Free radicals and aging of anterior segment tissues of the eye

The anterior segment of the eye consists of the cornea, anterior chamber, trabecular meshwork, iris, and ciliary body (Fig. 1). The cornea usually contains five layers. From anterior to posterior they are: the 50mm-thick epithelium; Bowman’s membrane (10mm); stroma (500mm); Descemet’s membrane (5mm); and endothelium (5mm). The values given for the thickness of each component of the cornea reflect those of humans, where the total corneal thickness is approximately 550mm. Other species have proportional thicknesses of the component layer of stroma relative to their total thickness (bovine .0.8 mm; rabbit .0.4 mm; mouse .0.1 mm). The epithelium is a fiveor six-cell layered membrane that is covered by the 7to 10-mm tear film and thus represents the first barrier against invasion from the environment or the first line of defense (including the tear film) against environmental stress. The cells are of the squamous type at the surface, while the most posterior layer of the epithelium consists of columnar basal cells: wing cells fill the space between the other cell layers. The surface cells are desquamated into the tear film (1,2) on a regular basis as a result of the shear forces of lid movements, with replacement centripetally from both conjunctiva at the corneal periphery and underlying basal cells (3–5). Replacement of epithelial cells occurs on a 5to 6-day cycle such that total epithelial replacement occurs over about a 2-week period (2,6–9).