Anatomy of the outer layer of the eyeball/cornea, sclera
- Cornea: clear transparent and elliptical structure with a smooth shining surface; Average diameter ~ 11-12 mm.
- Its responsible for transmission of light. Its convex shaped allows it to refract light at an angle to help it focus into the retina. Transparency is maintained by:
- Regular arrangement of stroma
- Avascularity
- Relative state of dehydration.
- Made up of dense connective tissue, giving it the function of: Maintaining the structural integrity of the eye, protection from mechanical damage and from sunlight.
- The thickness of the central part is 0.52 mm and the peripheral part is 0.67 mm. The central one-third is known as the optical zone.
- Refractive index of cornea is 1.37. The dioptric power of the cornea is approximately + 43 to + 45 D.
Structure
The cornea consists of five layers namely:
- The epithelium (Cellular layer): Stratified squamous type of epithelial cells consists of three cell types namely the basal columnar cells, two or three layers of wing cells and surface cells.
Presence of a collection of stem cells called “limbus” on the edges, allows regeneration, healing and nutrition of the epithelial cell within 7 days of damaged. Capture water to hydrate the cornea and prevent drying.
- Bowman’s membrane (Acellular): It is made up of collagen condensed connective tissue. Since it’s acellular, it does not regenerate when damaged. Healing is achieved by fibrosis leading to scaring and formation of permanent corneal opacity.
- Substantia propria or stroma: It forms 90% of corneal thickness but since its made up of regularly arranged “fine collagen fibres” which allows it to remain transparent. It also contains ground substance and fibrocytes. This structure allows it to remain dehydrated. In corneal pathologies, hydration of the collagen fibers disturbs the regular arrangements of the collagen fibers ➔ edema and loss of transparency.
- Elastic fibres (Descemet’s membrane): It is a thin but strong homogeneous elastic membrane which can regenerate.
- The endothelium: Inner most layer arranged into a single layer of cells. The cells contain two way pumps, bringing in nutrients from the aqueous humor and pumping water out of the stromal layer to prevent edema. The cell density is about 3000 cells mm2 at birth which decreases with advancing age. Corneal decompensation occurs only when more than 75% cells are damaged. It is measured by specular microscopy.
Nutrition of the Cornea
Cornea is an avascular structure. It derives nutrition from: