FRACTURE FACES OF ZONULAE OCCLUDENTES FROM "TIGHT" AND "LEAKY" EPITHELIA

TLDR

Epithelia differ in transepithelial permeability, with leaky epithelia allowing paracellular flux through zonula occludentes and tight epithelia exhibiting high resistance to this pathway. The study aimed to determine whether zonula occludens morphology contributes to epithelial leakiness by examining freeze‑fracture replicas from tight and leaky epithelia. Freeze‑fracture replicas of zonulae occludentes were examined, revealing branching networks of strands on A and B membrane faces, and morphological measurements were compared with published physiological data. The proximal convoluted tubule of mouse kidney displayed a shallow, single‑strand zonula, whereas the frog urinary bladder had a deep, multi‑strand zonula, with intermediate epithelia showing correspondingly intermediate or variable junctional morphology.

Abstract

Epithelia vary with respect to transepithelial permeability. In those that are considered "leaky", a large fraction of the passive transepithelial flux appears to follow the paracellular route, passing across the zonulae occludentes and moving down the intercellular clefts. In "tight" epithelia, the resistance of the paracellular pathway to passive flux is greatly increased. To see whether differences in the morphology of the zonula occludens could contribute to this variability in leakiness among epithelia, replicas of zonulae occludentes in freeze-fractured material from a variety of tight and leaky epithelia were examined. The junctions appear as a branching and anastomosing network of strands or grooves on the A and B membrane fracture faces, respectively. It was found that the zonula occludens from a "very leaky" epithelium, the proximal convoluted tubule of the mouse kidney, is extremely shallow in the apical-basal direction, consisting in most places of only one junctional strand. In contrast, the "very tight" frog urinary bladder exhibits a zonula occludens that is relatively deep (>0.5 microm) in the apical-basal direction, and consists of five or more interconnected junctional strands interposed between luminal and lateral membrane surfaces. Epithelia of intermediate permeabilities exhibited junctions with intermediate or variable morphology. Toad urinary bladder, mouse stomach, jejunum, and distal tubule, rabbit gallbladder, and Necturus kidney and gallbladder were also examined, and the morphological data from these epithelia were compared to physiological data from the literature.