Microvilli are microscopic, finger-like projections extending from the surface of certain cells. These tiny extensions significantly increase the surface area of cells without substantially increasing cellular volume. This structural adaptation facilitates efficient interactions with the surrounding environment, playing a role in various bodily processes.
Understanding Microvilli Structure
Microvilli are slender, cylindrical membrane protrusions, measuring about 0.1 to 0.2 micrometers in diameter and 1 to 2 micrometers in length. Each microvillus is an extension of the cell’s plasma membrane, enclosing a core of cytoplasm. Unlike other cellular extensions, microvilli contain few or no cellular organelles.
Their internal stability comes from a dense bundle of cross-linked actin filaments. These filaments are tightly packed and held together by bundling proteins. This bundled actin core anchors to a cytoskeletal network called the “terminal web” at the base of the microvillus, providing structural support. In many epithelial tissues, thousands of microvilli are densely packed on the apical surface of cells, forming a “brush border.” This structure greatly increases the cell’s surface area.
Key Locations and Their Specific Roles
Microvilli are found in several locations throughout the human body, adapted for enhanced surface interaction or specialized sensory capabilities.
In the small intestine, microvilli form a dense brush border on the surface of epithelial cells. This arrangement vastly increases the surface area for absorbing digested nutrients, allowing for efficient uptake into the body.
Within the kidney tubules, microvilli also form a brush border. Their role involves reabsorbing water, ions, glucose, and amino acids from filtered blood back into the bloodstream. This process is important for maintaining the body’s fluid and electrolyte balance.
Specialized microvilli, known as stereocilia, are found on hair cells within the inner ear. These stereocilia are involved in mechanotransduction, converting mechanical vibrations from sound waves and head movements into electrical signals for sound and balance. Microvilli are also found on the plasma surface of egg cells, aiding in sperm anchoring during fertilization, and on taste buds, contributing to taste signal detection.
The Role of Microvilli in Absorption
Microvilli play a prominent role in absorption, especially within the small intestine, where their structure greatly enhances nutrient uptake. The large number of microvilli on each intestinal epithelial cell creates an enormous absorptive surface, significantly increasing the area for contact with digested food.
The membranes of these microvilli are embedded with specific digestive enzymes, known as “brush border enzymes.” These enzymes perform the final stages of nutrient breakdown at the site of absorption. For example, they break down complex carbohydrates into simpler sugars for immediate absorption.
Following digestion, the microvillar membrane is equipped with various transport proteins. These transporters facilitate the movement of absorbed nutrients from the intestinal lumen into the epithelial cells. From these cells, nutrients pass into the underlying capillaries or lymphatic vessels for distribution throughout the body. This integrated system ensures highly efficient nutrient absorption, which is important for energy, growth, and overall bodily function.