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Repeated UVR exposures cause keratocyte resistance to apoptosis and hyaluronan accumulation in the rabbit cornea.

PURPOSE
To evaluate hyaluronan (HA) production and level of apoptosis of corneal cells after repeated UVR exposures.


METHODS
Fifteen albino rabbit corneas were exposed to 310 nm UVR at a dose that causes biomicroscopically significant keratitis (0.47 J/cm2). Nine rabbits received a single dose of UVR. Six rabbits were irradiated 3 times at 7-day intervals. Rabbits exposed to the single dose of UVR, were sacrificed 24 hours, 7 and 14 days after irradiation. Rabbits exposed to the repeated doses of UVR, were sacrificed 24 hours and 14 days after the last irradiation. The corneal tissue specimens were processed for histological analysis using specific staining for HA, and the TdT-dUTP terminal nick-end labeling (TUNEL) assay.


RESULTS
Corneas exposed to a single UVR dose showed extensive positive TUNEL staining 24 hours after exposure. Almost all basal epithelial cells, keratocytes throughout the entire thickness of the stroma, and endothelial cells were TUNEL-positive. No HA was found 24 hours after exposure. Extracellular HA staining of high intensity was found at day 7 throughout the entire central stroma, except the anterior one-fourth. At day 14 only a faint HA staining was detected in the posterior stroma, close to Descemet's membrane. Corneas exposed to repeated UVR doses showed at 24 hours positive TUNEL staining only in epithelial cells and in very few stromal cells. The majority of stromal cells and endothelial cells were unaffected. At the same time HA staining of very high intensity was found both at 24 hours and day 14, and it was evenly distributed throughout the entire thickness of the stroma.


CONCLUSION
Repeated UVR exposures lead to increased production and accumulation of HA in the corneal stroma. The repopulated keratocytes are much more resistant to apoptosis than the native ones. HA accumulation may be a sign of long-term changes in the cornea.