The eye works in much the same way as a camera – it detects incoming light reflected from objects in our environment and transforms them into images. At the back of the eye, a delicate, light-sensitive tissue called the retina, which is only the thickness of a piece of paper, senses these images and sends them to the brain for processing of these pictures of the outside world. As light travels into the eye, it has to be focused so that it creates an accurate image.
This involves bending or ‘refracting’ the light rays inward so that they reach a point at which a clear image is formed. In people with perfect vision, this ‘focal point’ is exactly on the retina – if not the image will be blurred, which is what happens when you are short or long sighted.
The first bit of the eye that light passes through is the cornea, which is the transparent bit which covers the iris and the pupil. The cornea contributes to most of the eye’s focussing power and is a very specialised tissue without which our eyes cannot function. Because transparency is of prime importance, the cornea does not have blood vessels. It receives nutrients via diffusion from the tear fluid at the outside and the aqueous humour at the inside and also from neurotrophins supplied by nerve fibres that innervate it. In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery.
Transparency, avascularity, the presence of immature resident immune cells, and immunologic privilege makes the cornea a very special tissue. The cornea has no blood supply; it gets oxygen directly through the air. Oxygen first dissolves in the tears and then diffuses throughout the cornea to keep it healthy.
Layers in the Cornea
The human cornea, like those of other primates has five layers; the corneas of cats, dogs, wolves, and other carnivores only have four. From the anterior to posterior the five layers of the human cornea are:
- Corneal epithelium: a thin epithelial multicellular tissue layer (non-keratinized stratified squamous epithelium) of fast-growing and easily-regenerated cells, kept moist with tears. Irregularity or oedema of the corneal epithelium disrupts the smoothness of the air-tear film interface, the most significant component of the total refractive power of the eye, thereby reducing visual acuity. It is continuous with the conjunctival epithelium which is composed of about 6 layers of cells which are shed constantly on the exposed layer and are regenerated by multiplication in the basal layer.
- Bowman's layer (also erroneously known as the anterior limiting membrane, when in fact it is not a membrane but a condensed layer of collagen): a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers.
- Corneal stroma (also substantia propria): a thick, transparent middle layer, consisting of regularly-arranged collagen fibers along with sparsely distributed interconnected keratocytes (the cells used for general repair and maintenance). They are parallel and are superimposed like pages in a book.Up to 90% of the corneal thickness is composed of stroma.
- Descemet's membrane (also posterior limiting membrane): a thin layer without cells that serves as the modified basement membrane of the corneal endothelium, from which the cells are derived.
- Corneal endothelium: a simple squamous or low cuboidal monolayer comprised of mitochondria-rich cells. These cells are responsible for regulating fluid and solute transport between the aqueous and corneal stromal compartments. The corneal endothelium is bathed by aqueous humour, not by blood or lymph, and has a very different origin, function, and appearance from vascular endothelia. Unlike the corneal epithelium the cells of the endothelium do not regenerate. Instead, they stretch to compensate for dead cells, which reduces the overall cell density of the endothelium and has an impact on fluid regulation. If the endothelium can no longer maintain a proper fluid balance, stromal swelling due to excess fluids and subsequent loss of transparency will occur.