The effects of ultraviolet radiation on the repair and maintenance of the cornea

Abstract

Epidemiological studies have implicated UVR as a causative agent in a variety of ocular diseases. Keratoconus is the bulging of the cornea due to it becoming thinner, leading to poor vision. Although UVR has recently been identified as a likely contributing factor, the pathogenic mechanisms remain to be elucidated. This thesis aims to provide a model of keratoconus and subsequently study the components of corneal maintenance in response to UVR. The corneas of mice were chronically exposed to physiologically relevant doses of UVR, and studied in vivo using a slit lamp and ex vivo histopathologically to identify features of keratoconus. Acute cellular and molecular effects of UVR on the corneal epithelium was studied using genetically modified reported strains of mice to trace corneal epithelial cell linages and to measure their unique spoke-like pattern of growth as well as epithelial cell stratification. UVR-induced changes in the epithelium were studied using assays for proliferation, DNA damage and cell death. Keratoconus-like symptoms were induced after 20 weeks of chronic UVR including thinning of the stroma, loss of keratocytes, loss of epithelial cell layers, fragmentation of the basement membrane and stromal collagen disorganisation.Acute effects of low dose UVR included an increase in epithelial clonal growth rate and proliferation. UVR-induced photolesions were repaired quickly whereas apoptosis was not significantly increased, indicating that proliferation was not a response to cell death. Multiphoton imaging showed that delamination of epithelial cells was significantly increased after acute UVR exposure and was sustained for 3 days after irradiation. As a result of cell shedding and delamination, proliferation and centripetal migration are increased in the limbal stem cells and corneal epithelial cells. Potentially, corneal conditions from chronic UVR arise as a consequence of the compounding of these biological effects of acute UVR exposures.