TIGHT JUNCTIONS OF THE HUMAN CILIARY EPITHELIUM - REGIONAL MORPHOLOGYAND IMPLICATIONS ON TRANSEPITHELIAL RESISTANCE

The tight junctions of the human ciliary epithelium have been studied using the freeze-fracture technique with particular regard to regional differences in the tight junction networks and implications on the tr ansepithelial resistance. The tight junctions of the non-pigmented epi thelium always form continuous networks and consist of from two to mor e than 20 superimposed strands (mean 3 to 4), suggesting that relative ly tight and leaky sites of the paracellular pathway coexist within th e same cell perimeter. The tight junction morphology is more complex i n the anterior pars plicata than in the posterior pars plicata, but it s complexity increases again towards the pars plana. The application o f an electrical circuit analysis to morphometric freeze-fracture data suggests that the transepithelial resistance decreases from the anteri or pars plicata (32 Ohm cm(2)) towards the posterior pars plicata (26 Ohm cm(2)) and becomes lowest in the pars plana (19 Ohm cm(2)) and tha t the transepithelial resistance of the native epithelium is lower tha n most reported calculations based on electrophysiological measurement s. We conclude that the human ciliary epithelium is a leaky layer, but that the anterior ciliary processes are less leaky than the posterior processes and the pars plana despite the occurrence of the breakdown of the blood-aqueous barrier predominantly in the anterior pars plicat a.