The study by Claude and Goodenough examines the ultrastructural differences in zonulae occludentes (tight junctions) between "tight" and "leaky" epithelia. The authors use freeze-fracture replicas to analyze the morphology of these junctions in various epithelial tissues, including the mouse kidney, stomach, jejunum, rabbit gallbladder, Necturus kidney, and toad urinary bladder. They find that tight epithelia, such as the frog and toad urinary bladders, have deep zonulae occludentes with complex networks of interconnected strands, while leaky epithelia, like the mouse proximal convoluted tubule, have shallower and simpler zonulae with fewer strands. The number of strands in a junction appears to be more important for determining transepithelial permeability than the total junctional depth. The study also discusses the limitations of freeze-fracturing techniques and the need to consider geometrical and chemical variations when interpreting transepithelial permeability measurements.The study by Claude and Goodenough examines the ultrastructural differences in zonulae occludentes (tight junctions) between "tight" and "leaky" epithelia. The authors use freeze-fracture replicas to analyze the morphology of these junctions in various epithelial tissues, including the mouse kidney, stomach, jejunum, rabbit gallbladder, Necturus kidney, and toad urinary bladder. They find that tight epithelia, such as the frog and toad urinary bladders, have deep zonulae occludentes with complex networks of interconnected strands, while leaky epithelia, like the mouse proximal convoluted tubule, have shallower and simpler zonulae with fewer strands. The number of strands in a junction appears to be more important for determining transepithelial permeability than the total junctional depth. The study also discusses the limitations of freeze-fracturing techniques and the need to consider geometrical and chemical variations when interpreting transepithelial permeability measurements.