Tight junctions and the epithelial barrier
Tight junctions are connections that join neighbouring epithelial cells (cells that cover surfaces and line tubes) and seal the space between them.
Remember
Tight junctions seal the space between neighbouring epithelial cells and limit what passes between them, so substances have to cross through the cells, where transporters control them.
Memory aid
tight junctions seal, desmosomes stick and gap junctions talk
Remember each junction by its job: tight junctions seal the space between cells, desmosomes anchor cells together and gap junctions let small substances pass between cells.
Cells in an epithelial sheet sit packed together and are joined by specialised connections called junctions. Each type of junction has its own job, and the tight junction is the one that forms the barrier.
Tight junctions sit at the apical end of the cells. Apical means the side facing the outside world or the lumen, the hollow space inside a tube or organ. There they close the gap between neighbours, so very little gets between the cells.
The narrow space between neighbouring cells is called the paracellular route. When tight junctions block it, very little can slip through that gap. Substances have to cross through the cells instead, using channels and transporters, which are membrane proteins that carry specific substances. Because the cell controls those proteins, the epithelium can regulate tightly what crosses it, as in the kidney nephron (the working unit of the kidney) and the gut. The junctions also regulate the paracellular route itself, controlling which ions and small solutes can pass between the cells.
Tight junctions also stop integral membrane proteins (proteins embedded in the membrane) from drifting between the apical surface and the sides of the cell, so the two surfaces stay different. If tight junctions fail, the sheet becomes leaky and loses its barrier function.
The other junctions do different jobs. Desmosomes anchor cells to one another and resist stretching and twisting. Gap junctions are channels between neighbouring cells that let ions and small molecules pass. Hemidesmosomes anchor the cell to the basement membrane, the thin protein layer beneath the epithelium. Adherens junctions connect neighbouring cells through their actin filaments, which are part of each cell's internal scaffold.
Key facts
- Where
- tight junctions sit at the apical end of epithelial cells, the side facing the outside world or the lumen.
- Main job
- they seal the space between neighbouring cells and limit what passes between them.
- Route taken
- substances have to cross through the cells, using channels and transporters that the cell controls.
- Cell surfaces
- they stop membrane proteins drifting between the top and side surfaces, which keeps the two surfaces different.
- Example
- the epithelium lining the nephron, where tight junctions make it possible to control tightly which ions pass across it.
- If they fail
- the epithelial sheet becomes leaky and loses its barrier function.
Exam traps
- Desmosomes hold epithelial cells together firmly, but they do not seal the gap against ions and molecules, so they are not the junction that limits passage between cells.
- Gap junctions are not a barrier. They form channels between cells, so they let ions and small molecules pass directly between cells rather than limiting passage.
- Hemidesmosomes anchor an epithelial cell to the basement membrane beneath it rather than to its neighbours, so they do not control what passes between cells.
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