The Anatomy of the Cornea

The cornea is the clear dome of tissue at the front of the eye. It connects smoothly to the sclera (the white part of the eye). The cornea refracts light through the lens below it, which allows the eye to process images. Injuries to the cornea can impair a person’s vision.

Anatomy

In most people, the cornea is somewhat oval in shape, and it is thicker at the edges than it is at the center. The cornea sits above the iris and the lens.

Structure

Although it is very thin and transparent, the cornea is made up of five separate tissue layers.

• Epithelium: These cells produce a thin, glistening “skin” layer on the outer cornea.
• Bowman’s layer: Also called Bowman’s membrane, this thin tissue layer is made up of collagen cells that provide structure to the cornea.
• Stroma: The thickest layer of the cornea, the stroma is composed of collagen cells.
• Descemet’s membrane: This very thin layer of cells provides some elasticity to the cornea's structure.
• Endothelium: A single layer of cells on the innermost part of the cornea, the endothelium maintains the cornea’s crystal clearness.

Anatomical Variations

Congenital (present at birth) abnormalities of the cornea do occur, and they usually cause the cornea to be cloudy instead of clear. When these abnormalities occur, they often appear in conjunction with other medical conditions, including:

• Congenital brain abnormalities
• Heart defects
• Abnormalities of craniofacial (head and face) development
• Inherited corneal defects

Function

The cornea’s convex (domed) shape serves to refract (bend) light before it passes through the iris and the lens. The lens further refracts the light to refine the image projected onto the retina (the tissue that lines the back of the inner eyeball).

The degree of curvature of the cornea greatly affects its ability to refract light. Abnormalities of corneal curvature, such as keratoconus (a cone-shaped cornea), can make image processing poor or even impossible.

Associated Conditions

Because of its prominence at the front of the eye, the cornea is subject to the risk of abrasion (scratching, scraping). Minor abrasions that affect only the epithelial layer usually heal themselves within 24 hours. However, deeper scratches that reach Bowman’s layer often produce corneal scarring that can impair vision. Injuries that extend below Bowman's layer, such as a penetrating eye wound, can cause blindness.

In addition to trauma, the cornea can be affected by a wide variety of congenital conditions and other diseases that develop over time, a few of which include the following.

• Epithelial Basement Membrane dystrophy (EBMD): A disorder of the epithelial cells in the cornea
• Congenital hereditary endothelial dystrophy: This is an inherited condition in which the corneal tissues become swollen with fluid.
• Peter’s anomaly: This is a condition in which the developing cornea sticks to the iris or the lens, causing corneal cloudiness.
• Sclerocornea: In this condition, the corneal tissue doesn’t fully differentiate from the scleral tissue (white part of the eye) during development, resulting in an opaque cornea.
• Corneal tumors: These cancerous lesions are rare in childhood but can occur in adulthood.
• Posterior polymorphous dystrophy: This is a disease of the cornea’s endothelium that usually occurs in people around 30 years of age.
• Keratoconus: This is the development of a cone-shaped cornea, usually by adolescence.
• Corneal ulcers: These are sores on the cornea that may develop due to infection.
• Keratitis: This is inflammation of the cornea, often due to contact lens use.
• Fuch's corneal dystrophy: An inherited disorder of the cornea
• Lattice corneal dystrophy: An inherited disorder of the eye characterized by amyloid deposits in the eye. This can result in a steadily progressive loss of vision. 
  

Tests

Most corneal conditions can be detected by an eye doctor through a visual exam. The doctor may shine a bright light into the eye to check the cornea for abnormalities. In a slit lamp examination, ophthalmologists will evaluate not just the cornea but also underlying structures. Sometimes, they may also use special eye drops that stain microscopic abrasions to make them visible when a blue light shines on them.