Fig. 6. Diagram showing how the basal epithelial cells are continually replenished by a stem cell population that resides in the limbus. The basal epithelial cells migrate forward from the periphery to the center of the cornea. When they undergo mitosis, both daughter cells move into the anterior layers of the epithelium and continually move up in epithelium until eventually being shed. (From Thoft RA, et al. The X, Y, Z hypothesis of corneal epithelial maintenance. Invest Ophthalmol Vis Sci 24:1442, 1983.)