Corneal epithelial wound healing.

The corneal surface is, without question, the most specialised 123 mm2* of the body's surface. It forms an integral part of the ocular surface with which it shares several anatomical and physiological attributes. Like the rest ofthe body surface, it is in a state of constant 'healing'. Squamous epithelial cells are continually shed into the tear pool and simultaneously replenished by cells moving centrally from the limbus and anteriorly from the basal layers of the epithelium. This concept was propounded by Thoft' in his X, Y, Z hypothesis and is substantiated by several observations and experiments. X represents the proliferation of basal epithelial cells, Y is the proliferation and centripetal migration ofthe limbal cells, and Z the epithelial cell loss from the surface. For a state of equilibrium to be maintained X+Y must equal Z. It is estimated that the corneal epithelium is constantly renewed every 7 to 10 days.2 When this equilibrium is disturbed, as occurs dramatically after an abrasion, the corneal epithelial wound healing response sets in. This is essentially an exaggeration of the normal physiological process involving cellular and subcellular events occurring under the influence of extracellular matrix proteins and growth factors.