Tight junctions of the human corneal endothelium: morphological and electrophysiological features.

The corneal endothelium controls the hydration and nutrition of the avascular corneal stroma. To analyze the role of the tight junctions (TJ) for these functions, we examined human corneal endothelium by thin-section and freeze-fracture electron microscopy and by impedance analysis. On thin sections, tannic acid was seen to mark the external leaflet of the lateral plasma membrane also beyond the location of the TJ, indicating a significant macromolecular porosity of the TJ. On freeze-fracture images, the TJ surrounded the entire apicolateral plasma membrane but were found to be focally incomplete, suggesting a nonhomogeneous seal of the lateral intercellular space. Impedance analysis revealed a very leaky endothelial layer with a transendothelial resistance of 9.0 +/- 1.4 omega cm2. These findings are consistent with the hypothesized "pump-leak" model of the corneal endothelium: the TJ allow an effective dehydration of the corneal stroma, whereas the interruptions in the TJ network may be the morphological correlate for the passage of nutrients into the corneal stroma. Our data corroborate the assignment of the corneal endothelium to the group of "very leaky epithelia" that exhibit high transport rates of water and solutes against only minimal osmotic gradients.