Paracellular permeability of corneal and conjunctival epithelia

The paracellular permeability of normal rabbit cornea and conjunctiva was studied in vivo and in vitro. After intravenous administration, horseradish peroxidase was found to percolate to the intercellular space of conjunctival epithelia and was restricted by the tight junctions of the superficial epithelium. Only minimal tracer was present in the limbus and cornea. The difference between corneal and conjunctival paracellular pathway was further compared in vitro by tissue perfusion studies using various tracers from subepithelial space to apical surface. The intact full-thickness cornea was permeable to mannitol (MW 182) but not to inulin or dextran. The conjunctiva was permeable to mannitol, inulin and FITC-dextran (MW 20,000). The quantitative permeability to ³H-mannitol (X10^-8 cm/sec) of adult rabbit cornea was 0.12±0.02, which is about 55-fold and 50-fold lower than that of conjunctiva (6.78±0.21) and peritoneum (6.12±0.63), respectively. Removal of the corneal epithelium increased the permeability 40-fold; however, removal of the endothelium had little effect on the solute permeation. When both corneal epithelium and endothelium were debrided, the bare stroma became edematous and the permeability increased 70-fold. The permeability of 1-week-old rabbit cornea was 1.32±0.18, which decreased to 0.46±0.06 in 2-week-old rabbits, and became similar to the adult level at 4 weeks of age. When Tenon's capsule was included in the perfusion, the conjunctival permeability decreased 2.5-fold. With the apposition of bare corneal stroma to the conjuctiva and Tenon's capsule, the permeability decreased further (4-fold). These results confirm the notion that epithelium is the major limiting component of corneal and conjunctival permeability and further indicate that the solute permeation can be affected by corneal stromal hydration and by alteration of the subconjunctival tissue.