The transparent front part of the eye which covers the iris, pupil and the anterior chamber is called the cornea. Along with the lens, the cornea distracts the light in which the cornea is responsible for two-third of the eye’s total visual power. In humans, the refractive power of the cornea is about 43 dioptres. Though the cornea is responsible for most of the visual power of the eye, its focus is fixed. The curvature of the lens is adjustable to “tune” the focus as it depends on the distance of the object.
The cornea consists of unmyelinated nerve endings which are very sensitive to temperature, chemicals and touch. Therefore the touch causes a reflex which is when we blink. The cornea does not have blood vessels, it is therefore transparent. It gets its nutrients through the tear fluid from outside and from inside through aqueous humour as well asneurotrophins from nerve fibres.
The diameter of the cornea is 11.5 mm and it has a thickness of 0.5-0.6 mm in the middle and 0.6-0.8 mm at the periphery in the human eye. Avascularity, transparency, the presence of immature immune cells and immunologic privilege make cornea a special tissue. The cornea gets oxygen from direct air and it does not have any blood supply. When the tears get oxygen supply it helps in keeping the cornea healthy throughout. Albumin is the most abundant soluble protein found in humans and animals.
In humans, the cornea has five layers while cats, wolves and dogs have only four corneas. The five layers from the anterior to the posterior in humans are as follows:-
This is a fine epithelial multicellular tissue layer. It consists of easily regenerated and fast growing cells which are kept moist with tears. Any irregularity of the corneal epithelium disturbs the smoothness of the air-tear film interface. The air-tear film interface is one of the most important components of the eye and hence it may reduce the visual power.
This is a tough protective layer which protects the corneal stroma. It consists of irregularly arranged collagen fibers which basically consist of type I collagen fibrils. These fibrils attach themselves with each other. It has a 14 micrometres thick layer and is not present in carnivores.
This is a transparent thick middle layer which consists of regularly arranged collagen fibers. These fibers are sparsely distributed with interconnected keratocytes and these are cells for maintenance and repair. The corneal stroma consists of about 200 layers of type I collagen fibrils.
It is a cellular layer which is thin and is like a modified basement membrane of the corneal endothelium. This layer mainly consists of type IV fibrils which are not very rigid as compared to collagen type I fibrils.
It is a low cubodial layer of about 5μm thick consisting of mitochondria rich cells. These cells help in regulating solute transport and fluid between the corneal stromal compartments and aqueous. The cells of the endothelium do not regenerate unlike corneal epithelium. It stretches to fill up for dead cells and thus the overall cell density is reduced and it therefore impacts the fluid regulation. In case if the endothelium is does not have a proper fluid balance, due to excess fluid the stromal swells and there is a loss of transparency in it.