The Cornea

The human eye has three major components . One is the Cornea, which is a transparent film like structure that transmits the image into the eye. The transmitted light rays/image cross the Lens which is the second major component that focuses the received image/light rays appropriately on to the Retina. Retina is the third major component, responsible for the conversion of the physical light rays/images into a neural signal perceivable by the brain. All these three major components and their accessories should be structurally and functionally intact for a clear and correct vision. Unless all these three components are properly intact and are functioning vision may be affected3.


Fig. 1. Basic anatomical structure of the human eye

Cornea

The cornea is the transparent dome shaped outermost part of the eye.

The cornea has five layers

  • The epithelium
  • Bowman’s Layer
  • Stroma
  • Descemet’s membrane
  • Endothelium
  • Fig. 2. Layers of the cornea

Epithelium
The epithelium is the outermost layer which serves as an effective barrier against the entry of foreign substances like dust, bacteria etc into the eye. It is composed of layers of non-keratinized stratified squamous epithelium in which the cells of the superficial layer are shed constantly and are replaced by the cells generated by multiplication in the basal layer. The epithelium is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched. The part of the epithelium that serves as the foundation on which the epithelial cells anchor and organize themselves is called the basement membrane.
Bowman’s Layer
It is a tough layer that is composed of collagen fibres and serves to protect the underlying stroma
Stroma:
The stroma is the thick transparent middle portion of the cornea which is made up of Collagen fibres arranged in a regular lattice arrangement. It is made up of nearly 200 layers of Type-I collagen fibrils .It also contains a sparse population of interconnected keratinocytes which serve for the repair and maintenance. . Collagen gives the cornea its strength, elasticity, and form
Descemet's Membrane

It is a thin acellular layer also made up of collagen fibres of a different form from that of stroma and they serve as a basement membrane of the corneal endothelium beneath it. It can regenerate readily after injury
Endothelium:
 The corneal endothelium is made up of a  simple squamous or low cuboidal monolayer of mitochondria-rich cells that serve to maintain the fluid balance between the corneal stromal compartments. The corneal endothelium is bathed with the aqueous humor. Unlike the corneal epithelium the cells of the endothelium do not regenerate. Instead, they stretch to compensate for dead cells which results in reduced overall cell density which in turn will have an impact on the fluid balance of the cornea
Corneal Facts:

  • Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. Instead, the cornea receives its nourishment from the tears and aqueous humor that fills the chamber behind it.
  • As the cornea does not have a vascular supply it gets its Oxygen directly from the air. The Oxygen first dissolves in the tears and then diffuses throughout the cornea to keep it healthy. The waste product of a healthy cornea is Carbon Dioxide which must be disposed of. This diffuses out of the cornea and into the atmosphere in the reverse process
  • The cornea must remain transparent to refract light properly, and the presence of even the tiniest blood vessels can interfere with this process. All layers of the cornea must be free of any cloudy or opaque areas for the vision to function properly.
  • Once the Corneal endothelial cells are destroyed by disease or trauma, they are lost forever with corneal transplantation being the only available therapy.
  • The cornea acts as the eye's outermost lens. It functions like a window that controls and focuses the entry of light into the eye .The cornea contributes to approximately 2/3 of the refracting power of the eye along with the tear film. It contributes ±43 diopters. Refractive errors occur when the curve of the cornea is irregularly shaped (too steep or too flat). When the cornea is of normal shape and curvature, it bends, or refracts, light on the retina with precision. However, when the curve of the cornea is irregularly shaped, the cornea bends light imperfectly on the retina which leads to reduced vision