A brush border (or striated border) is the name for the microvilli-covered surface of pseudostratified columnar epithelium and simple columnar epithelium found in multiple locations of the body. Microvilli are approximately 100 nanometers in diameter and their length varies from approximately 100 to 2,000 nanometers in length. Because individual microvilli are so small and are tightly packed in the brush border, individual microvilli can only be resolved using electron microscopes; with a light microscope they can usually only be seen collectively as a fuzzy fringe at the surface of the epithelium. This fuzzy appearance gave rise to the term brush border, as early anatomists noted that this structure appeared very much like the bristles of a paintbrush.
They are found in two main locations:
- The small intestine tract: This is where absorption takes place. The brush borders of the intestinal lining are the site of terminal carbohydrate digestions. The microvilli which constitute the brush border have enzymes for this final part of digestion anchored into their apical plasma membrane as integral membrane proteins. These enzymes are found near to the transporters which will then allow absorption of the digested nutrients.
- The kidney: Here the brush border is useful in distinguishing the proximal tubule (which possesses the brush border) from the distal tubule (which does not).