Epithelial wound healing of the rat cornea after excimer laser ablation

Rats with excimer corneal ablations (ArF 193 nm, 228 pulses, diameter 3,5 mm, 17.3 mJ per pulse, depth about 1/2 of corneal thickness) in one eye were killed 1, 2, 4, 6 and 13 days after treatment. The other eye served as control. The cell number per microscopic vision field, the labelling index (LI) and the mitotic rate (MR) were calculated for the peripheral, midperipheral and central areas of the corneal epithelium. The cell number showed a uniform depression in the remaining corneal epithelium at Day 1, normalizing from the centre to the periphery. The LI was only significantly increased at Day 1, while the MR was statistically significantly increased peripherally at Day 2 and in all areas at Day 6. However, when the corneal epithelium was evaluated as a whole, the MR was significantly increased at days 1, 2 and 6. The proliferative response of the epithelium was very homogenous irregardless of the distance to the original lesion. Both the migratory and the proliferative phases of the healing process seemed to be delayed when compared to the healing of pure epithelial wounds. However, the initiation of an increase in DNA synthesis seems not to be delayed, indicating that it is primarily the G2 phase that has been prolonged with this ablation procedure. The stromal thickness was increased from about one half of the normal values immediately after the treatment to above normal values at Day 4, thereafter decreasing to normal values at Day 13. Thus, the regenerative ability of the stroma is more pronounced in rats than in humans, but also in humans regeneration of the stroma can possibly explain the regression of the myopic shift seen some time after excimer laser treatment.