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K. M. Meek, A. J. Quantock, J. Doutch; Changes in Visible Light Transmission Across the Cornea. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3506.
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To determine how transmission of visible light through the corneal stroma varies as a function of distance from the optical centre, and how this relates to known changes in ultrastructural parameters such as collagen centre-to-centre interfibrillar spacings and collagen fibril diameter.
Spectrophotometry was used to measure transmission in the 400nm to 700nm wavelength region, using a system in which the acceptance angle is suitably small to prevent scattered light reaching the photodiode. Corneal stromas were mounted by the scleral rim within special sample cells. Fresh bovine corneas and human corneas from tissue culture were used. The epithelial and endothelial cell layers were removed from all corneas which were then equilibrated to an approximate physiological hydration. The sample cell chambers were filled with Dow Corning 200/5cS silicon oil to minimise reflections from the cornea at the surface. Translational measurements of transmission were then taken at 1mm sample intervals, from the geometric centre of the corneas, at 10nm wavelength steps with a beam slit width of 1mm. The Direct Summation of Fields corneal transparency model was used to model stromal transmission at different points across the cornea. For this, the relative positions of the collagen fibrils were obtained from electron micrographs. The average fibril diameter and fibril number density, both as a function of position, were obtained from previously published X-ray diffraction data.
Results from bovine and human corneas suggest that corneal transmission in the central pre-pupillary region decreases gradually from the optical centre outwards. This was best modelled by the gradual increase in stromal thickness caused by additional lamellae, known to occur away from the centre of the stroma. Outside the central 3mm (human cornea) and central 6mm (bovine cornea) of the stroma, transmission decreased more than can be explained by the increase in thickness alone. Therefore, there appears to be an extra contributory term to light scattering outside the prepupillary zone. We modelled the theoretical effects of increase in tissue thickness, changes in fibril number density and changes in fibril radius, and found that the effects on light transmission of the small decrease in fibril number density at the periphery outweighed by that of the small increase in the radius of the fibrils that occurs in the peripheral cornea.
The decrease in transmitted light towards the periphery of the cornea is best modelled by the small increase in collagen fibril diameter and the well known increase in stromal thickness at the periphery.
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