Like cartilage, the cornea is avascular tissue; that is, without a blood supply. Therefore, wounds may not heal, because there is a limitation on the proliferation and migration of new cells to the corneal site. Ultraviolet light can damage the cornea and retina, providing impetus for people to wear sunglasses. There are many diseases and congenital conditions that attack corneas, but the most common threat is simply age. Seriously diseased or injured corneas do not regenerate well, but rather form scar tissue that obstructs vision.