Diverse types of junctions containing tight junction proteins in stratified mammalian epithelia

Molecular compositions and functions of tight junctions (TJs), that is, continuous, cell-cell-connecting zonulae occludentes serving as barrier structures for the paracellular transport of molecules and particles, have hitherto been determined for simple epithelia and for endothelia. In 2002, special TJ structures with barrier functions were identified in the stratum granulosum of mammalian epidermis. In addition, using biochemical and immunocytochemical methods, various types of TJ-type junctions have also been described that also contain claudins and/or occludin as well as typical TJ plaque proteins, in cell layers of all stratified squamous epithelia (e.g., various types of epidermis, gingiva, lingual, and other kinds of oral mucosa, pharynx, esophagus, trachea, vagina, and exocervix), including tissues without a lumen, such as the reticulum and Hassall corpuscles of the thymus, and tumors derived from such epithelia, notably squamous cell carcinomas. Biological and pathological aspects of TJ-related structures in such tissues are discussed.