Microvilli are tiny, finger-like projections extending from the surface of certain cells. These microscopic protrusions significantly expand the cell’s surface area, which is important for various cellular processes. Ranging from 0.1 to 1.0 micrometers in length and about 0.1 micrometers in diameter, their primary role is enhancing the efficiency of substance exchange across the cell membrane.
The Building Blocks of Microvilli
The structural integrity of each microvillus stems from a core of protein filaments. A dense bundle of actin filaments forms the fundamental framework, typically containing 20 to 30 filaments tightly packed together. These actin filaments are held in their bundled arrangement by specific cross-linking proteins like villin and fimbrin. These proteins are important for linking the actin filaments laterally, providing stability to the bundle and assembling the microvillar architecture. The entire internal actin core is enveloped by the cell’s plasma membrane. This membrane is continuous with the cell’s outer surface and is composed of a lipid bilayer embedded with various proteins. Lateral arms, made of proteins like myosin 1a and calmodulin, extend from the actin core to attach to the plasma membrane, further securing the structure.
How Structure Supports Function
The unique composition and arrangement of microvilli directly support their functional capabilities. Their finger-like shape and sheer number allow for a significant increase in the cell membrane’s surface area. This increased surface area is important for processes like absorption and secretion, as it provides more space for molecules to pass into or out of the cell. In many locations, the dense packing of microvilli creates a characteristic “brush border.” This brush border boosts the efficiency of molecular exchange, allowing cells to absorb nutrients or reabsorb water and solutes more effectively. The plasma membrane covering the microvilli contains specialized transport proteins and enzymes. These proteins facilitate the breakdown of complex substances and the movement of digested molecules, directly linking the structural design to the cell’s metabolic needs.
Common Locations in the Body
Microvilli are found in various parts of the body where increased surface area for exchange is beneficial.
They are abundant in the small intestine, lining epithelial cells. Their extensive brush border is important for efficient absorption of digested nutrients, such as glucose and amino acids, into the bloodstream.
Microvilli are also found in the kidney tubules, specifically the proximal convoluted tubule. In this region, microvilli contribute to the reabsorption of water, ions, and other solutes to maintain fluid and electrolyte balance. They also possess a mechanosensory function, sensing fluid flow to help regulate sodium reabsorption.
Modified microvilli, known as stereocilia, are found in the inner ear within sensory hair cells. While sharing structural similarities with typical microvilli, including an actin filament core, stereocilia are specialized for mechanotransduction. Their movement in response to fluid shifts, caused by sound waves or head movements, is converted into electrical signals, enabling the senses of hearing and balance.