Cornea

The Human Cornea. Credit: BSIP/UIG

What is the cornea?

The cornea is the transparent, dome-like structure on the front part of the eye. The cornea gives the eye two-thirds of its focusing or refracting power. One-third is produced by the internal crystalline lens.

What is the function of the cornea?

The cornea functions like a camera lens, helping to focus light coming into the eye onto the retina. The cornea is also filled with nerves that alert us to irritations that could potentially harm our vision and eye health.

What is the cornea made of?

The cornea is a remarkable piece of tissue made up of specialized cells. One thing unique to the cornea is that there are no blood vessels in the cornea to provide it nourishment. The cornea receives most of its nourishment directly from the tears on the surface and through the aqueous humor (a fluid that fills the anterior chamber of the eye) from inside the eye. Because the cornea is like a lens, it must be completely transparent and blood vessels would interfere with the focusing process.

The corneal tissue is arranged in six different layers:

  • Epithelium - The epithelium is the very top, outermost layer and makes up about 10 percent of the total corneal thickness. The epithelial cells serve to protect the cornea, transport nutrients to the rest of the cornea and provides a smooth optical surface for which light to focus. The epithelial cells also have thousands of small nerves. These nerves are very pain sensitive and allow us to know when something is causing harm to the eye. The epithelial cells grow on top of a basement membrane.
  • Bowman’s Layer - Bowman's layer is directly below the epithelial layer. It is composed collagen tissue. Bowman's layer is tough, but if it is damaged, scar tissue may form and recurrent openings of the injury can occur.
  • Stroma - The stroma makes up the bulk of the cornea. It consists of water and collagen. The collagen allows for the transparency and shape of the cornea. The layers of collagen are referred to as lamellae because of the way they are ordered and layered throughout the stroma. The anterior part of the stroma is layered differently than the posterior part of the stroma. This gives the cornea its strength, shape, and transparency. The stroma is the part of the cornea that is reshaped when refractive surgery, such as LASIK.
  • Dua's Layer - Dua's layer of the cornea is a recent discovery and research is pending. However, it is believed by most eye care scientists to be a previously unknown layer just 15 microns thick. Professor Harminder Dua discovered the layer between the corneal stroma and Descemet’s membrane. it is thought to provide strength to the cornea and knowledge of Dua's layer will provide insight to improve the safety of surgical procedures and why some people develop certain types of corneal disease.
  • Descemet’s Membrane - The next layer under the stroma is Descemet's membrane. Descemet's membrane is a strong but thin layer that protects the cornea against injuries and infection. Descemet’s membrane is also made of a different type of collagen and is produced by the endothelial layer directly below it.
  • Endothelium - The endothelium is a thin layer of cells that function to keep the cornea clear at all times. Endothelial cells act as little pumps to transfer excess fluid out of the stroma and back into the eye. If endothelial cells do not function well, the cornea because cloudy and difficult to see through. Endothelial cells are vital to the health of the cornea. If they are damaged or destroyed, the cornea will decompensate.

    How is the cornea measured and examined?

    Eye doctors can view the cornea under a slit lamp biomicroscope. A slit lamp is basically an upright microscope. A special slit beam is used to illuminate the different layers of the cornea. To measure the thickness of the cornea, doctors perform pachymetry. Pachymetry can be performed using an ultrasonic pachymeter or by using an optical coherence tomographer. The power and the curvature of the cornea can be measured by a corneal topographer

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