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-t hreshold exposure to ultraviolet radiation (UVR). METHODS. Sprague-Dawley rats received 5 kJ/m(2) UVR (lambda(MAX) = 300 nm, lambda(0.5) = 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 inves tigated 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 exposu re. 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 WR 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 re moved from the epithelium by phagocytosis.