Microvilli

Microvilli are thin, finger-like projections on the apical surface of epithelial cells that create a brush border appearance. They are predominantly found lining the small intestine, large intestine, kidney tubules, and certain cells in the respiratory and reproductive systems. These structures significantly enhance the cell's capacity to exchange materials with its surroundings.

Microvilli increase the plasma membrane's surface area approximately twenty-five-fold, allowing for greater nutrient and water absorption. The cell membrane covering microvilli contains various transport proteins and enzymes that assist in moving substances across the cell surface. This expanded surface area enables epithelial cells in the small intestine to absorb nutrients from digested food more effectively.

Microvilli are composed of a core of actin filaments that provide structural support. Bundling proteins like villin and fimbrin crosslink the actin filaments, promoting the formation of tightly bundled parallel arrays. These protein bundles orient the microvilli so they project outward from the apical cell surface in an organized manner.

In individuals with celiac disease, an immune response to gluten damages microvilli, hampering nutrient absorption. This damage results in malnutrition, cramping, and diarrhea. The destruction of these crucial structures prevents the intestinal epithelial cells from effectively absorbing nutrients from food.

Microvilli contribute to sensory perception in specialized cells such as hair cells in the inner ear. These microvilli help detect mechanical stimuli and convert them into electrical signals that transmit to the nervous system. This sensory function demonstrates that microvilli serve purposes beyond absorption and secretion.

Numerous mitochondria are positioned at the basal ends of cells with microvilli, providing the energy required for transport processes. These organelles generate ATP to power the active transport of substances across the cell membrane through transport proteins and enzymes located on the microvillus surface.

Microvilli are specialized structures that enhance cell functionality by increasing surface area and enabling cells to interact more effectively with their environment. Different cell types exhibit microvilli in varying densities and configurations, contributing to cell diversity across tissues like the intestine, kidney, and inner ear.