Microvilli are microscopic, finger-like projections that extend from the surface of certain cells. These structures play a significant role in various bodily functions by expanding the cell’s surface area. This increased surface area allows for more efficient interaction with the cell’s environment, facilitating processes like absorption and secretion.
What Are Microvilli
Microvilli are extensions of the cell membrane. Their internal structure is supported by a dense bundle of cross-linked actin filaments, which provide rigidity and shape. They are quite small, typically measuring about 0.1 micrometers in diameter and ranging from a fraction of a micrometer to about 2 micrometers in length.
When numerous microvilli are present on a cell’s surface, they create a textured appearance similar to the bristles of a brush, often referred to as a “brush border.” This brush border is a clear indication of a cell specialized for absorption or secretion, as the increased surface area allows for a greater number of transport proteins and enzymes. Despite being cellular extensions, microvilli contain few or no cellular organelles, focusing their structure on surface area enhancement.
Where Microvilli Are Found and Their Roles
Microvilli are found in several locations throughout the human body, each with specific functions. In the small intestine, they are abundant on the surface of enterocytes, which are the absorptive cells lining the digestive tract. Here, along with larger folds called intestinal villi, microvilli vastly increase the surface area available for nutrient absorption, enhancing it by approximately 25 times. This expanded surface allows for the efficient uptake of digested food molecules, including monosaccharides and amino acids, into the bloodstream. The microvillar membrane also contains enzymes like glycosidases and peptidases that break down complex nutrients into simpler forms for easier absorption.
Another prominent location for microvilli is within the proximal tubules of the kidneys. These tubules are responsible for reabsorbing water, ions, and other valuable molecules from the filtered blood back into the body. The brush border of microvilli in the kidney tubules significantly increases the surface area for this reabsorption, playing a role in maintaining the body’s fluid and electrolyte balance. Beyond absorption, these microvilli also function as mechanosensors, detecting fluid flow within the tubules and signaling adjustments in sodium reabsorption to maintain proper kidney function.
Specialized microvilli, known as stereocilia, are found on the hair cells within the inner ear. These structures are longer than typical microvilli, measuring about 10–50 micrometers, and are arranged in graded bundles. Stereocilia are mechanosensing organelles that convert mechanical energy, such as the vibrations caused by sound waves or head movements, into electrical signals. This conversion leads to an electrical signal that is transmitted to the brain for sound and balance perception.
Microvilli and Health
Disruptions to the normal structure or function of microvilli can have significant health consequences. In the small intestine, damaged or defective microvilli can lead to malabsorption syndromes, where the body struggles to absorb nutrients from food. For example, Celiac disease can cause damage to the intestinal microvilli, leading to nutrient deficiencies.
A rare genetic disorder called Microvillus Inclusion Disease (MID) is characterized by abnormally formed microvilli that are often trapped inside intestinal cells instead of being on the surface. This condition results in severe, chronic watery diarrhea, malnutrition, and dehydration, often appearing shortly after birth. Infants with MID frequently require intravenous nutrition to support their growth and prevent severe complications.
In the kidneys, microvilli dysfunction can impact the reabsorption of substances from the filtrate, potentially leading to imbalances in water and electrolyte levels. Issues with the specialized stereocilia in the inner ear can result in problems with hearing and balance. For instance, mutations in genes that encode components of stereocilia can lead to hereditary forms of deafness.