The cornea is the outermost layer of your eye. Covering the front surface of your eye, it’s dome-shaped and clear. Your cornea helps focus your vision, providing 65 to 75 percent of the focusing power of your eye. Even though your cornea is clear and looks simple, it contains specialized tissue that differs from most tissues in your body because it contains no blood vessels to protect it from infection. The cornea is nourished by your tears and aqueous humor, which is the fluid behind the cornea in the front part of your eye.
Your cornea serves as a barrier against dirt, germs, and other things that can harm your eye. It shares the protection of your eye with the sclera (the white of your eye), eye sockets and eyelids. The cornea works as a filter to keep the damaging UV rays of the sun from harming your retina. Your cornea has five layers with membranes in between. Each layer has its own important function: Epithelium: The cornea’s outside layer, the epithelium functions primarily as a shield to protect the eye from foreign objects like bacteria, dust and water. It’s also a smooth surface that absorbs nutrients and oxygen from your tears for the other corneal layers. The epithelium has thousands of tiny nerve endings, which is why rubbing or scratching your eye causes pain. The epithelial cells are attached to an anchor called the basement membrane. Bowman’s Membrane: The second layer, behind the basement membrane, Bowman’s membrane is a transparent film made up of protein fibers known as collagen. If you injure your Bowman’s membrane, it can become scarred. Depending on the size and location of the scars, this can result in vision loss. Stroma: The next layer down, the stroma is the thickest layer within the corneal structure. It’s made up mostly of collagen and water. The collagen helps the cornea keep its form, elasticity and strength. The stroma’s unique arrangement, shape and spacing of collagen proteins produce the light-conducting ability of your cornea.
The innermost layer of your cornea, the cells of the endothelium function to keep the cornea clear by maintaining a fluid balance. If the endothelium didn’t perform this function, the stroma would become cloudy. The endothelium also adds collagen to the basement membrane. Endothelial cells that are damaged or destroyed are not repaired or replaced by your body.