THE CORNEA



LAYERS OF THE CORNEA

The cornea contains most of the eye’s power—precisely two thirds of it.  It is the transparent and is shaped much like a dome which covers the front of the eye including the iris, pupil and the anterior chamber.   The cornea is comparable to the face of a watch since it is the part you look through.   It is transparent primarily due to the fact that it lacks pigment and does not have any blood vessels.   It is extremely sensitive as the cornea contains many nerve endings.  The entire cornea is approximately 0.5mm in thickness and is comprised of 5 layers: Epithelium, Bowman’s Layer, Stroma, Descemet’s Membrane and the Endothelium

 The cornea contains layers of tissue called lamellae which are arranged in a ‘lattice’ pattern.  This pattern allows light to pass through thus giving the cornea its transparency.

5 layers of the Cornea:

i.                 Epithelium:  The  epithelium forms 10% of the corneal thickness.  It is a membranous, multicellular layer which is not waterproof and therefore absorbs some of the tear film.

The epithelium has the ability to regenerate itself within a week, if scratched, abraded or damaged.  However, if the injury is deeper than the epithelial layer, then an opaque  scar results which causes the cornea to appear cloudy.   It is comprised of three cell layers: 

a.       Bottom/Basal Layer:  Columnar shaped, single layer of cells that move to the top

b.      Middle/Wing Cell Layer:  Polyhedral (various shapes) cells that flatten near the top

c.       Top/Surface Cell Layer:  These are squamous or flat cells

ii.               Bowman’s Membrane:  This is the condensed outer layer of the stroma.  It is made up of layered protein fibres called collagen.   Although the Bowman’s membrane is very resistant to injury, if it does sustain injury, there appears an opaque scar which may result in vision loss.

iii.              Stroma:  The stroma makes up 90% of the corneal thickness and is mostly made up of water and collagen.   There are no blood vessels in the stroma.   Bands of collagen fibres called lamellae are embedded in a ‘ground substance.’  This keep s the collagen arranged in a pattern to maintain transparency.

iv.              Descemet’s Membrane:  Located under the stroma, is a thin sheet of tissue which helps protect against infection, trauma and injuries.  It is secreted by the endothelium and able to regenerate itself after an injury.

v.                Endothelium:  This is a single layer of flattened cells attached to the Descemet’s Mebrane.   It is a very thin, innermost layer of the cornea.  These cells do not regenerate themselves in case of injury.   If too many cells are destroyed, blindness may occur leaving the only available option to be a corneal transplant.    The remaining cells spread out to fill in the space.  This layer’s primary purpose is to pump extra fluid out of the stroma

When light enters the cornea, it is refracted or bends onto the lens in the eye.  The lens also further focuses the light rays onto the retina, a layer of light receptor cells in the back of the eye.  In order for a person to see a clear image, light rays must bend and be focused by the cornea to fall on the retina.  The retina has an important job of converting the light rays into impulses via the optic nerve to the brain.  The bran converts these impulses to images and we are able to translate light into vision.  The cornea also serves as a filter to block out the most damaging ultraviolet light.