The cornea serves as the primary refractive medium of the eye, functioning like a convex lens with a refractive power of approximately 43 diopters (D). The corneal tissue exhibits a highly organized and orderly structure, conferring transparency as well as robust self-protective and reparative properties. The cornea is richly innervated with sensory nerves, primarily supplied by the ophthalmic branch of the trigeminal nerve via the long posterior ciliary nerves. Within the cornea, nerve endings lose their myelin sheath and branch into the epithelial cell layer after passing through Bowman’s membrane, resulting in highly sensitive sensation.
The cornea is avascular, with nutrients primarily derived from the aqueous humor, tear film, and the vascular network at the limbus. Oxygen supply to the epithelial cells comes from the tear film, while the endothelium receives oxygen from the aqueous humor. The primary energy substrate is glucose, most of which is obtained through the endothelium from the aqueous humor, with approximately 10% supplemented by the tear film and vascular supply at the limbus.
Corneal epithelial cells have a strong regenerative capacity, allowing for rapid repair following injury without leaving scars. However, if the basement membrane of epithelial cells is involved, the healing process becomes significantly prolonged. The basal cell layer of limbal epithelium contains limbal stem cells, which play a crucial role in epithelial renewal and repair. Bowman’s membrane is formed during embryonic development via secretion from keratocytes within the stroma and does not regenerate after injury.
The corneal stroma is primarily composed of type I collagen fibers (with diameters of 24–30 nm) and extracellular matrix components. The highly regular and orderly arrangement of these collagen fibers allows approximately 98% of incident light to pass through. It is generally believed that stromal injuries result in the formation of newly repaired collagen fibers with altered diameters and inter-fiber spacing, disrupting the original cross-linked structure and leading to scarring.
Descemet’s membrane is secreted by endothelial cells and is primarily composed of type IV collagen fibers. It is highly elastic, exhibits substantial resistance, and has the capacity to regenerate following injury. This layer is thin at birth but thickens with age.
The corneal endothelium comprises approximately one million cells, which decrease in number with age. These cells are connected by tight junctions that prevent aqueous humor from entering the extracellular space. The endothelium functions as part of the cornea-aqueous humor barrier and actively pumps out excess water, maintaining the corneal state of relative dehydration and transparency. The endothelial cells rarely undergo mitosis, and injury is primarily compensated for by the expansion and migration of neighboring cells to fill the defect. However, significant loss of endothelial cells can overwhelm the compensatory mechanism, resulting in corneal edema and bullous keratopathy.