Corneal endothelium

The corneal endothelium is a single layer of endothelial cells on the inner surface of the cornea. It faces the chamber formed between the cornea and the iris. The corneal endothelium are specialized, flattened, mitochondria-rich cells that line the posterior surface of the cornea and face the anterior chamber of the eye. The corneal endothelium governs fluid and solute transport across the posterior surface of the cornea and maintains the cornea in the slightly dehydrated state that is required for optical transparency. The corneal endothelium is embryologically derived from the neural crest. The postnatal total endothelial cellularity of the cornea (approximately 300,000 cells per cornea) is achieved as early as the second trimester of gestation. Thereafter the endothelial cell density (but not the absolute number of cells) rapidly declines, as the fetal cornea grows in surface area, achieving a final adult density of approximately 2400 - 3200 cells/mm2. The number of endothelial cells in the fully developed cornea decreases with age up until early adulthood, stabilizing around 50 years of age. The normal corneal endothelium is a single layer of uniformly sized cells with a predominantly hexagonal shape. This honeycomb tiling scheme yields the greatest efficiency, in terms of total perimeter, of packing the posterior corneal surface with cells of a given area. The corneal endothelium is attached to the rest of the cornea through Descemet's membrane, which is an acellular layer composed mostly of collagen IV. The principal physiological function of the corneal endothelium is to allow leakage of solutes and nutrients from the aqueous humor to the more superficial layers of the cornea while at the same time pumping water in the opposite direction, from the stroma to the aqueous. This dual function of the corneal endothelium is described by the "pump-leak hypothesis." Since the cornea is avascular, which renders it optimally transparent, the nutrition of the corneal epithelium, stromal keratocytes, and corneal endothelium must occur via diffusion of glucose and other solutes from the aqueous humor, across the corneal endothelium.