Apoptosis in the rat lens after in vivo threshold dose ultraviolet irradiation.

PURPOSE To investigate DNA damage in the rat lens after in vivo close-to-threshold exposure to ultraviolet radiation (UVR). METHODS Sprague-Dawley rats received 5 kJ/m2 UVR (lambdaMAX = 300 nm, lambda0.5 = 10 nm) unilaterally for 15 minutes. Animals were killed at 1, 6, and 24 hours and at 1 week after exposure. DNA-strand breaks were investigated in sagittal paraffin sections using the TdT-dUTP terminal nick-end labeling (TUNEL) technique and propidium iodide for counterstaining. Other lenses were prepared for transmission electron microscopy (TEM). RESULTS TUNEL-positive nuclei were found at only 24 hours after UVR exposure. About one tenth of the epithelial cell nuclei were TUNEL positive, and affected cells were scattered over the entire epithelium. No TUNEL-positive cells were found at 1 or 6 hours or at 1 week after UVR exposure or in the nonexposed lenses. TEM verified the occurrence of programmed cell death and showed the breakdown of the apoptotic cells by adjacent cells. No signs of necrosis were found. CONCLUSIONS Threshold-dose UVR induces programmed cell death that peaks 24 hours after exposure and involves the entire epithelium. Dead cells are removed from the epithelium by phagocytosis.