Microvillus

Microvilli are microscopic, finger-like projections of the plasma membrane that extend from the apical surface of certain epithelial cells, most prominently intestinal enterocytes and proximal tubular cells in the kidney. They form the brush border, greatly increasing the cell’s surface area to enhance nutrient absorption and digestion. Each microvillus is about 0.5 to 1.0 micrometers long and exhibits a core bundle of actin filaments that run along its length.

The actin core is cross-linked by actin-binding proteins such as fimbrin, villin, and espin, and is anchored at the base to a structure known as the terminal web, a network of intermediate filaments and other cytoskeletal components. The apical membranes of microvilli display a dense glycocalyx containing brush border enzymes, including disaccharidases like lactase and sucrase-isomaltase, as well as other hydrolases, which facilitate extracellular digestion.

Functionally, microvilli increase the absorptive surface area and host membrane-bound enzymes that complete nutrient digestion at the cell surface. They also contribute to the stability and turnover of the apical membrane through associated cytoskeletal and motor proteins, such as myosin-1 family members, which help anchor the membrane and regulate surface dynamics.

Clinical relevance includes disorders affecting microvilli structure or trafficking. Mutations in trafficking-related proteins, notably MYO5B, can cause microvillus inclusion disease, a severe congenital diarrheal disorder marked by malabsorption. Damage to the brush border from diseases like celiac disease or infections can similarly reduce absorptive capacity.