Purpose: The aim of this study was to determine the structure and complexity of the intercellular junctions between trabecular endothelial cells and Schlemm's canal endothelial (SCE) cells as they exist in the normal human eye. Despite the probable relevance of these junctions to aqueous outflow, examination of these junctions has been limited to monkey eyes.
Methods: Human eyes (< 24 hours after death) were fixed by immersion in modified Karnovsky's fluid. Radially oriented tissue-chopper sections (190 microns) were trimmed to contain only the trabecular meshwork, Schlemm's canal, and a narrow strip of the sclera. Specimens were processed for conventional electron microscopy and freeze-fracture. Replicas were produced in a freeze-fracture apparatus operated at -115 degrees C and 10(-7) torr. Thin sections were stained with uranyl acetate and lead citrate. Micrographs were taken on a transmission electron microscope.
Results: The trabecular endothelial cells were joined by gap junctions and short discontinuous junctional strands that partitioned predominantly with P-face. The strand pattern varied from short and isolated undulating strands to radiating arrays of short junctional strands. No continuous zonulae occludentes were observed. The SCE cells were joined by continuous tight junctions (tj) composed of discontinuous strands that rarely branched or anastomosed. The tj strands fractured preferentially with the E-face, where they were positioned at the bases of shallow valleys. On the P-face, a complementary system of shallow ridges was observed with few particles at their crests. Often, only a single continuous strand was present for long distances, with occasional short discontinuous strands running parallel to the single strand. Less often, short lengths of remarkably complex junctions, which occasionally branched or anastomosed, were also encountered. When more than one strand was present, numerous free endings and transjunctional pathways, described in monkey eyes as "slit-pores," were evident.
Conclusion: The overall structure of the junctions between endothelial cells of Schlemm's canal in the human eye was more complex, and thus less permeable, than that reported in the monkey eye. The role of slit-pores and other junctions of SCE and trabecular endothelial cells will require further investigation under conditions of flow and in glaucoma to determine if these junctions change in a manner that might influence outflow resistance.