Tight junctions: facts and models.

Tight junctions (TJ) are specialized membrane structures found in cell-cell contact areas where the membranes of the neighbouring cells come into a close proximity. TJs have a characteristic ultrastructure on thin cross-section and freeze-fracture images and are regarded one of the diagnostic features of epithelial and endothelial cells where TJs form a transepithelial/transendothelial paracellular filter permeable for molecules of a certain size and also separate apical and basolateral domains of the plasma membrane of an epithelial cell. Two molecular models of TJs seek to explain structural and functional properties of TJs, a lipid model and a protein model. Due to the recent advances in biochemistry and molecular biology of TJs the protein model is most widely accepted. This review compares once again the ability of the two models to explain the phenomenology of TJs. The data on TJ ultrastructure and distribution are recalled here, including the data on TJs in fibroblastic cells where TJs appear fragmented or focal. TJ functions are considered, among which may be an intercellular communication due to cell-to-cell diffusion of lipophilic molecules. TJ modulators are listed and their activity in respect of the lipid phase transitions is noted. The major advantages and drawbacks of the two molecular models of TJs are discussed and a "synthetic" version of the lipid and protein models, reconciling the achievements of them both, is suggested.