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C. K. Palaniappan, T. J. Millar; Effects of Cholesterol, Cholesteryl Oleate and β Carotene on Meibomian Lipid Films. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4156.
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Studies indicate that levels of cholesterol and its esters might be critical to the stability of the lipid layer of tear film. It has also been suggested that retinoids taken orally e.g. β-carotene, affect lipids of the eye. Retinoids also change the structure and dynamics of biomembranes. β-carotene has no polar groups and therefore may influence the outer part of the lipid layer whereas cholesterol being polar is likely to affect the aqueous lipid interface. Hence, the effect of these compounds on human Meibomian lipid films was investigated in vitro.
Films of known molar ratios of cholesterol, cholesteryl oleate or β-carotene mixed with human Meibomian lipids spread on an aqueous subphase were monitored by measuring surface pressure (Π) in a Langmuir trough. The free energy of mixing was estimated. Structural differences in the films were observed using fluorescence microscopy.
Cholesterol and cholesteryl oleate up to 10% molar ratios had minimal effect on Π in mixed films and hence the free energy of mixing was close to 0. In contrast, β-carotene, even <1% molar ratio, markedly reduced the surface area occupied by the film and decreased the free energy of mixing, particularly at low Π. Dynamic Π-area isocycles showed that the surface area where Π changes first occurred was very reduced with β-carotene in the film, but the maximum Π was unchanged. Cholesterol mixed Meibomian lipid films were structurally similar to normal Meibomian lipid films. They were amorphous in appearance with mixtures of regions with more intense and less intense fluorescence. Films with β-carotene at low Π, were overall low in fluorescence intensity with darker stringy regions surrounding the brighter regions. At high Π the darker stringy regions became much darker and then disappeared from the film - possibly being pushed off the surface.
The results were unexpected because cholesterol mixtures in other lipid systems show a considerable lowering of free energy (stabilisation of lipid films). Surface pressure and possibly structural integrity of human Meibomian lipid films is resistant to large changes in free cholesterol and cholesterol esters but sensitive to small changes in carotenoids.
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