Microvilli are microscopic, finger-like projections that extend from the surface of various cells, significantly expanding a cell’s outer boundary. By increasing this surface area, microvilli enhance the cell’s ability to interact with its surroundings, facilitating processes like absorption and secretion.
Understanding Microvilli Structure
Each microvillus is a slender projection, typically measuring around 0.1 micrometers in diameter and up to 2 micrometers in length. They possess a defined internal structure that provides support. The core of each microvillus contains a dense bundle of tightly packed actin filaments, a protein that offers structural stability. These actin filaments are cross-linked by bundling proteins such as fimbrin and villin, maintaining their organized arrangement.
The entire structure of a microvillus is an extension of the cell’s plasma membrane, enveloping the actin filament core. Along its length, the actin core is connected to the surrounding plasma membrane by lateral arms, which include proteins like myosin 1a and calmodulin. These connections help anchor the microvillus to the cell. When numerous microvilli are present on a cell’s surface, they create a dense, brush-like appearance, often called a “brush border.”
Diverse Functions in the Body
The primary function of microvilli is to vastly increase a cell’s surface area, particularly for absorption. In the small intestine, enterocytes are lined with thousands of microvilli, forming a brush border that expands the absorptive surface by approximately 25 times. This expanded area allows for more efficient uptake of digested nutrients, such as carbohydrates, proteins, and fats, into the bloodstream. Enzymes like glycosidases and peptidases are often embedded within the microvillar membrane, aiding in the breakdown of complex nutrients into simpler, absorbable forms.
Microvilli also play a significant role in the kidneys, specifically in the proximal convoluted tubules. They contribute to the reabsorption of water, glucose, vitamins, and other substances back into the blood from the glomerular filtrate. The increased surface area provided by these microvilli facilitates this reabsorption, preventing the loss of essential compounds.
Beyond absorption, microvilli are involved in sensory perception. Specialized microvilli known as stereocilia, found in the hair cells of the inner ear, detect fluid motion caused by sound vibrations and head movements. They convert these mechanical stimuli into electrical signals sent to the brain for hearing and balance. While functionally distinct, stereocilia share structural similarities with other microvilli.
Implications of Microvilli Dysfunction
When microvilli do not function correctly, it can lead to health issues, primarily affecting nutrient absorption. Impaired microvilli can result in malabsorption, where the body struggles to take in sufficient nutrients from food. This can manifest as chronic diarrhea, weight loss, and malnutrition.
One rare genetic disorder directly affecting intestinal microvilli is congenital tufting enteropathy (CTE), also known as microvillous inclusion disease (MVID). Infants with this condition experience severe, intractable diarrhea from birth due to defective intestinal epithelial differentiation and an inability to absorb nutrients.
While not a direct microvilli disorder, conditions like celiac disease can cause damage to the villi, which contain microvilli. This leads to villous atrophy and subsequent malabsorption when gluten is consumed. The damage reduces the surface area available for nutrient uptake, resulting in symptoms such as diarrhea, cramping, and malnutrition.