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Biophysical Journal 85:2205-2212 (2003)
© 2003 The Biophysical Society

Changes in the Refractive Index of the Stroma and Its Extrafibrillar Matrix When the Cornea Swells

Keith M. Meek, Sally Dennis and Shukria Khan

Structural Biophysics Group, Department of Optometry and Vision Sciences, Cardiff University, Cardiff, United Kingdom

Correspondence: Address reprint requests to Keith M. Meek, Redwood Bldg., King Edward VII Ave., Cathays Park, Cardiff University, Cardiff CF10 3NB, UK. Tel.: 44-292-087-6317; E-mail: meekkm{at}cf.ac.uk.

The transparency of the corneal stroma is critically dependent on the hydration of the tissue; if the cornea swells, light scattering increases. Although this scattering has been ascribed to the disruption caused to the arrangement of the collagen fibrils, theory predicts that light scattering could increase if there is an increased mismatch in the refractive indices of the collagen fibrils and the material between them. The purpose of this article is to use Gladstone and Dale's law of mixtures to calculate volume fractions for a number of different constituents in the stroma, and use these to show how the refractive indices of the stroma and its constituent extrafibrillar material would be expected to change as more solvent enters the tissue. Our calculations predict that solvent entering the extrafibrillar space causes a reduction in its refractive index, and hence a reduction in the overall refractive index of the bovine stroma according to the equation n's = 1.335 + 0.04/(0.22 + 0.24 H'), where n's is the refractive index and H' is the hydration of the swollen stroma. This expression is in reasonable agreement with our experimental measurements of refractive index versus hydration in bovine corneas. When the hydration of the stroma increases from H = 3.2 to H = 8.0, we predict that the ratio of the refractive index of the collagen fibrils to that of the material between them increases from 1.041 to 1.052. This change would be expected to make only a small contribution to the large increase in light scattering observed when the cornea swells to H = 8.




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