Cell kinetics of the rat corneal epithelium

In the rat corneal epithelium the mitotic rate (MR) is almost equal throughout the epithelium in the morning (Haaskjold et al. 1988). The labelling index (LI) shows a marked reduction in the central cornea, which could suggest a lack of uptake of tritiated thymidine via the salvage pathway (Haaskjold et al. 1989). In the present study we have used [3H]deoxycytidine, and [3H]thymidine after prior treatment with a methotrexate regimen to elucidate this discrepancy. Deoxycytidine is incorporated into DNA independent of thymidine kinase, while methotrexate, which depletes the cells of reduced folates, makes the cells completely dependent on the salvage pathway. With both techniques the same pattern of labelling was observed, confirming that in the morning the ratio between the MR and the LI differs throughout the cornea. Based on previous observations, an analysis of the MR/LI ratio during 24 h was performed, showing that these parameters were strongly correlated. This suggests that there may be different circadian variations in the cell proliferation parameters throughout the corneal epithelium. The methotrexate regimen may be a useful tool to investigate the salvage pathway.