 30 densely bundled actin filaments by the bundling proteins fimbrin (or plastin-1), villin, and espin.

• The structural core of the enterocyte microvilli is connected to the plasma membrane along its whole length by myosin 1a and the Ca2+ binding protein calmodulin. On one end, myosin 1a has a filamentous actin binding domain, while on the other, it has a lipid binding domain.

• The intermicrovillous space is the space between microvilli at the surface of a cell. When myosin II and tropomyosin are contracting, the intermicrovillous gap expands, and when contraction stops, it shrinks.

Functions of microvilli

• In the gastrointestinal tract, microvilli serve as the major surface for nutrition absorption. 

• The microvillar membrane is packed with enzymes that aid in the breakdown of complex nutrients into simpler compounds that are more easily absorbed because of its critical role. 

• Glycosidases, for example, are carbohydrate-digesting enzymes found in high concentrations on the surface of enterocyte microvilli. 

• Microvilli enhance the amount of digestive enzymes that can be present on the cell surface, as well as the cellular surface area available for absorption. 

• Immune cells also have microvilli, which allow them to detect characteristics on infections’ and other antigen-presenting cells’ surfaces.

• Enzymes that help microvilli function are frequently found within them. Lactase and other carbohydrate-hydrolyzing enzymes, for example, are found on microvilli in intestinal epithelial cells.

• The amorphous dark staining tip of the microvilli is where these enzymes are found. They are thereby increasing not only the area available for absorption, but also the area available for digestive enzymes to anchor on the cell surface and execute final stages of extracellular digestion, such as breaking down tiny peptides and disaccharides for transport across the membrane.

Note

• On the apical surface of some epithelial cells, such as those in the small intestine, thousands of microvilli form a structure known as the brush border.

• Microvilli can be found on the plasma surface of eggs, which aid in the anchoring of sperm cells that have penetrated the egg cells’ extracellular covering. 

• The formation of a cluster of elongated microtubules surrounding a sperm allows it to be brought closer and held firmly in place, allowing fusion to take place. 

• They’re large objects with a lot of surface area to absorb things. White blood cells have microvilli on their cell surfaces, which help them migrate.

Conclusion

Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that enhance surface area for diffusion while minimising volume increase, and are involved in a range of processes such as absorption, secretion, cellular adhesion, and mechanotransduction.  Actin-based structures found on the apical surface of many epithelial cells are known as microvilli. Plasma membrane, which encloses cytoplasm and microfilaments, covers microvilli. Despite the fact that microvilli are cellular extensions, they contain few or no cellular organelles. The microvilli’s core is made up of 20 to 30 densely packed actin filaments that are cross-linked by the bundling proteins fimbrin (or plastin-1), villin, and espin.