Glycosaminoglycans and collagen fibril organization in corneal tissue

Hypothesis: The hypothesis of this research is that changes in the sulphation patterns of the glycosaminoglycans are directly related to changes in ultrastructure, and hence transparency of the cornea. Aims: The aims of this research were to investigate glycosaminoglycan sulphation patterns and collagen fibril ultrastructure from central to peripheral regions of the cornea, and to gain a greater understanding in the effects of keratan sulphate and its reliance on oxygen supply. Methods: The bovine corneal extracellular matrix composition and collagen fibril parameters (fibril diameter and interfibrillar spacing) were biochemically and biophysically evaluated. This involved taking measurements of corneal thickness and hydration, as well as the amount of hydroxyproline and sulphated glycosaminoglycan. Immunolocalization of proteoglycan protein cores (lumican and keratocan) and specific glycosaminoglycans, particularly their sulphation distribution were studied using specific antibodies. Sulphation patterns of keratan sulphate were also quantified using specific antibodies. Transmission electron microscopy coupled with synchrotron small angle x-ray fibre diffraction was also employed to gain a greater understanding of the corneas' collagen fibril architecture and its interaction with glycosaminoglycans across the depths of the cornea. Results: The bovine cornea is thicker in the outer peripheral regions of the cornea and accordingly an increased amount of hydroxyproline is found at this region of the tissue. Keratan sulphate is predominantly found in the bovine cornea and is particularly heavily sulphated across the cornea. The degree of sulphation of keratan sulphate decreases at the outer peripheral regions of the cornea, which, interestingly, is where a transition of collagen fibrils occurs in that fibrils become less uniformly arranged, changes in fibril diameter are seen, and interfibrillar spacing values alter. Depth- profiled synchrotron microbeam analyses show that at different radial positions, from the corneal centre outwards, fibril diameter is greater superficially than in deep stromal regions. This does not include Bowman's layer which is below the spatial resolution of the analysis. The mid-depth stroma has higher interfibrillar spacing than is seen in posterior regions of the stroma, where fibril spacing appeared more compact. Previous work has pointed to a link between glycosaminoglycan content and oxygen availability. Work presented here indicates that in rabbit corneas, after 24 hr in 2% atmospheric O2, the glycosaminoglycan sulphation pattern changes significantly, with a significant increase of the high sulphated epitope of keratan sulphate. Conclusion: My data reveal that collagen fibrils in the central regions of the cornea are more closely packed and uniform in diameter than those in the outer peripheral cornea, and this may have potential implications for the transparency of the tissue. Stromal architecture is likely governed by sulphated proteoglycans, and changes in the types and sulphation patterns of corneal glycosaminoglycans from the corneal centre to periphery might be linked to differences in collagen ultrastructure. Moreover, the findings of differences in collagen fibril ultrastructure with depth through the cornea are possibly linked to biochemical alterations in proteoglycans. Finally, it is hypothesised that detrimental conditions, such as hypoxia after contact lens wear, might have an effect on the type and sulphation status of glycosaminoglycan synthesized and in vitro evidence for this is presented and discussed. (Abstract shortened by UMI.