Tight-junction strand networks and tightness of the epithelial barrier.

Tight junctions are cell-cell junction structures critical for controlling paracellular permeability. On freeze-fracture replica electron microscopy, they appear as a continuous network of fibrils (tight-junction strands). Tight-junction strands function as zippers that create a physical barrier against paracellular diffusion of molecules. The morphology of the tight-junction strand network varies greatly between tissues and, in recent years, studies have highlighted the mechanisms regulating the morphology of tight-junction strand networks and on their relevance to barrier function. In this review, we discuss evidence regarding the components of the tight-junction strand and the mechanisms for creating the tight-junction strand network. Furthermore, we discuss and hypothesize how its morphology contributes to the establishment of the epithelial barrier.