Choroidal endothelial junctions in primates.

Junctional ultrastructure of endothelial cells of the choroid was studied with freeze-fracture electron microscopy. A network of multi-stranded linear aggregates was found on arterial endothelial membrane E faces in association with the apposing arterial membrane P face strands. The complexity of the junctional strands decreased as vessel diameter decreased. The choriocapillaris showed staggered junctional strands exclusively on the membrane P face. Junctions of venules and veins were represented by plasmalemmal folds with sparse intramembrane particles on the P face. Freeze-fracture cytochemistry with the membrane probe, filipin, revealed two dissimilar membrane domains: one, an area of membrane fluidity at the junctional strands; and the other, identified by the incorporation of cholesterol into the membrane lipid bilayer, a stable membrane domain. The latter was present throughout endothelial membranes, but was especially prominent on the rims of fenestrations of the choriocapillaris.