Tight junctions (TJs) are morphologically distinct and highly regulated areas of close contact between the plasma membranes of neighbouring epithelial and endothelial cells. They are the most apical component of the epithelial junctional complex (which also includes adherens junctions and desmosomes – see poster) and are connected to the actin cytoskeleton. In freeze-fracture electron microscopy, TJs appear as intramembrane strands that form a net-like meshwork that encircles cells (see poster). TJs contribute to the formation of polarised epithelial and endothelial barriers by controlling the extent and selectivity of diffusion along the paracellular pathway (the gate function) and by forming an apical/basolateral intramembrane diffusion barrier in the outer leaflet of the plasma membrane (the fence function) (Cereijido et al., 2008). Exciting research in recent years has also linked TJs to intracellular signalling mechanisms that guide epithelial-cell proliferation, polarisation and differentiation. Several types of protein components of TJs have been...

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