Tear film structure and dry eye

The outer layer of the ocular tear film is formed from meibomian gland lipid secretions (meibum). This lipid layer is highly structured with a polar phase adjacent to the aqueous-mucin layer and a nonpolar phase anterior to the polar phase. It is the structured polar phase, with its surfactant properties, upon which the nonpolar phase depends for integrity. Defects in the polar phase, such as low levels of structural phospholipids, can result in disease conditions. One such condition has signs consistent with dry eye and results when levels of the phospholipids phosphatidylethanolamine and sphingomyelin are low. These findings will stimulate development of better tear replacement therapies.