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Craig Boote, Sally Hayes, Tomas White, Ahmed Abass, Nick Terrill, Thomas Sorensen, Keith M Meek; A study of the relationship between corneal structure and hydration using multi-modal x-ray scattering.. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1941. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Normal physiological function of the cornea depends on appropriate hydration levels within the stromal extrafibrillar matrix and within the collagen fibrils themselves. Previous work has indicated that intrafibrillar and extrafibrillar water content critically determines collagen molecular and fibrillar spatial organization, impacting corneal transparency and biomechanics. The purpose of the current study was to quantify the relationship between corneal structure and hydration in human and porcine eyes.
A total of 23 post-mortem human corneas and 50 porcine corneas were obtained. Corneal hydrations (H) ranging between H = 0.5 (equivalent water content: 33%) and H = 6.6 (87% water content) were set by equilibration in varying concentrations of polyethylene glycol solution for 2 days. Beamline I22 at the Diamond Light Source was used to record small-angle x-ray scattering (SAXS) patterns from the centre of 26 of the porcine and 9 of the human pre-equilibrated specimens. Collagen fibril diameter (FD) and interfibrillar separation (IFS) were measured from each SAXS pattern. The remaining 24 porcine and 14 human corneas were examined by wide-angle x-ray scattering (WAXS) using Diamond Beamline I02. WAXS patterns recorded from the centre of each specimen were analysed to quantify the collagen intermolecular separation (IMS).
IFS2 increased linearly with H for both human and porcine cornea. In contrast, FD versus H obeyed a non-linear, bi-phasic trend: increasing up to a hydration of approximately H = 2 with minimal change thereafter. Porcine specimens (36nm) exhibited a marginally greater final FD than human (35nm). IMS also varied with H in a bi-phasic manner, but reached a maximum value at alower hydration (H = 1.5) than FD. Human corneas displayed a significantly higher IMS at all hydration levels, measuring 1.70nm at physiological hydration (H = 3.2) compared to 1.47nm for porcine specimens.
The differential swelling behaviour of human and porcine corneas is likely due to a combination of inter-species differences in collagen intrafibrillar (molecular) organization and variations in the electrolytic composition of the extrafibrillar space.
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