The cell shedding rate of the corneal epithelium — a comparison of collection methods

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Abstract
To determine the spontaneous shedding rate of cells from the rabbit corneal epithelium using different harvesting methods. Cells were collected from the rabbit corneal epithelium using two in vitro methods and three in vivo methods. Cells were counted and the shedding rate calculated after adjustment for the collection time. The shedding rates obtained from the in vitro methods were (cells/min/cornea, mean +/- SE): 78.0 +/- 9.4 (corneal superfusion), and 5.4 +/- 1.3 (whole-eye perfusion). For the in vivo corneas, the shedding rates were: 10.0 +/- 2.3 (corneal superfusion), 8.1 +/- 0.4 (corneal immersion), and 14.5 +/- 1.5 (corneal irrigation). In vitro corneal superfusion was significantly different from the other four methods (P < 0.01). The results suggest that the spontaneous cell shedding rate of the in vivo rabbit corneal epithelium is 5 to 15 cells/min/cornea. This is much lower than estimates of about 100 cells/min/cornea based upon in vitro corneal superfusion. One explanation of this slow shedding rate is that factors which were absent during our collection methods (such as blinking) would normally increase shedding. Another possibility is that cells in the corneal epithelium may have a much longer life span than previously reported; rather than a few days, the epithelium could take several months to completely replace all cells. Whatever the explanation, the measured spontaneous shedding rate does not complement the reported production rate of new cells. It is necessary to revise our understanding of the kinetics of epithelial homeostasis.