Anatomy of the Eye: Understanding the Cornea

The Cornea of the Eye

• The cornea is the clear, firm outermost layer of the eye that encounters the external environment.
• It is derived from the Latin word "cornu" or "cornea tela", meaning horn or horny tissue.

Cornea Function

• The cornea has three important functions:
  • to bend or refract light that enters the eye
  • to focus light entering the eye
  • to provide a physical barrier for the eye and protect it from germs, dust, etc.

Location and Structure of the Cornea

• The cornea is located at the anterior-most part of the eye at the surface that is exposed to the external environment.
• It covers the iris, pupil, and the anterior chamber of the eye.
• It is connected to the sclera and the junction between them is known as the corneal limbus.
• The structure of the cornea is mostly made of collagen and includes six distinct layers.

Layers of the Cornea

• The layers of the cornea from outermost to innermost are:
  • Corneal epithelium
  • Bowman's layer
  • Corneal stroma
  • Dua's layer
  • Descemet's membrane
  • Corneal endothelium

Corneal Epithelium

• The corneal epithelium is made of epithelial cells known as stratified squamous epithelium.
• It is non-keratinized tissue made of about six layers of cells.
• It regenerates quickly and easily at the basal layer allowing for quick healing of the outer surface of the eye.
• It stays wet due to lubrication from tears.
• Its function is to allow for refraction or bending of the light that enters the eye.

Bowman's Layer

• The Bowman's layer is a connective tissue layer which consists of randomly laid collagen fibrils.
• It also contains proteins like laminin, nidogen, and perlecan.
• It does not contain any cells and its function is to protect the corneal stroma underneath.
• This layer is not present in the eye of non-primates.

Stroma

• The corneal stroma is the thickest layer of the cornea making up approximately 90% of the entire corneal thickness.
• It contains connective tissue in the form of collagen fibrils that are laid down in a regular pattern.
• It contains keratocytes.
• These cells help with cellular regeneration.
• Its function is to provide bulk of corneal structure and to allow repair and regeneration of cells and tissue.

Dua's Layer

• The Dua's layer is a very thin layer of cornea.
• It is shown to be very strong withstanding high air pressure and having minimal effects to outside air.
• Its function is to provide protection to structures and layers underneath it.
• This layer was discovered in 2013 by Dr. Harminder Dua at University of Nottingham during transplant research.

Descemet's Membrane

• The Descemet's membrane is a layer of connective tissue and contains collagen Type 4 fibrils.
• Its function is to serve as basement membrane for the corneal endothelium underneath.
• The cells for the endothelium are derived from this layer.
• It is also known as the posterior limiting membrane.

Corneal Endothelium

• The corneal endothelium is the sixth and final layer of the cornea.
• It contains a single layer of simple squamous or cuboidal layer of cells.
• It is surrounded by fluid known as aqueous humor.
• Its function is to allow movement of fluid and salt between the stromal layer and the aqueous humor.
• This layer is different from the epithelium in that it does not regenerate.

Corneal Disorders

• The structure of the cornea is susceptible to physical damage due to its location on the surface of the eye.
• Some common problems associated with cornea include:
  • Corneal abrasion: loss of surface of cornea, esp. the epithelial layer, due to a scratch or another trauma to the outer surface of the eye.
  • Keratitis: corneal inflammation.
• Serious disorders that can cause permanent degeneration of the cornea include:
  • Keratoconus: a degenerative disease where the cornea undergoes thinning and changes shape from a semicircle to a cone.
  • Fuch's dystrophy: a genetic condition characterized by cloudy vision isolated to morning time only.