The cornea is essentially the ' window' of the eye. It represents the clear outer surface which forms the front of the eye. The cornea is made up of several layers. The epithelium is the surface layer and the endothelium is the innermost layer. The two layers are separated by the stroma, which helps maintain the shape of the eye. The endothelial cells are water pumps and are responsible for maintaining the transparency of the cornea. If they become damaged, the cornea will become water-logged and cloudy and vision will be impaired. Patients with advanced endothelial cell failure may develop painful blisters under the epithelial cell layer, called bullae.