Functions Of The Cornea
The cornea performs a number of functions; most importantly:
- the cornea is the first focussing element of the eye. Incoming light passing through the central cornea is refocussed before passing through a second focussing structure, the lens. This allows incoming light to be focussed on or near the retina. To perform this focusing well, the cornea needs to be transparent, clear from opacities or scars and both front and back corneal surfaces have to be smooth.
- it provides a protective barrier to the eye, both mechanical and a barrier to prevent micro-organisms such as bacteria and viruses from entering.
Structure of the cornea
It consists of a number of layers, each layer performing a different function. From front to back the important layers (and their thicknesses) are:
Epithelium And Basement Membrane (50µm)
The epithelium is a multilayer of cells which originate in the periphery of the cornea and grow into the centre and are sloughed off. There is a continual process of the cells being turned over and regenerated.
This is a relatively inert structure bounded in front by epithelium/basement membrane and behind by Descemet’s Membrane and endothelium.
Clarity of the cornea is maintained by an endothelial cell layer on the back surface, which sit on something called Descemet’s Membrane. Collectively these cells are called the endothelium: they pump fluid from the cornea into the anterior chamber, thereby keeping the cornea clear. Endothelial cells are dependent on oxygen for this function. This removal of fluid from the cornea maintains its transparency.
Where there is only a mild reduction in endothelial cell number, the patient is asymptomatic. With a further reduction in number, the first symptom is blurred vision on waking in the morning. Vision clears over a period of minutes to hours each day. As further cell loss occurs, blurred vision lasts longer and ultimately lasts throughout the day. In late stages, the eye may even become painful.