Tight Junctions and Cell Boundaries
Tight junctions are the most apical of the epithelial junctions, forming a belt-like seal that fuses the outer leaflets of adjacent plasma membranes near the top of the lateral cell border. They define the boundary between the apical and basolateral cell surfaces and regulate what can pass between cells, making them the structural basis of selective epithelial and endothelial barriers.
Definition
A tight junction (zonula occludens) is an occluding cell junction in which integral membrane proteins of adjacent cells form a continuous, anastomosing network of sealing strands that limits the diffusion of solutes through the paracellular space and separates the apical from the basolateral membrane domain.
Scope
This topic covers the structure of tight junctions as occluding junctions, their two principal functions (the paracellular barrier and the apical-basolateral fence), the main protein families that build them (claudins, occludin, and zonula occludens scaffolds), and their role in defining cell boundaries in epithelia and endothelia. It is a reference and educational entry, not clinical guidance.
Key concepts
- Occluding junction / zonula occludens
- Paracellular barrier
- Apical-basolateral fence function
- Claudins
- Occludin
- Zonula occludens (ZO) scaffold proteins
- Selective paracellular permeability
- Blood-brain and blood-tissue barriers
Mechanisms
Tight junctions are built from rows of transmembrane proteins—chiefly claudins and occludin—whose extracellular loops from neighbouring cells meet to form continuous sealing strands; cytoplasmic zonula occludens (ZO) scaffold proteins link these strands to the actin cytoskeleton. The resulting seal performs two functions: a barrier function that restricts movement of ions and molecules through the space between cells, and a fence function that prevents membrane proteins and lipids from diffusing between the apical and basolateral domains, thereby maintaining cell polarity. The barrier is not absolute but selectively permeable, and claudin composition tunes which ions cross; loss of specific claudins, as shown for claudin-5 at the blood-brain barrier, loosens the seal in a size-selective way.
Clinical relevance
Tight junctions underlie the barrier properties of epithelia and endothelia, including the intestinal and blood-brain barriers, so their structure and regulation are central to understanding how tissues compartmentalise their environment. This entry describes mechanism for reference; it does not provide diagnostic or therapeutic recommendations.
History
Tight junctions were first resolved as the most apical element of the epithelial junctional complex by Farquhar and Palade (1963). The molecular era began when the integral proteins occludin and then the claudin family were identified, recasting the junction as a defined protein assembly; claudin-knockout studies, such as the claudin-5 work of Nitta and colleagues (2003), then linked individual proteins to specific barrier properties.
Key figures
- Mikio Furuse
- Shoichiro Tsukita
- James Anderson
- Marilyn Farquhar
- George Palade
Related topics
Seminal works
- farquhar-palade-1963
- anderson-vanitallie-2009
- furuse-2010
Frequently asked questions
- What does a tight junction do?
- It seals the space between adjacent epithelial or endothelial cells to control paracellular passage of molecules, and it acts as a fence that keeps apical and basolateral membrane components separate, maintaining cell polarity.
- Which proteins form tight junctions?
- The sealing strands are built mainly from claudins and occludin, which are linked inside the cell to zonula occludens (ZO) scaffold proteins that connect the junction to the actin cytoskeleton.