The cornea has a diameter of approximately 11.5 millimetres, and its thickness usually varies between 500 and 600 micrometres. It is one of the most nerve-packed and sensitive tissues in the body. In contrast to other tissues, it does not contain any blood vessels or lymph vessels. This contributes to the brightness and the optics of the cornea. As a result, it does not benefit from a supply of oxygen or other nutrients. The cornea absorbs these things from the air and the anterior eye chamber.
The cornea consists of different layers (also known as lamella), starting from the outside with the epithelium, followed by the Bowman layer, the stroma, the Descemet membrane and the endothelium. Scroll down for a more detailed description of the different parts of the cornea.
The cornea’s outer layer consists of a thin ‘skin’-like’ layer, the epithelium. This layer makes up 10 per cent of the cornea thickness and consists of 5 to 6 layers of cells. It protects against infection and trauma and can repair itself fairly quickly without forming any scar tissue, unlike the other layers of the cornea. This is because the epithelium renews itself every 7 to 10 days using epithelial stem cells.
During this process of continuous cell rejuvenation, the thickness and structure of the epithelium and the arrangement of the cells remain the same. This is necessary to ensure an unchanging corneal strength with good optical quality. Moreover, the surface epithelial cells and the mucus layer of the tear film create a smooth, stable optical surface which is resistant to bacterial bonding.
The Bowman layer is located directly underneath the epithelium. This layer is 8 to 14 micrometres thick, contains no cells, and is rich in connective tissue. It is a sturdy layer that potentially plays a role as a biological barrier against the infiltration of pathogens, especially viruses. Additionally, this layer is presumably involved in the healing of wounds to the anterior stroma. Due to the sturdiness of the Bowman layer, it appears to be important for retaining the shape of the cornea. If this layer is damaged, it is not repaired, and scar tissue will be formed.
Approximately 85-90% of the entire cornea consists of the stroma. The stroma primarily consists of water (78%), connective tissue (15%) and protein (7%). Furthermore, the anterior part of the stroma also contains ‘stromal cells’ (known as keratocytes).
The connective tissue in the stroma is arranged in parallel bundles (fibrils), packaged in parallel layers or lamella. Each lamella is positioned perpendicular to the fibres in the adjacent lamellae. This lamellar structure helps retain the general form and sturdiness of the cornea. Additionally, the unique structure and organisation of the collagen fibrils in the lamellae keep the cornea transparent. A disruption of this structure can reduce the transparency of the cornea and can thus result in functional loss.
The Descemet membrane is located under the stroma. This membrane is created by the corneal endothelium and acts as a basal membrane for it. This membrane has a thickness of approximately 10-13 micrometres; the thickness increases over the years. An extremely strong and thin pre-Descemet layer can be found on the border between the stroma and the Descemet membrane.
The endothelium is a monolayer approximately 4 micrometres thick and primarily consists of hexagonal cells on the back of the cornea. It forms a semipermeable barrier which regulates the transfer of nutrients and fluid from the anterior eye chamber to the cornea and which simultaneously pumps fluid from the cornea to keep it clear. The entry and exit of fluid to and from the cornea are balanced when the endothelium is working properly.
If endothelial cells are lost or damaged, they have little or no ability to divide or multiply. Instead, these cells expand to compensate for lost cells. When this process occurs, the remaining cells increase in size (polymegathism) and lose their hexagonal form (pleomorphism).
Speculaire microscopiefoto van de endotheelcellaag
People are born with about 6000 endothelial cells per square millimetre. This number is reduced to approximately 3500 cells per square millimetre by the age of 5. The cell density gradually decreases with an average rate of 0.6% per year over an individual’s lifetime.
Accelerated cell loss may occur following an internal eye operation (like a cataract operation or a cornea transplant), trauma, a corneal disease (like Fuchs endothelial dystrophy) or glaucoma. The cornea generally remains clear until an endothelial density of approximately 500-700 cells per square millimetre is reached. The moisture balance in the cornea will become disrupted if even fewer cells are present; this results in corneal swelling, as well as reduced transparency and optical quality.
