On tight-junction structure.

We have analyzed previous thin-section and freeze-fracture observations of the tight junction. We propose that the tight-junction strands represent intramembranous, cylindrical, inverted micelles. At the junctional site, the exoplasmic halves of the plasma membranes are fused into a continuous leaflet. Therefore, topologically and structurally the tight junction is viewed as the outcome of a process of linear fusion between the plasma membranes of epithelial cells. The extracellular spaces delimited by the junction are separated by two distinct exoplasmic membrane halves and the cylindrical micelles. Junctional stability, fostered by the environmental symmetry of the cytoplasmic milieux of contiguous cells, may be maintained by transmembrane integral proteins at the junctional site, interacting at the cytoplasmic surface with cytoskeletal components.