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J. P. McCulley, I. A. Butovich, J. C. Wojtowicz; Historical Brief on Human Meibum Lipids. Invest. Ophthalmol. Vis. Sci. 2008;49(13):83. doi: https://doi.org/.
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Meibomian gland (MG) secretes an oily substance, meibum that spreads across the eye surface, mixes with secretions produced by other ocular structures and creates a thin film. It plays a critical role in protecting the ocular surface. Since lipids are poorly miscible with water, they tend to locate at the air/aqueous interface. The protective efficacy of the tear film is believed to be related to the chemical composition of the lipid layer. The aim of this study is to review the literature on the composition of human MG secretions, provide an overview on methods of collecting meibum samples, methods of lipid analyses and the results obtained in previous studies.
Compilation of human meibum lipid composition analyses from 26 studies.
The usefulness of the data obtained about meibum and its quality depends on proper sampling and the analytical technique. Meibum sample collection has not yet been standardized. Historically, several methods have been developed which have yielded contradictory data regarding meibum expression vs. dissection of MG with surrounding tissue being homogenized. Analytical results have also varied by animal species. The amount of the collected sample typically is very small, between 0.1 and 2 mg. Therefore, in many studies pooled samples were analyzed instead of individual ones. Some of the analytical techniques used in the past have their shortcomings and are not well suited for analyzing small and complex samples such as meibum.Based on the reviewed literature, the major lipids present in meibum are of nonpolar origin: waxes, sterols and steryl esters, followed by triacylglycerides and fatty acids (free and long chain). As to the amphiphilic lipids, diacylglycerides were reported in fewer number studies, while monoacylglycerides - only in two. On the other hand, information on the composition of the polar lipids is more controversial. Meibum Phospholipids were found in small amounts (16% or less) in some studies, but not in the others.
Meibum contains a complex mixture of lipid molecules. Considering the limitations of past analytical procedures, inconsistencies arose due to variations in the methods of sample collection, preparation and analysis. It could be very difficult in certain cases to interpret and compare the results on MG lipids obtained in different laboratories. Therefore, it is important to continue their qualitative and quantitative characterization taking into account the limitations of the techniques used in the past. Historical analytical inconsistencies may be partly explained by the consideration that the tear film lipid layer may have contributing sources other than meibum.
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