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Structural and biochemical investigations of the cornea and the trabecular meshwork

Koudouna, Elena ORCID: 2013. Structural and biochemical investigations of the cornea and the trabecular meshwork. PhD Thesis, Cardiff University.
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The experiments which comprise this thesis focused on structure-function relationships in two distinct collagen-rich connective tissues of the eye, the cornea and the trabecular meshwork. The cornea is the transparent tissue that covers the front of the eye and has the ability to transmit and refract light. Corneal transparency is the result of the unique organisation of collagen fibrils in the corneal stroma matrix, of which sulphated proteoglycans are key regulators, owing to the presumed importance of the sulfation pattern of corneal proteoglycans. The trabecular meshwork is the sponge-like tissue located around the cornea through which the bulk of the aqueous humor flows towards the juxtacanalicular tissue and the inner wall of Schlemm’s canal to exit the eye and control intraocular pressure. First part of the current research examined the chemical composition and sulphur speciation during corneal embryogenesis in order to elucidate important changes in the biochemical signature of the corneal matrix associated with the acquisition of transparency. It also investigated the content and distribution of distinct sulphur species through the depth of the mature corneal stroma and assessed biochemical-functional relationships that ultimate render tissue transparency. The research also studied the three-dimensional ultrastructure of the human trabecular meshwork, particularly the ultrastructure of the juxtacanalicular tissue that lies adjacent to the inner wall of Schlemm’s canal and the three-dimensional assembly of collagen type VI in the trabecular meshwork itself. X-ray fluorescence microscopy revealed key differences in the chemical composition of the cornea of the developing chick. In particular, the chemical signature of phosphorus, chlorine, sulphur, potassium and calcium were observably different during the developmental period from day 12 to day 16. S k-edge x-ray near edge structure spectroscopy showed that the main sulphur species present in the embryonic cornea were thiols, organic monosulfides, ester sulphate and inorganic sulphate. The chemical signature of these sulphur species was also noticeably different during embryonic corneal development. The changes in the chemical signature of phosphorus with development are believed to underline changes in the presumptive keratocyte population within the embryonic corneal stroma. Chlorine, potassium and calcium are important elements involved in the regulation and balance of the net negative or positive charge of the embryonic cornea and may influence the interactions of corneal matrix molecules. The changes in the sulphur speciation character amongst different developmental corneas are associated with changes in the sulphation status of corneal proteoglycans which play a fundamental role in governing tissue structure and function, and thus transparency. With regards to the sulphur speciation across the depth of the mature corneal stroma, it was found that there is an inconsistency in the sulphur content and distribution throughout the depth of the tissue, from the stromal region closest to the epithelium against the deeper stromal regions near the endothelium. The heterogeneity of the sulphur species in the most anterior part of the mature corneal stroma, at the interface with the Bowman’s layer supports the view that the differentiation and the transition between these two corneal layers is not very abrupt. The rest of the mature corneal stroma depth does not show any differences regarding its content in the sulphur-containing compounds indicating that the distribution and sulfation status of the corneal glycosaminoglycans have very little impact on the overall sulfur speciation. The three-dimensional ultrastructure of the human trabecular meshwork in large volumes and at high resolution identified giant vacuoles in the endothelial cell layer of the inner wall of Schlemm’s canal and these were grouped into four categories based on whether they formed pores, basal and apical, or not. Interestingly, the distribution of these vacuoles was found to be non-uniform. It was discovered that the juxtacanalicular tissue was not homogenous with respect to the proportion of the electron lucent, matrix free spaces throughout the tissue’s depth away from the inner wall of Schlemm’s canal. Three-dimensional reconstructions of collagen type VI in the trabecular meshwork showed that there is no structural regularity in the organisation of type VI collagen assemblies, or their association with sulphated proteoglycans, suggesting a role in aqueous humor outflow. This data allow us to propose a model of aqueous humor outflow and how this is funneled through the juxtacanalicular tissue towards the lumen of Sclemm’s canal.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Optometry and Vision Sciences
Subjects: R Medicine > RE Ophthalmology
Uncontrolled Keywords: Cornea; Trabecular meshwork; Collagen VI; Chemical composition; Ultrastructure
Date of First Compliant Deposit: 30 March 2016
Last Modified: 05 Jan 2024 05:45

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