Abstract
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. Within the stroma, the specific arrangement of superimposed lamellae provides the tissue with tensile strength, whilst the spatial arrangement of individual collagen fibrils within the lamellae confers transparency. In keratoconus, this precise stromal arrangement is lost, resulting in ectasia and visual impairment. In the normal cornea, we previously characterised the three-dimensional arrangement of an elastic fiber network spanning the posterior stroma from limbus-to-limbus. In the peripheral cornea/limbus there are elastin-containing sheets or broad fibers, most of which become microfibril bundles (MBs) with little or no elastin component when reaching the central cornea. The purpose of the current study was to compare this network with the elastic fiber distribution in post-surgical keratoconic corneal buttons, using serial block face scanning electron microscopy and transmission electron microscopy.We have demonstrated that the MB distribution is very different in keratoconus. MBs are absent from a region of stroma anterior to Descemet's membrane, an area that is densely populated in normal cornea, whilst being concentrated below the epithelium, an area in which they are absent in normal cornea. We contend that these latter microfibrils are produced as a biomechanical response to provide additional strength to the anterior stroma in order to prevent tissue rupture at the apex of the cone. A lack of MBs anterior to Descemet's membrane in keratoconus would alter the biomechanical properties of the tissue, potentially contributing to the pathogenesis of the disease.
Highlights
The transparency and strength of the cornea arise from the highly organised arrangement of the stromal extracellular matrix
Normal cornea stained with tannic acid displayed an extensive Microfibril bundle (MB) system in the central and peripheral cornea, concentrated immediately anterior to Descemet's membrane (Fig. 2AeB)
SEM Serial block face scanning electron microscopy (SBF) SEM data sets obtained from regions of stroma overlying Descemet's membrane in keratoconic button 1 appeared very dark with little contrast, so it appeared that no MBs were present, and it was difficult to produce 3D reconstructions
Summary
The transparency and strength of the cornea arise from the highly organised arrangement of the stromal extracellular matrix. Lamellae are highly interlaced (Radner et al, 1998a) and randomly orientated (Komai and Ushiki, 1991) in the anterior stroma when viewed en face, whereas they are more organised in the posterior stroma, showing two preferred orientations (Abahussin et al, 2009; Aghamohammadzadeh et al, 2004; Meek et al, 1987), and form a circum-corneal annulus at the limbus (Newton and Meek, 1998) This specific arrangement of collagenous lamellae throughout the stroma provides the cornea with the ability to resist tensile strain. In advanced stages of keratoconus, the organisation of the collagen lamellae is severely disrupted, resulting in a loss of tensile strength and the progression of ectasia (Daxer and Fratzl, 1997; Meek et al, 2005; Radner et al, 1998b)
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