A microvillus is a microscopic, finger-like projection of the plasma membrane found on the surface of certain epithelial cells. These structures are primarily composed of a core of actin filaments, which provide structural support and maintain their shape. Microvilli are covered by a glycocalyx, a carbohydrate-rich layer that protects the membrane and facilitates specific interactions. Their primary purpose is to increase the surface area of the cell, enhancing its capacity for absorption and secretion.
Location
Microvilli are most commonly found on the apical surface of epithelial cells in tissues specialized for absorption and secretion. They are abundant in:
- Small Intestine: On enterocytes lining the intestinal villi, forming the brush border.
- Kidneys: On proximal tubule cells, aiding in reabsorption.
- Other Locations: Certain sensory cells, such as in the inner ear, also have microvilli for specialized functions.
Structure
Microvilli are highly specialized structures that enhance the surface area of epithelial cells. Their complex organization and arrangement are crucial for their role in absorption and secretion. Below is a detailed breakdown of the microvillus structure:
Plasma Membrane
- Outer Layer:
The microvillus is covered by the plasma membrane, which is an extension of the cell’s apical membrane.- It is smooth and continuous with the rest of the cell surface.
- Embedded with specific transport proteins, ion channels, and enzymes that facilitate absorption and digestion.
- Glycocalyx:
The plasma membrane is coated with a carbohydrate-rich layer called the glycocalyx.- It provides protection to the microvillus and plays a role in trapping enzymes and other molecules for digestion.
Core Structure
- Actin Filaments:
The core of the microvillus consists of a bundle of parallel actin filaments.- These filaments are cross-linked by actin-binding proteins such as fimbrin and villin, which maintain the stability and integrity of the microvillus.
- The actin filaments extend from the tip of the microvillus down into the cell, anchoring to the terminal web.
- Terminal Web:
At the base of the microvillus, the actin filaments are connected to a dense network of filaments called the terminal web.- The terminal web, composed of intermediate filaments and myosin, provides support and anchorage for the microvilli.
Associated Proteins
- Cross-Linking Proteins:
Proteins like fimbrin and espin stabilize the parallel arrangement of actin filaments within the microvillus. - Anchoring Proteins:
Proteins such as myosin I anchor the actin filaments to the plasma membrane, ensuring the microvillus maintains its structure and connection to the cell surface. - Enzymatic Proteins:
Enzymes like disaccharidases and peptidases are embedded in the plasma membrane or attached to the glycocalyx, contributing to localized digestion.
Dimensions and Arrangement
- Size:
Microvilli are extremely small, typically about 0.1 micrometers in diameter and 1–2 micrometers in length. - Density:
They are densely packed on the apical surface of the cell, forming structures like the brush border in the small intestine and kidney proximal tubules.
Variations
- Length and Density:
The length and number of microvilli vary depending on the cell type and its absorptive requirements.- Enterocytes in the small intestine have a dense array of microvilli to maximize nutrient absorption.
- Kidney tubule cells also exhibit microvilli for reabsorption of solutes and water.
Functions of Microvilli
Microvilli perform a range of essential functions that are vital to the proper functioning of various tissues and organs. Their primary role is to increase the surface area of cells, enabling more efficient absorption, secretion, and interaction with the surrounding environment.
Enhanced Absorption
Microvilli dramatically increase the surface area of the apical membrane of epithelial cells, facilitating the absorption of nutrients, water, and ions.
- Small Intestine: On enterocytes, microvilli form the brush border, which is essential for the absorption of carbohydrates, proteins, fats, and micronutrients.
- Kidneys: Microvilli on proximal tubule cells aid in the reabsorption of water, electrolytes, glucose, and amino acids from the filtrate.
Localization of Enzymes
Microvilli provide a surface for the attachment of digestive enzymes and transport proteins, which optimize the breakdown and uptake of nutrients.
- Enzymes such as disaccharidases (e.g., lactase, maltase) and peptidases are anchored in the glycocalyx, allowing localized digestion of carbohydrates and proteins directly at the site of absorption.
Secretion
Microvilli also play a role in secretion, facilitating the release of cellular products into the surrounding environment.
- Goblet Cells: In certain epithelial cells, microvilli aid in the secretion of mucus to lubricate and protect mucosal surfaces.
- Kidney Tubules: They participate in the secretion of substances involved in electrolyte and acid-base balance.
Signal Transduction
The plasma membrane of microvilli contains specialized receptors and ion channels that participate in signal transduction.
- These structures detect environmental changes, such as the presence of specific nutrients or mechanical stimuli, and initiate appropriate cellular responses.
- In sensory systems, such as in taste buds or auditory cells, microvilli are involved in detecting stimuli like taste or sound.
Defense and Barrier Function
The glycocalyx layer covering the microvilli acts as a protective barrier:
- Physical Protection: Shields the underlying epithelial cells from mechanical damage and harmful substances.
- Immune Defense: Traps and interacts with antigens, toxins, and microbes, contributing to the mucosal immune defense.
Maintenance of Cellular Homeostasis
Microvilli participate in regulating the cell’s interaction with its environment:
- They aid in the exchange of ions, water, and small molecules to maintain osmotic and ionic balance.
- They facilitate the removal of waste products from the epithelial surface.
Specialized Roles in Sensory Functions
In certain tissues, microvilli have adapted to perform specialized sensory functions:
- Taste Buds: Microvilli on taste receptor cells detect specific chemical stimuli, contributing to the sensation of taste.
- Auditory System: In the inner ear, microvilli-like structures called stereocilia detect sound vibrations and contribute to hearing.
Clinical Significance
Microvilli are essential for efficient absorption and cellular function, and their dysfunction or damage can lead to significant clinical conditions. Some examples include:
- Celiac Disease: Damage to the microvilli in the small intestine occurs due to an autoimmune response triggered by gluten. This leads to malabsorption of nutrients, causing symptoms like diarrhea, weight loss, and nutrient deficiencies.
- Microvillus Inclusion Disease: A rare genetic disorder where microvilli fail to form properly on intestinal cells, resulting in severe diarrhea and malnutrition from infancy.
- Infections: Certain pathogens, such as Escherichia coli (E. coli), produce toxins that damage microvilli, impairing nutrient absorption and causing gastrointestinal symptoms.
- Kidney Disorders: Damage to microvilli in renal tubules can impair reabsorption processes, contributing to conditions like renal tubular acidosis.
- Cancer: Alterations in microvilli structure are observed in some cancers, affecting cell communication and adhesion, which may promote metastasis.
The health of microvilli is critical for nutrient absorption and overall homeostasis, making them a key focus in the diagnosis and treatment of gastrointestinal, renal, and systemic diseases.
