Separation of distinct body, organ and tissue compartments, and maintenance of epithelial cell polarity require tight junctions (TJ)-cell-cell junctions located in the apicolateral regions of epithelial and endothelial cells. Studies on the protein components of vertebrate TJ have revealed an intricate network of membrane, sub-membrane, cytoskeletal, and signalling molecules. How these molecules functionally interact to provide TJ with their functions, and what roles these molecules play in control of cell growth and differentiation is a fundamental problem in cell biology.