Introductory remarks on the brush border

The specialized surface of the luminal border of small intestinal epithelial cells was first described by J. Henle in 1837 and for many years controversy raged about the nature of this region. Was it a plate rendered porous by perforating canals or was it composed of an array of rods with their long axes normal to the surface? Because the diameter of the microvilli was below the limits of resolution by optical microscopy, the arguments could not be settled until the region had been observed under the electron microscope in 1950. In 1961, the brush border membrane (also known as the free border or microvillus membrane) was separated for biochemical study and results on transport using vesicles were first described in 1973. The increase in surface area due to the microvilli is about 40 times and the surface: volume 'ratio' for a single microvillus is 4 X 10(5) cm-1, about 20 times that for an erythrocyte. An important but unresolved question concerns the relationships between transporter proteins in the membrane and proteins that have digestive functions; the physiological role of the glycocalyx is not yet resolved.