The cornea is the transparent, dome-shaped outer surface of the eye, covering the iris and pupil. It functions much like the eye’s front window, allowing light to enter. This clear structure also plays a significant role in focusing light, contributing to about two-thirds of the eye’s total focusing power. The cornea’s transparency and specific curvature are fundamental for clear vision.
The Outer Protective Layers
The outermost layer of the cornea is the epithelium, serving as the eye’s primary defense against foreign particles and germs. This layer of cells, similar to skin, regenerates rapidly, allowing minor abrasions to heal quickly. The epithelium is densely packed with nerve endings, making the cornea one of the most sensitive areas of the body and explaining the intense discomfort from small scratches. It also absorbs oxygen and nutrients from tears, supplying them to the rest of the cornea.
Just beneath the epithelium lies Bowman’s layer, a tough, transparent sheet composed mainly of collagen fibers. This layer provides structural support and helps the cornea maintain its shape. Unlike the epithelium, Bowman’s layer does not regenerate after injury. Damage to this layer can result in permanent scarring, potentially impairing vision.
The Central Structural Layer
The stroma forms the thickest part of the cornea, accounting for approximately 80% to 90% of its total thickness. This substantial layer is composed of meticulously arranged collagen fibers and water, creating a highly organized matrix. The precise, uniform spacing and alignment of these collagen fibers allow light to pass through the cornea without scattering, maintaining its clarity.
This structural integrity gives the cornea its strength and flexibility, helping it withstand external pressures while maintaining its precise curvature for light refraction. Conditions such as keratoconus involve a weakening and thinning of this stromal layer, leading to a cone-shaped bulging of the cornea and distorted vision. A thin but strong sheet, Dua’s layer, is situated at the back of the stroma.
The Inner Regulatory Layers
Beneath the stroma and Dua’s layer is Descemet’s membrane, a thin, strong basement layer that provides support for the innermost corneal cells. This membrane is elastic and plays a role in the overall structural integrity of the cornea. It acts as a barrier, offering some protection against injuries and infections reaching deeper into the eye.
The innermost layer of the cornea is the endothelium, a single layer of cells that performs a specialized function. These cells act like miniature sump pumps, removing excess fluid from the stroma. This active pumping mechanism prevents the stroma from swelling and becoming cloudy, thus maintaining corneal transparency. Endothelial cells do not regenerate; their number naturally decreases with age or due to conditions like Fuchs’ dystrophy, which can lead to fluid accumulation, corneal swelling, and subsequent vision loss.