Definition of Corneal Transplantation

The cornea is a transparent tissue that does not contain blood vessels and forms the front fifth of the wall of the eyeball. The cornea is in contact with the external environment through a layer of tears. The function of the cornea is as follows: 1- Allowing light to enter the eye. 2- Forming a clear image on the cornea. 3- Protecting the rest of the eye from external factors such as microbes. The dimensions of the cornea in adults are about 11-12 mm in the horizontal axis and 9-11 mm in the vertical axis. Its thickness is 5/0 mm in the middle and gradually increases to reach 7/0 mm at the edge. The curvature of the cornea provides two-thirds of the refractive power of the eye, equivalent to 40-44 diopters. The transparency of the cornea depends on three factors: A- Loss of blood vessels in the cornea B- The orderly arrangement of collagen fibers, which form the main tissue of the cornea C- Regulation of the water content of the cornea, which is done through the correct role of the corneal endothelial cells. Any defect in the aforementioned factors leads to corneal opacity and poor vision. The cornea is oxygenated and nourished through three sources, including the tear layer, aqueous humor, and blood vessels at the edge of the cornea. The cornea consists of five different layers, and a group of researchers consider the tear layer, which is located in front of the cornea and plays an important role in maintaining its health, to be one of the layers of the cornea. However, the main layers of the cornea from outside to inside are as follows: 1- The epithelial layer that covers the surface of the cornea and protects it from bacterial factors, and therefore proper repair of the epithelial layer after corneal transplantation is necessary for the success of the operation. 2- Bowman's layer, which is a collagen layer devoid of cells, with a thickness of 12 microns and is located under the epithelial layer. 3- The stromal layer, which constitutes more than 90% of the thickness of the cornea and contains corneal cells and collagen fibers. The transparency of this layer is due to the regular arrangement of collagen fibers and the regular water content by endothelial cells and the loss of blood vessels. The corneal layer transplantation operation is performed in this layer. 4- Descemet's membrane is a thin and flexible membrane with a thickness of 8-10 microns and this layer acts as a basement membrane for the endothelial cell layer. 5- The endothelial layer is a single-cell layer with a thickness of 5 microns and consists of hexagonal cells that lack the ability to reproduce and their number is greater at birth and gradually decreases with age. This layer plays a role in regulating the water content of the cornea and maintaining its transparency. Any damage to this layer as a result of some surgical operations inside the eye leads to corneal swelling and decreased transparency.