Pilot study on the time course of apoptosis in the regenerating corneal epithelium

To determine whether apoptosis contributes to regeneration of the corneal epithelium following erosion and following ultraviolet irradiation.

Central corneal erosions were made on one eye of 16 rats. One eye of another set of 16 rats was exposed to UVB irradiation. The rats were killed at time intervals varying from 12 hours to 7 days after treatment. Enucleated eyes were fixed in buffered formaldehyde and evaluated by terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate (dUTP) nick-end labelling (TUNEL) and morphology. The total number of cells and the number of TUNEL positive cells were counted in perpendicular sections using light microscopy.

Following central erosion the total epithelial cell number was restored by day 3. During the first 5 days, TUNEL positive cells were observed only in small numbers, but an increase occurred by days 6 and 7. After UVB, an increase in TUNEL positive cells was noted for at least 3 days, and by day 7 there was a small increase of TUNEL positive cells. This differed from the results seen in control animals.

The present study indicates that after injury, apoptosis occurs in two distinct phases. There is an initial early phase of apoptosis which subsides at about the time the cell mass is being restored and after damaged cells have been removed. A later phase of apoptosis occurs suggests it has a homeostatic role which contributes to the regulation of the cell population.