The endothelium is the extremely thin, innermost layer of the cornea. Endothelial cells are essential to keep the cornea clear. Normally, fluid leaks slowly from inside the eye into the stroma. The endothelium’s primary task is to pump this excess fluid out of the stroma. Without this pumping action, the stroma would swell with water, become hazy, and eventually opaque.

Once endothelium cells are destroyed by disease or injury, they are lost forever. If a person loses too many endothelial cells, he or she might need a corneal transplantation.