brushborder

The brush border refers to the dense layer of microvilli on the apical surface of certain epithelial cells, most notably the absorptive enterocytes of the small intestine and the cells lining the proximal tubule of the kidney. This specialized surface area enhances the cell’s capacity to digest, absorb, and reabsorb nutrients and solutes.

Structure and composition: Microvilli are slender, actin-rich projections whose cores consist of parallel actin filaments cross-linked by proteins such as villin, fimbrin, and espin. They are anchored to a terminal web beneath the apical membrane, which includes a network of cytoskeletal proteins. The apical membrane hosts a range of brush border enzymes, including disaccharidases and peptidases, as well as transport proteins essential for nutrient uptake.

Function: The primary role of the brush border is to greatly increase surface area for efficient absorption and reabsorption. In the intestinal brush border, enzymes such as lactase and sucrase–isomaltase break down carbohydrates, while peptidases process peptides. Transporters within the membrane then absorb sugars, amino acids, and ions. In the kidney, the brush border on proximal tubular cells contributes to the reabsorption of filtered substances, aiding fluid and solute recovery.

Distribution and clinical significance: The intestinal brush border is most prominent in the small intestine, whereas a similar apical surface is present in the proximal tubule of the kidney. Damage or loss of the brush border—seen in conditions such as celiac disease, infectious gastroenteritis, microvillus disorders, or nephrotoxin exposure—reduces absorptive capacity and enzyme activity, contributing to malabsorption and impaired reabsorption.