The cornea is the eye’s outermost layer, a clear, dome-shaped structure that covers the iris, pupil, and anterior chamber. Its transparency and curvature are essential for vision, as it acts as the eye’s main refractive surface, accounting for approximately 65 to 75 percent of the eye’s total focusing power. The cornea also serves a protective function, shielding the delicate inner structures of the eye from dust, germs, and other harmful matter. This highly organized tissue is composed of distinct layers that allow it to perform its dual role of protection and vision.
The Essential Composition
The cornea is primarily composed of water and collagen, with its unique properties arising from the precise arrangement of these components. Water maintains a highly regulated hydration level of approximately 78% of the total weight. This specific hydration is necessary to preserve the tissue’s transparency, as excessive swelling or dehydration can lead to cloudiness and impaired vision.
Collagen is the main structural protein, making up about 16% of the cornea’s composition and providing its strength, elasticity, and form. The majority of this collagen is Type I, forming fibrils that are uniform in diameter and arranged in a highly ordered lattice pattern. These collagen fibrils are interspersed with specialized cells called keratocytes, which maintain and repair the collagen matrix. Keratocytes also produce non-aqueous components, such as proteoglycans, which help regulate the spacing and hydration of the collagen fibers.
The Five Structural Layers
The corneal tissue is organized into five layers. The outermost layer is the Epithelium, which provides a smooth surface for the tear film and acts as a barrier to block the passage of foreign materials, such as bacteria and dust, into the eye. This layer is composed of multiple cell layers and contains a dense network of nerve endings, making the cornea highly sensitive to touch.
Directly beneath the epithelium is Bowman’s layer, a thin, acellular sheet composed of randomly arranged collagen fibrils. This layer provides structural integrity and resistance to trauma. If damaged, it does not regenerate, meaning injuries that penetrate this layer can result in permanent scarring.
The thickest layer is the Stroma, accounting for about 90% of the total corneal thickness. It consists of approximately 200 flattened layers, called lamellae, where the collagen fibers are stacked and arranged in a precise, orthogonal pattern. This structure is essential to the cornea’s transparency.
Beneath the stroma lies Descemet’s membrane, a strong, yet thin, basement membrane that serves as the foundation for the innermost layer. This membrane is secreted by the cells of the inner layer and becomes progressively thicker with age, providing a resilient protective barrier. The innermost layer is the Endothelium, a single layer of specialized cells that faces the anterior chamber of the eye. These cells are metabolically active and do not typically regenerate in adults, making their health important for long-term corneal function.
Maintaining Cornea Health and Transparency
The cornea is an avascular tissue, meaning it contains no blood vessels. This unique characteristic allows it to maintain transparency by eliminating light scattering. Because it lacks blood vessels, the cornea must receive its necessary nutrients and oxygen from alternative sources. The outer layers obtain oxygen primarily from the tear film, while the inner layers receive nutrients like glucose from the aqueous humor, the fluid that fills the anterior chamber.
The endothelial layer plays an important role in maintaining corneal clarity by regulating its hydration level through the “pump-leak” system. Fluid from the aqueous humor naturally leaks into the stroma, causing it to swell slightly due to its hydrophilic proteoglycan content. Endothelial cells actively pump excess fluid out of the stroma and back into the anterior chamber using active ion transport, preventing the tissue from swelling and becoming cloudy.
Transparency is also preserved by the highly regular spacing and uniform diameter of the collagen fibrils within the stroma. This precise lattice arrangement prevents light from scattering as it passes through the tissue. Any disruption to this organization, such as swelling from endothelial dysfunction or scarring from injury, can lead to increased light scattering and a loss of corneal clarity.