Epithelial cell polarity as reflected in enterocytes

Absorptive cells are the main cells present, in the intestinal epithelium. The plasma membrane of these tall columnar cells reflects their high degree of polarization, by dividing into apical and basolateral domains with diff erent compositions. The most characteristic structure of these cells consis ts of closely packed apical microvilli with the same height, looking like a brush, which is why they were named the brush border. The concentrated pat tern of some apical markers observed in a restricted brush border domain sh ows that mature enterocytes are hyperpolarized epithelial cells: the filame ntous brush border glycocalyx is anchored at the top of the microvilli and the annexin XIII is concentrated in the lower three fourths. Many studies h ave been carried out on the biosynthesis and intracellular pathway of domai n markers. The results show clearly that the basolateral markers take a dir ect pathway from the trans-Golgi network to the basolateral membrane. Howev er, the two apical pathways, one direct and one indirect pathway via the ba solateral membrane, are used, depending on the apical protein involved. Eff icient protein sorting and addressing are essential to the establishment an d maintenance of cell polarity, on which the integrity of the epithelial ba rrier depends. Microsc. Res. Tech. 49:353-362, 2000. (C) 2000 Wiley-Liss. I nc.