Collagen fibril characteristics at the corneo‐scleral boundary and rabbit corneal stromal swelling

Abstract

Background: The aim of this investigation was to reassess the impact of the scleral rim on the swelling of the mammalian corneal stroma and to investigate the ultrastructural features of the scleral rim and corneal stromal tissues. Methods: The epithelium and endothelium were removed from corneas excised from three‐month‐old female rabbits. The resulting preparations consisted of the corneal stroma plus a surrounding scleral rim, excised corneal stroma or a nine‐millimetre button of central corneal stroma. These preparations were immersed in a 35 mM bicarbonatebuffered mixed salt solution (equilibrated with five per cent CO2‐air, pH 7.54 at 37 degrees Centigrade for nine hours. Some sclero‐corneal preparations were fixed for light or transmission electron microscopy. Results: The initial rate of swelling of corneal stromal buttons was greatest at 127 ± 8 per cent per hour, less for complete stromal preparations (118 ± 9 per cent per hour) and least for sclerostromal preparations (76 ± 12 per cent per hour). The swelling continued over three to nine hours but sclera preparations swelled up to 40 per cent with no further swelling. Light microscopy demonstrated that the sclero‐corneal rim tissue limited the swelling of the posterior corneal stroma. TEM sections of the episclera and sclera indicate that most fibril bundles show a radial orientation to the cornea. There are marked anterio‐posterior differences in the collagen fibrils of the scleral surround that are distinctly different from previous reports. Average fibril diameters were 62.8 ± 7.9 nm in the episclera, 122.4 ± 18.9 nm, 133.5 ± 51.9 nm and 56.5 ± 11.2 nm in the anterior, mid‐ and posterior scleral stroma, compared to an average fibril diameter of 33.5 ± 3.5 nm for the posterior corneal stroma. Conclusions: When there is a scleral rim in place, the swelling of the corneal stroma is substantially less than for isolated corneal stroma. The effect can be attributed to the a b sence of a cut‐edge effect for the sclero‐corneal stromal preparation but the unique and largely radial arrangement of the collagen fibrils in the scleral rim plays a part in limiting the swelling of the adjacent corneal stroma. The heterogeneous nature of this sclero‐corneal interface requires further investigation to define the mechanism of the effect.