One of the main features associated with the lipid layer of the tear film is that it lowers surface tension of the tear film, and this assists spreading of the tears across the ocular surface.
1–3 The ability of the lipid layer to withstand the stresses put on it during a blink cycle and resist collapse onto the ocular surface depends upon its structure and, therefore, its composition. Although there is variation in the data from different laboratories, overall their results show that meibomian lipids comprise approximately 90% cholesteryl and wax esters (CE; WE), 2% triglycerides, 4% omega hydroxy acyl fatty acids (OAHFA), and a small amount of free fatty acids (FA).
4–10 Within these lipid groups, the acyl chains vary in length and saturation, and oleic acid (C18:1; 18 carbons with one double bond) is a very commonly found acyl chain.
6,7,9 Approximately 80% of the waxes have a monounsaturated fatty acid chain, and this is oleic acid in 90% of cases.
9 The fatty alcohol always appears to be saturated and a very common WE is arachidyl oleate (note that the nomenclature for the WEs, Cxy:z-Cqr:s, indicates the alcohol chain first with xy carbons and z double bonds, and the acid component second with qr carbons and s double bonds; C20:0-C18:1).
9 With age it appears that the ratio of CEs to WEs (0.57) increases, whereas with meibomian gland disease, it (0.34) decreases.
8 In addition, infrared spectroscopy used to measure phase transitions (melting points) of meibomian lipids (not as a spread film) indicated that higher melting points were a feature of meibomian gland disease compared with age-matched normals. By correlating these findings with the phase transitions of WE and CE mixtures, it was inferred that the degree of saturation of WE had a disproportionate influence on increasing the melting point and led to more order in the meibomian lipid structure.
11 It was also shown
8 that infant meibum had a relative low CE to WE ratio (0.24), and since infants have very stable tear films, the question arises as to how the components of meibomian lipids add to its stability. In order to understand the influence of particular components of meibomian lipids on the physical properties of the lipid layer, different individual lipid species were examined in terms of their surface activity and in some cases when mixed with meibomian lipids. Measuring the surface activity of meibomian lipids in vitro has been used by several groups and has been useful to glean information about the structure of the lipid layer.
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