Tight junction

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Tight junctions are the seals that connect neighboring epithelial cells and block most water and solutes from slipping between them. They can also act as small, selective channels for tiny ions or water in some situations.

What they’re made of
- Tight junctions are built from many strands of transmembrane proteins that span both cell membranes and meet outside the cells.
- There are about 40 different proteins in these junctions, with the main players being occludin, claudins, and JAM proteins.
- Inside the cell, these proteins connect to ZO-1 and the actin cytoskeleton, anchoring the junction to the cell’s structure.

How they’re studied
- Freeze-fracture electron microscopy helps reveal the extent and arrangement of tight junctions in membranes.

What they do
- Barrier and gatekeeper: they prevent most substances from passing between cells, so materials must enter cells to cross tissue barriers.
- Polarity support: by blocking lateral diffusion of proteins, they help maintain the distinct apical (top) and basolateral (bottom/sides) surfaces of cells, enabling specialized functions like targeted transport and secretion.
- Signaling roles: tight junction proteins also influence cell behavior, including differentiation, movement, and growth. This makes them an important area of cancer research.

Epithelia status
- Tightly sealed tissues are “tight”; leakier tissues have looser junctions.

Other notes
- Some viruses, like herpes simplex, can spread by crossing tight junctions to evade immune defenses.
- In the kidney, a model suggests paracellular transport may involve a combination of large gaps and many tiny pores.
- Invertebrates use septate junctions instead of tight junctions.

Summary: Tight junctions are crucial multiprotein structures that seal cells together, regulate what passes between them, help maintain cell polarity, and participate in cellular signaling.