Paracellular route of aqueous outflow in the trabecular meshwork and canal of Schlemm. A freeze-fracture study of the endothelial junctions in the sclerocorneal angel of the macaque monkey eye.

The intercellular junctions of the endothelial cells of the trabecular meshwork and canal of Schlemm were examined with the electron microscope in the macaque monkey eye by both thin-sectioned specimens and the freeze-fracturing technique. The endothelial cells that line the beams of the meshwork are joined by gap junctions and short, isolated strands of tight junction; zonulae occludentes are absent. Thus aqueous humor can freely traverse the patent endothelial clefts of the trabecular meshwork. The endothelial cells of the canal of Schlemm are joined by zonulae occludentes and a small number of minute gap junctions. In 57% of their length, the tight junctions consist of one or two strands; the strands are rarely more than four. They remain preferentially associated with the E-face of the membrane, run parallel to one another, and only exceptionally branch or anastomose. Thus they are provided with free endings and do not form a bidimensional network. As a result of this organization, the zonula occludens is traversed by meandering channels of extracellular space or split pores, which connect the open endothelial clefts on the luminal and tissue fronts of the junction. The frequency of slit pores is 0.134 per micrometer of zonula occludens. They occupy 0.87% of the intercellular boundary and 0.0015% of the area of the endothelium. Estimates of the fluid conductance of the zonulae occludentes indicate that the intercellular clefts of the endothelium of Schlemm's canal filter but a small fraction of the amount of aqueous humor that leaves the anterior chamber through the conventional route.