microvili

Microvilli are microscopic, fingerlike projections from the apical surface of certain epithelial cells, notably enterocytes in the small intestine and proximal tubule cells in the kidney, forming the brush border that dramatically increases the surface area available for absorption and secretion. They occur in many epithelia, with density and length varying by tissue.

Each microvillus consists of a core bundle of actin filaments, typically 20–30 filaments, crosslinked by proteins such as fimbrin, fascin, and espin. The bundle is anchored at its base to the terminal web and connected to the apical plasma membrane by linker proteins from the ERM family (ezrin, radixin, moesin) and by myosin motors such as myosin-I. The apical membrane at the tip carries digestive enzymes and nutrient transporters essential for absorption.

The primary role of microvilli is to increase absorptive surface area to enable efficient uptake of nutrients and ions. Brush border enzymes such as lactase and sucrase-isomaltase digest disaccharides, and various transporters mediate the uptake of glucose, amino acids, and minerals during digestion.

Clinical relevance: Abnormalities or loss of microvilli are seen in certain diseases. Microvillus inclusion disease is a rare congenital disorder caused by defective trafficking of the microvillar membrane. Other enteropathies, including celiac disease, can cause brush border atrophy and malabsorption.