THE SEAL

Two ways across a cell layer

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Paracellular

Between neighboring cells, through the narrow gap that joins them

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Transcellular

Through a cell, either across its membrane or inside carriers and vesicles

In the brain, protein seals called tight junctions shut the paracellular route.

THE ZIPPER

Strands that lock cells together

Proteins on each cell reach across the gap and bind their partners on the neighboring cell, forming strands that act like a zipper.

Zip the gap shut, and nothing slips between cells.

THE PROTEINS

The parts list

Claudin-5 The key sealing protein in brain endothelium, pairing with partners on the neighboring cell

Occludin A junction protein that helps stabilize and tune the seal

ZO-1 A scaffold inside the cell that links the junction to the cell's internal skeleton

VE-cadherin Part of the neighboring adherens junction, which holds the cells mechanically together

Seal proteins plus scaffolds plus anchors make one strong junction.

THE EFFECT

What a good seal does

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Blocks traffic

Ions and many small polar molecules cannot slip between cells

⚡

Raises resistance

The sealed layer resists electrical current, which gives scientists a way to measure it

A tight barrier is both a chemical filter and an electrical resistor.

WHEN IT FAILS

Leaky junctions and how we spot them

Stroke, inflammation, infection, and several neurological diseases can loosen the junctions, and the barrier turns leaky. In a lab model, scientists stain for claudin-5 and ZO-1 and look for a continuous bright outline around every cell.

A continuous outline of junction proteins is the first sign of a good barrier in a dish.

UNIT 3 STUDY COMPLETE

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Ready for the Fab Challenge?

You've learned how tight junctions seal the gap between cells and how scientists check that they are intact.