Cornea

The cornea is a transparent avascular tissue that functions as a window and allows light to enter the eye. Thanks to its mechanical stability, it counteracts the intraocular pressure and ensures the shape of the eye remains stable. It is almost impossible for microorganisms to penetrate the cornea and reach the interior of the eye chamber.

The cornea consists of six layers:

• the corneal epithelium with underlying basement membrane
• the corneal stroma with Bowman’s layer
• the corneal endothelium with Descemet’s membrane 

The corneal epithelium is the outermost layer of the cornea. It is composed of approx. 5-7 layers of cells stacked on top of each other. The epithelium cells are all produced in the underlying basement membrane/layer by means of cell division. The epithelium is capable of full regeneration since it is renewed every seven days, which is important for healing eye wounds. This process is also referred to as “turnover” or apoptosis (programmed cell death).
 

The corneal stroma is the thickest layer of the cornea representing about 80% of total corneal thickness and consisting of hydrogel (hydrogel = “water with a rubbery consistency”). This type of hydrogel is composed of 80% water, 15% collagen (long protein chains with great tensile strength) and 5% GAG (glycosaminoglycane).
The main role of the stroma is to counteract the forces that impact the eyeball from the inside (IOP) or outside.
Stromal regeneration is limited due to its collagen structure. In the event of an injury, repair cells (fibroblasts) migrate into the tissue and create a new tight network of collagen structures (scar tissue) which is no longer transparent. 

The corneal endothelium is a single layer of cells with a hexagonal shape on the back of Descemet’s membrane. This cell layer is created only once and cannot be renewed. Once the endothelium cells are destroyed, they are lost forever. The human cornea has an endothelial cell density of about 6,000 cells/mm² at birth. By the age of 20, this number is down to about 2,500 cells/mm² and continues to decrease by approx. 25-30 cells per year and mm². Since the corneal endothelium plays a key role in the metabolic exchange and the nutrition of the cornea, the loss of endothelial cell density is compensated by two special mechanisms – polymegathism and polymorphism. The remaining cells change in size and shape to counteract this decrease in the number of cells and maintain the metabolism and nutrient supply.