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Peter King-Smith, Melissa Bailey, Richard Braun; A multi-lamellar sandwich model of the tear film lipid layer, TFLL. Invest. Ophthalmol. Vis. Sci. 2013;54(15):6007.
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© ARVO (1962-2015); The Authors (2016-present)
To develop a model of TFLL structure.
Creation of a model which is consistent with reported results including ocular evaporation, TFLL respreading after a blink, TFLL biochemistry, X-rays of meibum films, skin lipid studies, evaporation through monolayers, and compression-expansion cycles of lipid films.
The evaporation resistance of the normal TFLL is much higher than that of lipid monolayers used in evaporation control of reservoirs. High evaporation resistance in monolayers requires saturated hydrocarbon chains, and evaporation resistance increases greatly (exponentially) with chain length (Archer & La Mer, 1955, J Phys Chem 59, 200). Correspondingly, very long (e.g., 26 carbon) saturated chains are found in both meibum (Butovich, 2011, Prog Lipid Res 50, 278) and skin lipids. However, lipid films with saturated chains tend to be disrupted by compression-expansion cycles (Millar & King-Smith, 2012, IOVS 53, 4710). Fig. 1 shows a TFLL model which is compatible with these conflicting requirements of high evaporation resistance and respreading after the compression-expansion cycle of a blink. “Lamellar sandwiches” of non-polar lipids are superimposed on a polar lipid layer. Proposed molecular structure is given in Fig. 2. Long chains are tilted based on X-ray studies (Leiske et al., 2012, Langmuir 28, 11858). Cholesteryl esters (Fig. 2A) are shown as a bilayer with interdigitation of the long saturated acid chains (Alonso et al., 2001, J Phys Chem B 105, 8563). Rectangles represent cholesterol rings with the cholesterol side chains represented by short lines at top and bottom. Circles in Fig. 2B represent the connecting oxygen atoms in wax esters, with the acid components (often oleate) represented by the outer kinked chains and long saturated alcohol components forming the central interdigitation. Possible arrangements of esters are shown in Fig. 2C, D and E. While the cores of the sandwiches provide evaporation resistance, the lamellae can slip over each other to provide the fluidity needed for blinking, e.g., as in shuffling a pack of cards. A similar sandwich model has been proposed for skin lipids (Bouwstra et al., 2003, Prog Lipid Res, 42, 1).
The model forms a basis for discussion, with implications for evaporative dry eye.
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