Tight junctions in the lens epithelia of human and frog: freeze-fracture and protein tracer studies.

For the first time, the existence of zonulae occludentes in the lens epithelia of human and frog has been demonstrated, using a "double mounting" method in freeze-fracture transmission electron microscope (TEM). The physiologic barrier function of zonulae occludentes in frog lens epithelium is determined by a "wash out" procedure in the protein tracer studies. It was found that within various time intervals of washing, horseradish peroxidase (HRP) was consistently restricted to the location of membrane fusions (zonulae occludentes), as seen with thin-section TEM. This corresponds to the location of zonulae occludentes found in the freeze-fracture studies. Thus, these data strongly suggest that there are zonulae occludentes in the frog lens epithelium and that these structures do provide a barrier function for the transepithelial diffusion of HRP with a molecular weight of 40,000 daltons.