Microvilli

Microvilli are microscopic, finger-like projections of the plasma membrane that extend from the surface of many types of cells. They are especially prominent on absorptive epithelial cells, such as enterocytes lining the small intestine. By increasing the amount of cell membrane exposed to the surrounding environment, microvilli provide more surface area for processes such as nutrient and solute absorption .^1,2

Each microvillus contains a central bundle of actin filaments surrounded by plasma membrane. Proteins that bundle the actin filaments and connect them to the membrane help maintain the projection's shape and mechanical stability. In intestinal enterocytes, thousands of closely packed microvilli cover the apical, or lumen-facing, surface of each cell. Together, this organized array forms the brush border, which contains membrane-associated enzymes and transport proteins involved in digestion and nutrient uptake .^1,2

Microvilli are distinct from intestinal villi. A villus is a much larger, multicellular projection of the intestinal lining that contains epithelial cells, connective tissue, blood vessels, and a lymphatic vessel. A microvillus, by contrast, is a subcellular projection of the membrane of a single cell. Individual intestinal villi are therefore covered by epithelial cells whose apical surfaces bear thousands of microvilli, creating two different levels of surface-area enlargement .^1,2