The transparent part of the eye(s) and it is located in the front of the eye(s), it is the part that covers the pupil, anterior chamber, and the iris. In combination with the lens and the anterior chamber it can refract light, however, around two-thirds of optical power in the eyes come from the cornea and it can also be reshaped by a procedure referred to as LASIK. The refractive power of humans is around 43 dioptres. Although it contributes to most of the focusing that the eye(s) is doing, it does not have a fixed focus. It focuses light in order to see objects that are up close, which is referred to as accommodation and this can be accomplished through a geometry change within the lens. The medical terms that are related to the cornea will usually begin with a prefix of ‘kerat-’, which is from the work ‘Kepac’ and is a Greek word meaning horn.
There are unmyelinated nerve endings in the cornea that are very sensitive, it can hurt from being touched, from hot and/or cold drinks, and even certain chemicals. It will have an involuntary reflex happens when it is touched that makes the eyelid(s) close automatically. A cornea which is healthy has no need for blood vessels, as it uses oxygen for dissolving tears, and afterwards, diffusing this entire part of the eye in order to keep it healthy. It is very close to the way that nutrients get transported though diffusion of the tear fluids to the area on the outer surface, including the aqueous humour, which is done by going through the inside surface, including the neurotrophins that the nerve fibres supply in order to innervate it. The diameter of a human’s cornea is around 11.5 mm, with a thickness of about 0.5to 0.6 mm at the center point and at the periphery, it is about 0.6 to 0.8 mm. The cornea is a unique tissue as it has immunologic privileges, consists of an immature resident immune cells, avascularity, and transparent. Albumin is the most abundant soluble protein found in the mammalian cornea.
In humans, it is a border with both, the corneal limbus and the sclera. It is mostly an extension for the sclera in lampreys, as it is detached from the skin just above it. However, with vertebrates that are more advanced it will be fused to the skin in a way that it forms just one structure (albeit, one made from many layers). Whereas, having virtually a refractive index, the same that water does, it does not have a need to focus light. The cornea of humans has six layers (if the layer of Dua is being included), otherwise, there are only five layers. Furthermore, the cornea of primates such as dogs, cats, others that are carnivores, and wolves will only have four.
The following is a list of the layers of the human cornea starting with the anterior, and going to the posterior.
- Corneal epithelium: A layer of tissue referred to as the multi-cellular epithelial layer, and a non-keratinized stratified which is exceedingly thin layer that grows rapidly and also regenerates cells without any problems, and it stays moist through tear fluids.
- Bowman’s layer: This is also referred to as an anterior limiting membrane and despite the subepithelial basement membrane, this layer is tough and is composed with collagen (mostly type I collagen fibrils), including nidogen, perlecan, laminin, and some more HSPG’s that has a purpose of protecting the corneal stroma. This layer can be described as being a condensed area in the apical stroma and acellular that is made up primarily from woven collagen fibrils, which interact and attach with one another. This particular layer is 8 to 14 micrometres (μm) in thickness, it is also very thin or absent of non-primates.
- Corneal stroma: (Also known as substantia propria), which is transparent and thick in the middle layer, and has collagen fibers arranged (these are the general repair and the maintenance cells), and are distributed via inter-connected keratocytes. They can be found parallel, with super-imposed the way that pages in a book are. It consists of around 200 layers, which are mostly type I collagen fibrils. The individual layers are 1.5 to 2.5 μm. With up to 90% of the thickness in the cornea being made up of stroma.
The following are two different theories as to how the transparency of the cornea comes into play:
Because of the lattice arrangement that occurs with collagen fibrils, the single fibrils’ light scatter is canceled due to destructive interference of different individual fibrils scattered light.
- In order for transparency to take place, neighboring collagen fibrils’ need space within the stroma needs to be under 200 nm (Goldman and Benedek).
- Because of the lattice arrangement that occurs with collagen fibrils, the single fibrils’ light scatter is canceled due to destructive interference of different individual fibrils scattered light.
- Descemet’s membrane: This is sometimes referred to as the posterior limiting membrane. Which is a thin layer of the acellular, which is a modified foundation of the endothelium, whee cells originated from. It is made-up mostly of collagen type IV fibrils. It is approximately 5 to 20 μm in thickness, which depends on the age of the subject. For the anterior of the Descemet’s membrane is a very strong, but thin layer, the Dua’s layer, which is around 15 microns in thickness and has the ability of withstanding of 1.5 to 2.0 bars.
- Cornea endothelium: This is a lower cuboidal mono-layer or merely a squamous, it is around 5 μm in thickness of cells rich in mitochondria. These cells help regulate the transport of fluids and solute among the aqueous and cornea. Meanwhile, the endothelium is washed in the aqueous humor, rather than the lymph or blood, as it consists of an origin that is different and has different functions, as well as its appearance being different which come from the vascular endothelia.
Being densely innervated the cornea falls in a category considered to be some of the body’s most sensitive tissues, as it is loaded with sensory nerve fibers by way of the ophthalmic division from the trigeminal nerve having 70 to 80 long ciliary nerves.