Cellular dynamics of corneal wound re-epithelialization in the rat. III. Mitotic activity.

The number and distribution of mitotic epithelial cells in the ocular surface during homeostasis and in response to abrasion of the mammalian cornea were determined. Normal rats and those receiving a 3 mm diameter centrally located epithelial defect, received an intraperitoneal injection of colchicine 6 h prior to sacrifice. Mitosis in the basal epithelium during homeostasis was comparable in magnitude across the ocular surface epithelium, with the exception of a few mitotic figures in the midline. Thirty percent of the mitotic figures were in the basal layer (layer 1), and 70% were in layer 2; the cells in layer 2 were often noted to retain connection to the basal lamina by cytoplasmic stalks. Mitosis was rarely noted in the regenerating epithelium. However, summation of M phase cells in both the basal and suprabasal epithelium adjacent to the wound showed increases of 3- and 5-fold at 30 and 36 h after abrasion, respectively, from levels at homeostasis and the time of injury. In striking contrast to homeostatic epithelium, 80% of the mitotic cells were located in layer 1 of the corneal epithelium, with normal distribution observed by 72 h. Mitosis in the limbus and conjunctiva was increased 3-fold at 30 h and 24 h, respectively, from values at homeostasis and the time of debridement. These results, using rigorous statistical analysis and precise topographic assessment, showed that mitosis is not impeded - but rather often accelerated - following denuding of the corneal epithelium and that the spatial distribution of mitotic cells is correlated with wounding. The data revealed that re-epithelialization of the corneal epithelium is not dependent on mitosis in the regenerating epithelium, but rather in the adjacent unwounded epithelium of the cornea, with most cells being located in the basal layer until re-epithelialization is completed. Mitotic cells in the limbus and conjunctiva may be related to replenishment of ocular surface epithelial cells used in the repair process rather than directly supplying the abraded surface.