Aging and the cornea

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 difficult to differentiate 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 differentiation.34Conditional renewal populations, which normally have an extremely low proliferation rate, can also produce dividing cells in response to extrinsic stimuli. Stromal keratocytes are a prime example of a conditional renewing population.5 Corneal endothelial cells retain the capacity to undergo mitosis and conditional renewal in humans although they very seldom do so.6-9 Non-dividing cells are those from static cell populations (exemplified by cerebral neurons) which never divide during adult life.3 Corneal aging produces both structural and functional changes. These changes in turn can affect the ability of the organ to refract light, to repair itself, and to protect itself and the internal structures of the eye.10 A variety of corneal aging changes have been reported. However, as it is difficult to distinguish age specific deterioration from degenerations modified by environmental and genetic factors, we think it is helpful to consider these alterations within the broader framework of the aging process. The study over the past 30 years of isolated cells in culture as a model system for aging changes has greatly advanced our understanding of …