Renewal of the rabbit corneal epithelium as investigated by autoradiography after intravitreal injection of 3H-thymidine

Purpose. To evaluate the renewal time of the rabbit corneal epithelium and to verify whether there are differences in the rate of renewal related to the topography of the cornea. Methods. After the intravitreal injection of 20 mu Bq/eye of H-3-thymidine (H-3-TdR), the rabbits were killed at variable survival-time intervals (3 hours to 90 days), and their corneas were processed for autoradiography on paraffin and 0.75-mu m-thick Epon sections. The frequency of labeled nuclei, per each linear millimeter of epithelium, was estimated on paraffin sections of the whole length of the cornea of rabbits killed 6 hours after injection. The activity of unbound H-3-TdR was measured in the vitreous and aqueous humor. Results. The incorporation of H-3-TdR into newly synthesized DNA ceased between 1 and 2 days after the intravitreal injection. At the shortest time intervals (3 hours to 1 day), the overwhelming majority of the labeled nuclei were located in the basal stratum of the stratified squamous epithelium. Cells of the wing stratum were labeled from 3 days onward. At 14 days, labeled nuclei were visualized only in the inner portion of the surface stratum, whereas their outermost lavers were labeled only at II days and later. Lightly labeled nuclei could still be detected in the epithelium less than or equal to 90 days after injection. The counts of labeled nuclei did not show significant differences in their frequencies between the periphery and the center of the cornea. The existence of vascularization at the periphery of the corneal stroma and the lack of a conspicuous Bowman's laver made it difficult to characterize morphologically the rabbit limbus with the light microscope. Conclusion. It takes >14 days for a daughter cell to migrate from the basal stratum to the outermost layer of the epithelium. Labeled daughter cells were detected less than or equal to 90 days in several layers of the corneal epithelium. Therefore it takes >2 weeks for the complete renewal of the epithelium. Our data suggest that the proliferative capability of the centrally located basal cells is enough to guarantee the renewal of the corneal epithelium under physiologic conditions.