Tight-junction strand networks and tightness of the epithelial barrier

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