Mikrovilli

Mikrovilli are microscopic, finger-like projections on the apical surface of certain epithelial cells, most notably enterocytes in the small intestine and cells of the proximal tubules in the kidney. They form a brush border that dramatically increases the cell’s surface area, enhancing both absorption and secretion. They are typically non-motile and structured as uniform, densely packed extensions of the plasma membrane.

Each mikrovillus contains a core bundle of actin filaments. This core is crosslinked by actin-binding proteins such as fimbrin (plastin), villin, and espin, which help maintain stiffness and shape. The actin bundle is anchored to the underlying cytoskeleton by linker proteins like ezrin and connections to the terminal web, a dense network of actin filaments and associated proteins at the cell’s apical region. At the base, the microvilli attach to the terminal web, providing stability and coordinated organization across the cell surface.

The apical membrane of mikrovilli bears a glycocalyx rich in enzymes and carbohydrate residues. A key function of the brush border is hosting brush border enzymes (such as lactase, sucrase-isomaltase, and various peptidases) that complete the final steps of nutrient digestion and facilitate efficient uptake of monosaccharides, amino acids, and other nutrients.

Clinical relevance includes conditions that damage or shorten microvilli, leading to malabsorption, as seen in certain enteropathies and celiac disease. Genetic defects affecting motor or structural components (for example, myosin proteins) can impair microvillar maintenance, contributing to disorders like microvillus inclusion disease.