Apoptosis in the rat corneal epithelium during regeneration

In the present transmission electron microscopic study, regenerating rat corneal epithelium is examined at three different time points after chemical abrasion. Four days after the initial injury classical signs of regeneration are observed. At day 6 regeneration is less pronounced and epithelial differentiation dominates the morphological picture. Many basal cells are swollen and single electron-dense basal cells have a shrunken morphology (dark cells). Intercellular cytoplasmic sacs are common and some appear to be phagocytosed. At day 8 there are no signs of regeneration and neither dark cells nor swollen cytoplasmic sacs are seen. No signs of intraepithelial inflammation are seen at any time. "Odland"-like granules, which possibly play a role in epithelial desquamation, occur suprabasally in large numbers at all time points. We believe the simultaneous presence of dark cells and phagocytosis at day 6, without signs of inflammation, indicates ongoing apoptosis. Intraepithelial cell loss might explain the discrepancy between an increased proliferation rate and the absence of hyperplasia which has been reported at this time point.