Transmission electron micrograph of a section close to the luminal surface of the rat intestinal epithelium showing a tight junction (zonula occludens) extending diagonally across the image from upper left to lower right. The triple layer structure of the epithelial cell membrane (total thickness 110 A) is visible along the free cell margin and the microvilli, which are mostly cut in cross section. Image corresponds to Fig. 2 in J. Cell Biol. 17:375-412. Image made available by James D. Jamieson and the Department of Cell Biology, Yale University School of Medicine.