Tight Junctions and Epithelial Barrier Function
Epithelia separate compartments. They must stop free mixing of water, ions, and microbes, yet still allow selected transport. Tight junctions seal the paracellular space between neighboring cells. That seal is the core of barrier function.
The junction sits at the apical end of the lateral membrane. Claudins form the main strands that contact the next cell. Occludin and other MARVEL proteins help organize the strands. Zonula occludens proteins link the complex to the actin cytoskeleton. The belt can therefore change with cell shape and tension.
Barrier tightness is not the same in every tissue. A bladder epithelium is tighter than a small-intestine epithelium. Claudin composition explains much of that difference. Some claudins make cation pores. Others tighten the seal. The same structure can act as a barrier in one organ and a selective channel in another.
Two paths cross an epithelium. The transcellular path uses pumps and channels through the cell. The paracellular path uses the junction. When tight junctions weaken, leak rises. Antigens, bacteria, and solutes then move between cells. Inflammation can follow.
The gut is a clear example. Food, microbes, and immune cells meet at the lining. Cytokines can open junctions. A wider paracellular path is linked to diarrhea and chronic inflammation. The kidney is another example. Thick ascending-limb claudins recover ions. Mutations disturb salt handling.
The blood–brain barrier uses extreme tightness. Endothelial tight junctions limit plasma proteins and many drugs. That protection is useful. It also complicates therapy.
Barrier function is dynamic. Calcium, phosphorylation, and motor proteins remodel strands within minutes. Growth and repair require temporary opening. Permanent opening is injury. Physiologists therefore measure both resistance and permeability, not only histology.
Common assays include transepithelial electrical resistance and tracer flux. Live imaging can show strand breaks. Knockout models link a single claudin to a clinical phenotype. Those tools separate a structural gap from a transport-protein defect.
Tight junctions therefore make an epithelium more than a sheet of cells. They set how leaky the paracellular route is. Composition, signaling, and the cytoskeleton decide whether the barrier holds. When the seal fails, compartment identity fails with it.