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Understanding the molecular composition (e.g., proteins and lipids) of the tear film (TF) and the contribution of the meibomian gland to the TF is critical in gaining knowledge about TF instabilities, dry eye syndromes, contact lens (CL) incompatibilities, and other eye diseases. Among its functions, the lipid layer of the TF slows evaporation of the aqueous component, preserves a clear optical surface, and forms a barrier to protect the eye from microbial agents and organic matter, such as dust and pollen.1 The TF contains a complex mixture of proteins, enzymes, lipids, mucins, and salts that allows the TF to perform its functions (Fig. 1). Researchers believe the outer lipid layer is 5 to 10 molecules thick and is composed primarily of wax and sterol esters, possibly intercalated with each other and with proteins rather than forming distinct repeating layers of molecules.2,3 Evidence from interferometric studies indicate that the TF lipid layer thickness ranges from 20 to 160 nm.4 If the size of a lipid molecule is approximately 2.2 nm (22 Å), then the calculated thickness for one layer would be 11 to 44 nm. The addition of polar and nonpolar layers would add to the lipid thickness, which indicates that the lipid component of the TF may be multiple layers thick or have other contributing sources to correspond with reported thickness measurements.5    Figure 1.  A proposed model of the precorneal tear film showing the relationship and interaction of lipid-binding proteins and the outer lipid layer.    While the signs and symptoms of TF instability are reasonably well characterized, we are only beginning to understand the specific molecular components of the TF and their relationship with disease and TF stability. The purpose of this review is to examine the meibomian gland's contribution to TF lipids and lipid–protein interactions in health and disease.

investigative ophthalmology and visual science

The International Workshop on Meibomian Gland Dysfunction: Report of the Subcommittee on Tear Film Lipids and Lipid–Protein Interactions in Health and Disease