Abstract
The primary aim of this study was to quantify the relationship between corneal structure and hydration in humans and pigs. X-ray scattering data were collected from human and porcine corneas equilibrated with polyethylene glycol (PEG) to varying levels of hydration, to obtain measurements of collagen fibril diameter, interfibrillar spacing (IFS) and intermolecular spacing. Both species showed a strong positive linear correlation between hydration and IFS2 and a nonlinear, bi-phasic relationship between hydration and fibril diameter, whereby fibril diameter increased up to approximately physiological hydration, H = 3.0, with little change thereafter. Above H = 3.0, porcine corneas exhibited a larger fibril diameter than human corneas (p < 0.001). Intermolecular spacing also varied with hydration in a bi-phasic manner but reached a maximum value at a lower hydration (H = 1.5) than fibril diameter. Human corneas displayed a higher intermolecular spacing than porcine corneas at all hydrations (p < 0.0001). Human and porcine corneas required a similar PEG concentration to reach physiological hydration, suggesting that the total fixed charge that gives rise to the swelling pressure is the same. The difference in their structural responses to hydration can be explained by variations in molecular cross-linking and intra/interfibrillar water partitioning.
Highlights
The outer covering of the eye comprises a strong, transparent cornea and an opaque sclera
Discounting the outlying data from one human cornea (4% polyethylene glycol (PEG), H 1⁄4 5), the relationship between PEG concentration and stromal hydration was seen to be similar for all species
We developed a protocol to control accurately the hydration of the corneal stroma of several species, and applied it in a detailed analysis of the hierarchical structural response of human and pig stroma to changes in hydration
Summary
The outer covering of the eye comprises a strong, transparent cornea and an opaque sclera. The cornea comprises several layers, the largest being the stroma, which occupies approximately 90% of the total corneal thickness and is composed of water, collagen, proteoglycans and keratocytes. Thin collagen fibrils lie parallel to each other within stacked layers (lamellae) which are themselves interspersed with thin, flat keratocytes. The small diameter of the collagen fibrils and their regular separation distance, when observed at physiological hydration (Hphys), are believed to be regulated by the charge density on the proteoglycans [6,7,8,9]. As detailed in a review article by Meek & Knupp [10], the transparency of the cornea at Hphys can be primarily attributed to the specific arrangement of collagen fibrils within the corneal stroma and to the refractive index of its constituent cells [11].
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