Four characteristics and a model of an effective tear film lipid layer (TFLL)

Abstract

It is proposed that a normal, effective tear film lipid layer (TFLL) should have the following four characteristics: 1) high evaporation resistance to prevent water loss and consequent hyperosmolarity; 2) respreadability, so it will return to its original state after the compression-expansion cycle of the blink; 3) fluidity sufficient to avoid blocking secretion from meibomian glands; 4) gel-like and incompressible structure that can resist forces that may tend to disrupt it. These characteristics tend to be incompatible; for example, lipids that form good evaporation barriers tend to be disrupted by compression-expansion cycles. It is noted that clues about the function and organization of the TFLL can be obtained by comparison with other biological lipid layers, such as lung surfactant and the lipid evaporation barrier of the skin. In an attempt to satisfy the conflicting characteristics, a "multilamellar sandwich model" of the TFLL is proposed, having features in common with the skin evaporation barrier.

Keywords: X-ray analysis; blinks; evaporation resistance; evaporative dry eye; lipid layer structure; lipid monolayers; meibomian gland dysfunction; skin lipid barrier.

Copyright © 2013 Elsevier Inc. All rights reserved.

Figures

Figure 1

Evaporation resistance as a function of carbon chain length for saturated fatty acids at 25°C. An exponential curve has been fitted to the experimental results. (Redrawn from Archer and LaMer.)

Figure 2

Collapse of lipid monolayers. Circles represent polar headgroups of lipid molecules whereas attached lines represent hydrophobic hydrocarbon chains. A. Upward folding towards the air. B. Downward folding towards the aqueous sub-phase. C. A parallel fold above the main monolayer surface. D. A parallel fold below the main monolayer surface.

Figure 3

Possible structures of the tear film lipid layer. Red lines represent polar lamellae and blue lines represent nonpolar lamellae. A. “Plate” model of nonpolar lamellae. B. Folded model of nonpolar lamellae. See text for details.

Figure 4

Possible molecular arrangement of cholesteryl and wax esters in the proposed nonpolar lamellae. For both cholesteryl and wax esters, the center of the lamellar thickness consists of interdigitated long, saturated chains. A. Cholesteryl esters. Long, saturated chains come from the fatty acid component. Rectangles correspond to the cholesterol rings, and the cholesterol side chain is represented by short lines at top and bottom. B. Wax esters. Long, saturated chains come from the alcohol component. The combining oxygen atoms are represented by circles, while unsaturated fatty acids (eg, oleic) are represented by kinked chains at top and bottom. C. Overall structure consisting of “domains” of cholesteryl and wax esters. D. An alternative overall structure where cholesteryl and wax esters are densely mixed on a molecular scale. E. A structure in which cholesteryl and wax esters are arrayed on opposite sides of the interdigitated chains.