<P> In human physiology there are two main types of epithelia using distinct types of barrier mechanism . Epidermal structures such as skin form a barrier from many layers of keratinized squamous cells . Internal epithelia on the other hand more often rely on tight junctions for their barrier function . This kind of barrier is mostly formed by only one or two layers of cells . It was long unclear whether tight cell junctions also play any role in the barrier function of the skin and similar external epithelia but recent research suggests that this is indeed the case . </P> <P> Epithelia are classed as "tight" or "leaky", depending on the ability of the tight junctions to prevent water and solute movement: </P> <Ul> <Li> Tight epithelia have tight junctions that prevent most movement between cells . Examples of tight epithelia include the distal convoluted tubule, the collecting duct of the nephron in the kidney, and the bile ducts ramifying through liver tissue . </Li> <Li> Leaky epithelia do not have these tight junctions, or have less complex tight junctions . For instance, the tight junction in the kidney proximal tubule, a very leaky epithelium, has only two to three junctional strands, and these strands exhibit infrequent large slit breaks . </Li> </Ul> <Li> Tight epithelia have tight junctions that prevent most movement between cells . Examples of tight epithelia include the distal convoluted tubule, the collecting duct of the nephron in the kidney, and the bile ducts ramifying through liver tissue . </Li>

Where are tight junctions found in the body