Cellular Response and Reactive Hyaluronan Production in UV-Exposed Rabbit Corneas

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Abstract
The distribution of hyaluronan (HA) and the cellular response after photokeratitis induced by different ultraviolet (UV) wavelengths in the rabbit cornea was examined to help understand the mechanism of corneal injury and repair after UV damage. HA is a high molecular weight disaccharide polymer capable of binding considerable amounts of water. It is not normally found in the rabbit corneal stroma. The production of HA represents a generalized corneal response to injury. Twenty-four albino rabbit corneas were exposed to 270, 290, and 310 nm of UV radiant energy in 8-nm full wavebands in doses producing biomicroscopically significant keratitis (three corneal thresholds for keratitis (Hc): 0.016 J/cm2 for 270 nm, 0.04 J/cm2 for 290 nm, and 0.14 J/cm2 for 310 nm) and in subkeratitis doses (0.7 Hc: 0.004 J/cm2 for 270 nm, 0.008 J/cm2 for 290 nm, and 0.03 J/cm2 for 310 nm). The rabbits exposed to 270 and 290 nm of UV radiation were sacrificed 3 days after exposure. The rabbits exposed to 310 nm of UV radiation were sacrificed 3, 7, and 14 days after exposure, respectively. The corneal tissue specimens were double stained and examined morphologically and histochemically for HA by light microscopy. Evaluation of corneas exposed to 270 and 290 nm of UV radiant energy in both subkeratitis and keratitis doses and those corneas exposed to 310 nm of radiant energy in subkeratitis dose showed neither stromal changes nor production of HA by corneal cells. Corneas exposed to 310 nm of UV radiant energy in keratitis dose at 3 days after exposure showed disappearance of keratocytes in entire thickness of central cornea. Cells bordering this damaged area were staining for HA. By 7 days after exposure almost the whole damaged area, except one fourth of anterior stroma, was repopulated by new keratocytes staining positive for HA. The corneal structures became normal and HA almost completely disappeared 14 days after exposure. A keratitis dose of 310 nm of UV light irradiation is needed to cause keratocyte damage. A keratitis dose of the shorter wavelengths does not cause keratocyte cell damage at the light microscopic level. The keratocyte production of HA appears to be a sign of cell readiness to repopulate the damaged stroma devoid of keratocytes.

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