The Presence and Significance of Polar Meibum and Tear Lipids

The ocular tear film is a complex structure composed of a number of elements. While all of these components serve valuable functional and structural roles, the external lipid layer has been a focus because it is known to play a critical role in dry eye. Traditionally, meibomian gland phospholipids have been considered to be the vital amphiphilic molecules needed to create an interphase between the outer nonpolar lipid layer and inner aqueous layers, yet recent work has called this theory into question. The purpose of this review is to clarify the current understanding of the origins, identity, and significance of polar tear lipids. Studies indicate that both phospholipids and ω-hydroxy fatty acids likely play a critical role in tear film stability. Studies also indicate that polar lipids likely originate from multiple sources and that they are integrally involved in ocular surface disease. Additional studies are needed to fully understand the origins and significance of polar tear lipids, because to date only correlational evidence has described their hypothesized origins and functions.