It provides the sum or density of the endothelial cells in the inner layer of the cornea. At birth, the figure is close to 5,000 cells per square millimeter; by the age of 40, it drops to 3,000 cells per square millimeter and declines to about 2,000 cells per square millimeter by the age 80. Specific genetic changes cause premature corneal endothelial cell loss. The cell count is used to evaluate suspicious corneal appearance or thickness and for a wide range of procedures, particularly in cataract surgery and corneal transplantation.