The content of the meibomian gland lipid exprimate is known, but little is known about the phospholipids that comprise the glandular cells. The purpose of the present study is to identify and quantitate the phospholipid complement of the meibomian gland cells that produce the lipid secretion of meibomian oil and which is vital to tear film stability. Eyelids (n = 50) were excised from rabbits, and after surgical removal of surrounding tissues, the tarsal plates with and without expressing meibomian oil were extracted and phospholipids of the plates quantified by 31P nuclear magnetic resonance (NMR). Seventeen phospholipids were quantified from tarsal plates expressed of oil and tarsal plates containing meibomian oil: alkylacylphosphatidylcholine (AAPC), dihydrosphingomyelin (DHSM), dimethylphosphatidylethanolamine, diphosphatidylglycerol (cardiolipin), ethanolamine plasmalogen (EPLAS), lysoethanolamine plasmalogen, lysophosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylserine, phosphatidic acid, phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol, phosphatidylinositol, phosphatidylserine, sphingomyelin (SM), sphingosylphosphorylcholine. The six zwitterionic and neutral phospholipids, DHSM, EPLAS, PE, SM, AAPC, and PC together comprise 79.5% of the total meibomian gland phospholipid profile (in meibomian oil this value is 84.2%). The zwitterionic and neutral phospholipids dominate meibomian gland phospholipid profiles. Since the meibomian gland cells undergo holocrine secretion and form the meibomian glad secretion, such a composition is consistent with the hypothesis that a chemically stable lamellar surfactant layer phospholipids bind non-polar meibomian oil to the aqueous layer of the tear film.
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"It has been shown to be a very sensitive, repeatable and reliable method of analysis and thus was chosen for this experiment. Cholesterol was selected as a representative non-polar lipid as it has been widely cited to be one of the most prevalent deposited lipids [57,62,70-72] and phosphatidylcholine was chosen as a polar lipid species, due to its presence in the tear film [11,73,74]. "
[Show abstract][Hide abstract] ABSTRACT: To characterize various properties of a physiologically-relevant artificial tear solution (ATS) containing a range of tear film components within a complex salt solution, and to measure contact lens parameters and lipid deposition of a variety of contact lens materials after incubation in this ATS.
A complex ATS was developed that contains a range of salts, proteins, lipids, mucin, and other tear film constituents in tear-film relevant concentrations. This ATS was tested to confirm that its pH, osmolality, surface tension, and homogeneity are similar to human tears and remain so throughout the material incubation process, for up to 4 weeks. To confirm that silicone hydrogel and conventional hydrogel contact lens materials do not alter in physical characteristics beyond what is allowed by the International Organization for Standardization (ISO) 18369-2. The diameter, center thickness, and calculated base curve were measured for five different lens materials directly out of the blister pack, after a rinse in saline and then following a two week incubation in the modified ATS. To test the ATS and the effect of its composition on lipid deposition, two lens materials were incubated in the ATS and a modified version for several time points. Both ATS solutions contained trace amounts of carbon-14 cholesterol and phosphatidylcholine, such that deposition of these specific lipids could be quantified using standard methods.
This ATS is a complex mixture that remains stable at physiologically relevant pH (7.3-7.6), osmolality (304-306 mmol/kg), surface tension (40-46 dynes/cm) and homogeneity over an incubation period of three weeks or more. The physical parameters of the lenses tested showed no changes beyond that allowed by the ISO guidelines. Incubations with the ATS found that balafilcon A lenses deposit significantly more cholesterol and phosphatidylcholine than omafilcon A lenses (p<0.05) and that removing lactoferrin and immunoglobulin G from the ATS can significantly decrease the mass of lipid deposited.
This paper describes a novel complex artificial tear solution specially designed for in-vial incubation of contact lens materials. This solution was stable and did not adversely affect the physical parameters of the soft contact lenses incubated within it and showed that lipid deposition was responsive to changes in ATS composition.
"Quantitative comparison of compositional differ ences between lipids excreted from the meibomian gland (ML) and lipids in tear fluid is difficult because only three comprehensive human meibum lipid compositional stud ies have been published that encompass the percentage of all species of wax esters, cholesterol and choles terol esters, phospholipids and diglycerides (Cory et al., 1973; Nicolaides et al., 1981; Tiffany, 1978). Numer ous studies to quantify individual classes of lipid, such as phospholipids (Greiner et al., 1996) have been pub lished. The accuracy and precision of measurements of the lipid composition of meibum is complicated by the large variation in composition from person to person (Tiffany, 1978), the paucity of sample and the complexity of lipid species (Nicolaides et al., 1981). "
[Show abstract][Hide abstract] ABSTRACT: Infrared and fluorescence spectroscopies were applied to characterize the molecular conformational/structure and dynamics of human meibum (ML) and tear lipids (SSL). ML lipids contained more CC and CH3 moieties than SSL. SSL contained OH groups that were not apparent in the spectra of ML. The CO stretching band observed in the infrared spectra of SSL and ML revealed that the CO groups are not involved in hydrogen bonds. Bands due to the polar moieties CO and PO2- did not change significantly with increasing temperature, suggesting that they may not play an appreciable thermodynamic role in the lipid hydrocarbon chain phase transition. Components in tears bind to SSL and exclude water at the water-lipid boundary where the polar headgroups of phospholipids are located. If similar interactions occur in vivo at the tear film lipid-aqueous interface, they would reduce the rate of evaporation. The results provide a foundation for future studies to assess possible differences with age and sex in tears from normal and dry eye subjects.
Chemistry and Physics of Lipids 07/2007; 147(2):87-102. DOI:10.1016/j.chemphyslip.2007.04.001 · 2.42 Impact Factor
"For example, the presence of only 10% plasmalogen (a polar lipid similar to PC but with an ether bond and polyunsaturated fatty acid) reduced polar lipid surface tension by 50% and the monolayer viscosity by 80% (Tolle et al., 1999). Some evidence for the presence of plasmalogens in meibum has been reported (Greiner et al., 1996). Furthermore, monolayers with galactosphingolipids (cerebrosides) containing hydroxy-fatty acids readily form bilayers but if instead the fatty acid is unsaturated or a very long chain saturated fatty acid, bilayers are only formed with difficulty and after compression may not expand effectively again (Johnston and Chapman, 1988). "
[Show abstract][Hide abstract] ABSTRACT: The outer layer of the tear film--the lipid layer--has numerous functions. It is a composite monolayer composed of a polar phase with surfactant properties and a nonpolar phase. In order to achieve an effective lipid layer, the nonpolar phase, which retards water vapor transmission, is dependent on a properly structured polar phase. Additionally, this composite lipid layer must maintain its integrity during a blink. The phases of the lipid layer depend on both lipid type as well as fatty acid and alcohol composition for functionality. Surprisingly, the importance of the composition of the aqueous layer of the tear film in proper structuring of the lipid layer has not been recognized. Finally, lipid layer abnormalities and their relationship to ocular disease are beginning to be clarified.