[Functional morphology of the cornea]

The corneal epithelium serves as a barrier between tear film and stroma and maintains a smooth corneal surface. The optical features of the cornea, namely refraction and transparence, depend largely upon the characteristics of the superficial corneal layers. Any lesion of this superficial cell coat facilitates the penetration of particles of even small diameter (virus, bacteria) towards the stroma. In fact, viral corneal diseases usually start in the epithelium, the virus being replicated in transformed epithelial cells. Corneal wounds are rapidly covered by migrating epithelial cells, invading the lesion from the border of the epithelial defect. Cohesion between epithelium and underlying stroma remains very weak unless regenerating epithelial cells have synthesized a basement membrane, allowing thus a firm attachment between epithelial hemidesmosomes and basement membrane. Recurrent erosions occur readily in the absence of epithelial basement membranes. Presumably, corneal epithelium is involved in the biosynthesis of fibrillar collagen of the superficial stromal layers. Corneal wound healing is delayed by a pathological epithelium. Corneal hydratation is controlled by the endothelium. It maintains a bulk flow of water towards the aqueous humor side against a pressure gradient and regulates the movement of metabolites. In the case of endothelial malfunction, the corneal stroma thickens due to a swelling of mucopolysaccharides. The endothelium synthesizes a very thick basement membrane (Descemet's membrane). In penetrating corneal wounds, Descemet's membrane is synthesized by the endothelium.