The study examines the morphology of zonulae occludentes in tight and leaky epithelia using freeze-fracture techniques. It finds that the depth and complexity of these junctions correlate with transepithelial permeability. In "very tight" epithelia, such as the frog urinary bladder, the zonulae occludentes are deep and complex, consisting of multiple interconnected strands. In contrast, "very leaky" epithelia, like the mouse proximal convoluted tubule, have shallow, simple junctions. Intermediate permeability epithelia show intermediate junctional morphology. The study also notes that the number of strands and their arrangement are more important than overall junctional depth in determining permeability. Freeze-fracture provides a clearer view of the junctional structure compared to thin sections. The research highlights the importance of junctional morphology in determining transepithelial permeability and suggests that variations in junctional structure can explain differences in epithelial tightness. The study also discusses the limitations of current measurement techniques and the need for further research to fully understand the relationship between junctional morphology and permeability.The study examines the morphology of zonulae occludentes in tight and leaky epithelia using freeze-fracture techniques. It finds that the depth and complexity of these junctions correlate with transepithelial permeability. In "very tight" epithelia, such as the frog urinary bladder, the zonulae occludentes are deep and complex, consisting of multiple interconnected strands. In contrast, "very leaky" epithelia, like the mouse proximal convoluted tubule, have shallow, simple junctions. Intermediate permeability epithelia show intermediate junctional morphology. The study also notes that the number of strands and their arrangement are more important than overall junctional depth in determining permeability. Freeze-fracture provides a clearer view of the junctional structure compared to thin sections. The research highlights the importance of junctional morphology in determining transepithelial permeability and suggests that variations in junctional structure can explain differences in epithelial tightness. The study also discusses the limitations of current measurement techniques and the need for further research to fully understand the relationship between junctional morphology and permeability.