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Aging, the persistent decline in age specific fitness of an organism as a result of internal physiological deterioration, is a common process among multicellular organisms. 1 In humans, aging is usually monitored in relation to time, which renders it diYcult to diVerentiate between time dependent biological changes and damage from environmental insults. There are essentially three types of aging at work in any adult tissue; the aging of long lived proteins, the aging of dividing cells, and the aging of non-dividing cells. 2 Dividing cells may be derived from renewing populations in which the rate of cell loss and division is great. An example is the corneal epithelium in which complete turnover occurs within 5–7 days after terminal diVerentiation. 3 4 Conditional renewal populations, which normally have an extremely low proliferation rate, can also produce dividing cells in response to extrinsic stimuli. Stromal keratocytes …