Intact Tight Junctions Seal the Intestinal Epithelium
Claudins and occludin zip adjacent cells into a selective barrier.
- 100%: Junction Integrity (fully sealed apical complex)
- 4–8: Claudin Strands (parallel sealing strands per junction)
- <1 nm: Paracellular Gap (smaller than a water cluster)
- High: Barrier Resistance (transepithelial resistance at peak)
Claudin-occludin protein lattice
Claudins polymerize into strands that fuse neighboring membranes shut.
Over 20 claudin subtypes tune pore size per tissue.
Apical junctional complex
Tight junctions sit above adherens junctions near the cell apex.
ZO scaffold anchoring
Zonula occludens proteins tether junction strands to the actin cytoskeleton.
Selective Permeability Through Regulated Pores
Small ions slip through claudin pores while large molecules stay blocked.
- ~4 Å: Pore Diameter (sized for ions and water only)
- >180 Da: Molecules Blocked (sugars, peptides, toxins excluded)
- Charge-based: Ion Selectivity (claudin-2 pores favor cations)
- Pore + Leak: Flux Pathway (two distinct paracellular routes)
Pore pathway vs leak pathway
Charge-selective pores pass ions; a separate leak route restricts size.
Claudin-2 vs claudin-1 balance
Pore-forming claudins compete with sealing claudins for junction space.
Claudin-2 upregulation alone can double paracellular water flux.
Physiological regulation
Hormones and nutrients fine-tune pore claudin expression continuously.
Inflammation and Toxins Trigger Junction Loosening
Cytokines and bacterial toxins signal cells to unzip their tight junctions.
- TNF-α, IL-13: Key Cytokines (drive junction disassembly signaling)
- Elevated: Zonulin Release (endogenous modulator opens junctions)
- Increased: MLCK Activity (myosin contraction pulls junctions apart)
- Minutes–Hours: Onset Time (rapid signal-driven remodeling)
Cytokine signaling cascade
TNF-α activates MLCK, contracting the perijunctional actin ring.
Zonulin reversibly opens junctions within minutes of release.
Bacterial toxin exploitation
Pathogens hijack zonulin signaling to force their own entry.
Claudin redistribution
Sealing claudins internalize while pore-forming claudins stay put.
The Paracellular Space Widens Beyond Normal Limits
Actomyosin contraction physically pulls the intercellular gap wide open.
- 10–50 nm: Gap Width (up from under 1 nm sealed)
- >10,000 Da: Cutoff Raised (large proteins now fit through)
- Multiple: Strand Breaks (claudin strand network fragmenting)
- >70%: Resistance Drop (transepithelial resistance collapse)
Cytoskeletal ring contraction
Perijunctional actomyosin rings pull cell borders apart mechanically.
Strand fragmentation
Continuous claudin strands break into discontinuous, leaky segments.
A gap of just 10 nm already admits mid-sized proteins.
Loss of size selectivity
Pore and leak pathways merge into one open channel.
Barrier Dysfunction Lets Bacterial Fragments Leak Through
Undigested proteins and bacterial fragments cross freely into the bloodstream.
- Elevated: LPS Translocation (endotoxin enters systemic circulation)
- >100,000 Da: Size Blocked (almost nothing excluded now)
- Triggered: Systemic Inflammation (immune activation from leaked antigens)
- Leaky Gut: Clinical Link (associated with autoimmune conditions)
Bacterial fragment translocation
Lipopolysaccharide fragments cross the ruptured barrier into vessels.
Circulating LPS can trigger low-grade inflammation body-wide.
Immune activation cascade
Leaked antigens provoke innate immune responses far from the gut.
Chronic disease associations
Sustained barrier dysfunction is linked to IBD and metabolic disease.