Structural studies of the corneal stroma with focus on the elastic fibre network in health and disease

The optical and biomechanical properties of the cornea are largely governed by the collagen-rich stroma, a layer that represents approximately 90% of the total thickness. It has been postulated that a novel corneal layer exists in the posterior stroma, immediately above Descemet’s membrane, termed ‘pre-Descemet’s layer’. The main aim of this thesis was to determine if this region has different structural properties to the overlying stroma. A second aim was to examine the elastic fibre distribution throughout the depth of the stroma in healthy and diseased corneas, with focus on pre-Descemet’s layer. Techniques used include serial block face scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, amongst various other imaging techniques. Depth analysis revealed that centre-to-centre interfibrillar spacing was significantly lower in the first ~10µm of stroma distal to Descemet’s membrane compared to overlying regions in central cornea. Three-dimensional analysis revealed the presence of long elastic fibres running throughout the stroma, parallel to the surface of the cornea, which were concentrated in pre-Descemet’s layer. This elastic material seemed to originate from the limbus as fenestrated sheets before travelling radially into the cornea as small fibres. This data provides evidence for pre-Descemet’s layer containing altered biomechanical properties that may contribute to the formation of a variable cleavage plane observed during pneumodissection. Additionally, the elastic fibre network is likely to play an important role in the deformation and recovery of the cornea. Furthermore, the presence of elastic fibres in foetal cornea suggests a potential role in development. The distribution of elastic fibres was very different in keratoconic buttons. No fibres were located above Descemet’s membrane, whereas the elastic fibres appeared concentrated below the epithelium in thinned coned regions, potentially as a biomechanical response to prevent rupture. Attempts were made to elucidate a functional role for elastic fibres by studying the corneas from a mouse model for Marfan syndrome, where there was a ~50% reduction in elastic fibre quantity. These corneas were significantly thinner and flatter than wild types suggesting that elastic fibres play a role in maintaining the shape of the cornea. Overall, this thesis has characterised pre-Descemet’s layer, demonstrating that structural differences are present. It is likely that the network of elastic fibres described in this thesis play a multi-functional role in the cornea