Mesoscopic structure and surface behaviour of the tear film lipid layer

• Main Author: Bin Abdul Rahman, Irman
• Published: University of Exeter 2008
• Subjects: 617.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.484831

The tear film lipid layer (TILL) forms a continuous sheet that extends across the ocular surface from lid to lid. It acts as an effective barrier to water evaporation, decreasing the loss of the aqueous component of tears and aiding in the prevention of tear film breakup. It also provides a smooth optical surface for the cornea and aids in anti-microbial activity. The hypothesised structure of the TILL is a 3D multilayered structure composing of two lipid phases: an amphiphilic (polar) phase one molecule thick adjacent to the aqueous-mucin phase and a thicker overlaying hydrophobic (non-polar) phase intercalated with the aliphatic clIains of the amphiphilic phase. Although the hypothesised structure of the TILL has been extensively studied, there is little evidence that attests ·to the presence of this multilayered structure and resolves the lipid-lipid interactions that are responsible for the finer details ofits organisation. We applied a set of carefully selected techniques borrowed from the arsenal of membrane biophysics to investigate both the surface behaviour and mesoscopic structure of the natural and synthetic TILL systems, and subsequently'comparing the results to another artificial' system mimicking the inner leaflet layer of the red blood cell plasma membrane, that while sharing a significant proportion of their lipid composition, have different properties and serve distinctive biological functions. We demonstrate that the natural TILL and its synthetic replica do form a 3D multilayered structure, although the natural system displayed a much superior efficiency compared to the synthetic replica in forming and deforming this 3D structure. We also demonstrate that cholesterol ester (CE), a hydrophobic component of the TILL, promotes and sustains this 3D formation, providing a synergistic relation between the amphip4ilic and hydrophobic layers. Through fluorescence microscopy imaging, we demonstrated that the natural TFLL and the synthetic replica are separated into two major components (amphiphilic and hydr~phobic). At higher surface pressures, the hydrophobic domains cover the amphiphilic domains, reinforcing the conclusion of a multilayer formation. The natural TILL displays a more organised domain structure than the model system, encouraging us to speculate that these organised domains are similar to lipid rafts in the plasma membrane. Measurements of the TILL viscoelasticity demonstrate that the dilational modulus response is dominant in all the systems probed and is independent of. the oscillation frequency. The dilational modulus is however, dependent on the phase state in which the monolayer is in during oscillation.