Corneal structure and function:

The cornea is the clear oval window at the front of the eye – it is the anterior part of the fibrous coat (main structural supporting layer) of the eye.  The fibrous layer continues as the white part of the eye (sclera and bulbar conjunctiva) around the back of the eye to complete the spherical globe. 

The cornea has distinct layers.   The outer layer is the multicellular epithelium which rests on a supporting basement layer (basement membrane).  Cells on the surface are continually shed into the tear film and replaced by cells from beneath.  The layers sitting on the basement membrane gradually replaces the cells above it.   The epithelium acts as a barrier against infection from the outside and helps limit the amount of water entering the cornea from the tear film bathing its surface.

Beneath the epithelium and its basement membrane the bulk of the cornea is comprised of a fibrous stroma.  The stroma is made up of regular layers of collagen rather like the layers of an onion.  The fibrils which make up the collagen layers are separated by a matrix of water,  proteoglycans,  and various ionic constituents. The regular spacing and small size of the collagen fibrils is important in maintaining the transparency of the cornea (needed for normal vision).  Any increase in the water content of the cornea (edema) can result in a loss of transparency.

The deepest layers of the cornea are the single cell layer of endothelium and its basement membrane (Descemet’s’s membrane) which sits against the stromal layer.  The endothelium does not have regenerative capacity as the outer epithelium – if endothelial cells are damaged or lost they do not replace themselves.  The corneal endothelium is important in regulating the water content of the cornea.  If the cells are damaged fluid will accumulate in the corneal stroma (edema – loss of transparency)