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I. A. Butovich, J. P. McCulley; Lipidomic Analysis of Normal and Dry Eye Human Meibomian Gland Secretions - Historical Survey and New Developments. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1546.
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The goal of this report is to provide insight on newly emerging data and ideas pertinent to the field of dry eye research and critically analyze available data on the lipids found in human meibomian gland secretions (MGS). It is believed that these lipids are important contributors to the tear film lipid layer (TFLL). In previous studies, lipid deficiencies and abnormalities in MGS were connected to various forms of dry eye. Thus, a comprehensive lipidomic analysis of meibum could provide critical information on the mechanisms and progress of the disease. The report is concerned with meibum collected from healthy and dry eye subjects.
Samples of normal MGS collected from equal numbers of males and females and an equal number of samples from a matching group of dry eye patients were characterized using high-performance liquid chromatography (HPLC) and atmospheric pressure ionization ion trap mass spectrometry (MS).
The dominant lipid species in every sample of MGS were found to be oleic acid-based wax esters, cholesteryl esters, and various triacylglycerols. Small amounts of diacylglycerols and free cholesterol (less than 0.5%, w/w, dry weight) were also detected in MGS. Oleamide was detected in some, but not all samples in quantities less than 0.05% (w/w). No appreciable amounts of ceramides were found in MGS. HPLC and MS methods used in the study were capable of resolving and detecting model mixtures of authentic phospholipids (e.g. phosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, phosphatidylcholine, sphingomyelin etc.) at nanogram levels and below, but showed little or no presence of these species in the MGS samples. Phosphocholine-based lipids were found in MGS in quantities less than 0.01% (w/w), if any. A side-by-side comparison of major lipids in normal and dry eye MGS showed that there were only subtle differences between them with no clear marker of dry eye disease.
These observations suggest that MGS are a major source of the nonpolar lipids of the oleic acid-based wax esters and cholesteryl esters families for the TFLL, but not of the previously reported phospholipid, ceramide, and oleamide components of the TFLL. The high degree of similarity of lipid composition of normal and dry eye meibum suggests that the differences might be much subtler than originally thought and could involve other components of tear film. The impact of these findings on the current models of TFLL and dry eye will be discussed.
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